path: root/compiler/GHC/Parser/PostProcess.hs

{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE ViewPatterns #-}
{-# LANGUAGE DataKinds #-}

{-# OPTIONS_GHC -Wno-incomplete-record-updates #-}

--
--  (c) The University of Glasgow 2002-2006
--

-- Functions over HsSyn specialised to RdrName.

module GHC.Parser.PostProcess (
        mkRdrGetField, mkRdrProjection, Fbind, -- RecordDot
        mkHsOpApp,
        mkHsIntegral, mkHsFractional, mkHsIsString,
        mkHsDo, mkSpliceDecl,
        mkRoleAnnotDecl,
        mkClassDecl,
        mkTyData, mkDataFamInst,
        mkTySynonym, mkTyFamInstEqn,
        mkStandaloneKindSig,
        mkTyFamInst,
        mkFamDecl,
        mkInlinePragma,
        mkOpaquePragma,
        mkPatSynMatchGroup,
        mkRecConstrOrUpdate,
        mkTyClD, mkInstD,
        mkRdrRecordCon, mkRdrRecordUpd,
        setRdrNameSpace,
        fromSpecTyVarBndr, fromSpecTyVarBndrs,
        annBinds,
        fixValbindsAnn,

        cvBindGroup,
        cvBindsAndSigs,
        cvTopDecls,
        placeHolderPunRhs,

        -- Stuff to do with Foreign declarations
        mkImport,
        parseCImport,
        mkExport,
        mkExtName,    -- RdrName -> CLabelString
        mkGadtDecl,   -- [LocatedA RdrName] -> LHsType RdrName -> ConDecl RdrName
        mkConDeclH98,

        -- Bunch of functions in the parser monad for
        -- checking and constructing values
        checkImportDecl,
        checkExpBlockArguments, checkCmdBlockArguments,
        checkPrecP,           -- Int -> P Int
        checkContext,         -- HsType -> P HsContext
        checkPattern,         -- HsExp -> P HsPat
        checkPattern_details,
        incompleteDoBlock,
        ParseContext(..),
        checkMonadComp,       -- P (HsStmtContext GhcPs)
        checkValDef,          -- (SrcLoc, HsExp, HsRhs, [HsDecl]) -> P HsDecl
        checkValSigLhs,
        LRuleTyTmVar, RuleTyTmVar(..),
        mkRuleBndrs, mkRuleTyVarBndrs,
        checkRuleTyVarBndrNames,
        checkRecordSyntax,
        checkEmptyGADTs,
        addFatalError, hintBangPat,
        mkBangTy,
        UnpackednessPragma(..),
        mkMultTy,

        -- Token location
        mkTokenLocation,

        -- Help with processing exports
        ImpExpSubSpec(..),
        ImpExpQcSpec(..),
        mkModuleImpExp,
        mkTypeImpExp,
        mkImpExpSubSpec,
        checkImportSpec,

        -- Token symbols
        starSym,

        -- Warnings and errors
        warnStarIsType,
        warnPrepositiveQualifiedModule,
        failOpFewArgs,
        failOpNotEnabledImportQualifiedPost,
        failOpImportQualifiedTwice,

        SumOrTuple (..),

        -- Expression/command/pattern ambiguity resolution
        PV,
        runPV,
        ECP(ECP, unECP),
        DisambInfixOp(..),
        DisambECP(..),
        ecpFromExp,
        ecpFromCmd,
        PatBuilder,

        -- Type/datacon ambiguity resolution
        DisambTD(..),
        addUnpackednessP,
        dataConBuilderCon,
        dataConBuilderDetails,
    ) where

import GHC.Prelude
import GHC.Hs           -- Lots of it
import GHC.Core.TyCon          ( TyCon, isTupleTyCon, tyConSingleDataCon_maybe )
import GHC.Core.DataCon        ( DataCon, dataConTyCon )
import GHC.Core.ConLike        ( ConLike(..) )
import GHC.Core.Coercion.Axiom ( Role, fsFromRole )
import GHC.Types.Name.Reader
import GHC.Types.Name
import GHC.Unit.Module (ModuleName)
import GHC.Types.Basic
import GHC.Types.Error
import GHC.Types.Fixity
import GHC.Types.Hint
import GHC.Types.SourceText
import GHC.Parser.Types
import GHC.Parser.Lexer
import GHC.Parser.Errors.Types
import GHC.Parser.Errors.Ppr ()
import GHC.Utils.Lexeme ( isLexCon )
import GHC.Types.TyThing
import GHC.Core.Type    ( unrestrictedFunTyCon, Specificity(..) )
import GHC.Builtin.Types( cTupleTyConName, tupleTyCon, tupleDataCon,
                          nilDataConName, nilDataConKey,
                          listTyConName, listTyConKey )
import GHC.Types.ForeignCall
import GHC.Types.SrcLoc
import GHC.Types.Unique ( hasKey )
import GHC.Data.OrdList
import GHC.Utils.Outputable as Outputable
import GHC.Data.FastString
import GHC.Data.Maybe
import GHC.Utils.Error
import GHC.Utils.Misc
import Data.Either
import Data.List        ( findIndex )
import Data.Foldable
import qualified Data.Semigroup as Semi
import GHC.Utils.Panic
import GHC.Utils.Panic.Plain
import qualified GHC.Data.Strict as Strict

import Control.Monad
import Text.ParserCombinators.ReadP as ReadP
import Data.Char
import Data.Data       ( dataTypeOf, fromConstr, dataTypeConstrs )
import Data.Kind       ( Type )
import Data.List.NonEmpty (NonEmpty)

{- **********************************************************************

  Construction functions for Rdr stuff

  ********************************************************************* -}

-- | mkClassDecl builds a RdrClassDecl, filling in the names for tycon and
-- datacon by deriving them from the name of the class.  We fill in the names
-- for the tycon and datacon corresponding to the class, by deriving them
-- from the name of the class itself.  This saves recording the names in the
-- interface file (which would be equally good).

-- Similarly for mkConDecl, mkClassOpSig and default-method names.

--         *** See Note [The Naming story] in GHC.Hs.Decls ****

mkTyClD :: LTyClDecl (GhcPass p) -> LHsDecl (GhcPass p)
mkTyClD (L loc d) = L loc (TyClD noExtField d)

mkInstD :: LInstDecl (GhcPass p) -> LHsDecl (GhcPass p)
mkInstD (L loc d) = L loc (InstD noExtField d)

mkClassDecl :: SrcSpan
            -> Located (Maybe (LHsContext GhcPs), LHsType GhcPs)
            -> Located (a,[LHsFunDep GhcPs])
            -> OrdList (LHsDecl GhcPs)
            -> LayoutInfo
            -> [AddEpAnn]
            -> P (LTyClDecl GhcPs)

mkClassDecl loc' (L _ (mcxt, tycl_hdr)) fds where_cls layoutInfo annsIn
  = do { let loc = noAnnSrcSpan loc'
       ; (binds, sigs, ats, at_defs, _, docs) <- cvBindsAndSigs where_cls
       ; (cls, tparams, fixity, ann) <- checkTyClHdr True tycl_hdr
       ; tyvars <- checkTyVars (text "class") whereDots cls tparams
       ; cs <- getCommentsFor (locA loc) -- Get any remaining comments
       ; let anns' = addAnns (EpAnn (spanAsAnchor $ locA loc) annsIn emptyComments) ann cs
       ; return (L loc (ClassDecl { tcdCExt = (anns', NoAnnSortKey, layoutInfo)
                                  , tcdCtxt = mcxt
                                  , tcdLName = cls, tcdTyVars = tyvars
                                  , tcdFixity = fixity
                                  , tcdFDs = snd (unLoc fds)
                                  , tcdSigs = mkClassOpSigs sigs
                                  , tcdMeths = binds
                                  , tcdATs = ats, tcdATDefs = at_defs
                                  , tcdDocs  = docs })) }

mkTyData :: SrcSpan
         -> NewOrData
         -> Maybe (LocatedP CType)
         -> Located (Maybe (LHsContext GhcPs), LHsType GhcPs)
         -> Maybe (LHsKind GhcPs)
         -> [LConDecl GhcPs]
         -> Located (HsDeriving GhcPs)
         -> [AddEpAnn]
         -> P (LTyClDecl GhcPs)
mkTyData loc' new_or_data cType (L _ (mcxt, tycl_hdr))
         ksig data_cons (L _ maybe_deriv) annsIn
  = do { let loc = noAnnSrcSpan loc'
       ; (tc, tparams, fixity, ann) <- checkTyClHdr False tycl_hdr
       ; tyvars <- checkTyVars (ppr new_or_data) equalsDots tc tparams
       ; cs <- getCommentsFor (locA loc) -- Get any remaining comments
       ; let anns' = addAnns (EpAnn (spanAsAnchor $ locA loc) annsIn emptyComments) ann cs
       ; defn <- mkDataDefn new_or_data cType mcxt ksig data_cons maybe_deriv
       ; return (L loc (DataDecl { tcdDExt = anns',
                                   tcdLName = tc, tcdTyVars = tyvars,
                                   tcdFixity = fixity,
                                   tcdDataDefn = defn })) }

mkDataDefn :: NewOrData
           -> Maybe (LocatedP CType)
           -> Maybe (LHsContext GhcPs)
           -> Maybe (LHsKind GhcPs)
           -> [LConDecl GhcPs]
           -> HsDeriving GhcPs
           -> P (HsDataDefn GhcPs)
mkDataDefn new_or_data cType mcxt ksig data_cons maybe_deriv
  = do { checkDatatypeContext mcxt
       ; return (HsDataDefn { dd_ext = noExtField
                            , dd_ND = new_or_data, dd_cType = cType
                            , dd_ctxt = mcxt
                            , dd_cons = data_cons
                            , dd_kindSig = ksig
                            , dd_derivs = maybe_deriv }) }


mkTySynonym :: SrcSpan
            -> LHsType GhcPs  -- LHS
            -> LHsType GhcPs  -- RHS
            -> [AddEpAnn]
            -> P (LTyClDecl GhcPs)
mkTySynonym loc lhs rhs annsIn
  = do { (tc, tparams, fixity, ann) <- checkTyClHdr False lhs
       ; cs1 <- getCommentsFor loc -- Add any API Annotations to the top SrcSpan [temp]
       ; tyvars <- checkTyVars (text "type") equalsDots tc tparams
       ; cs2 <- getCommentsFor loc -- Add any API Annotations to the top SrcSpan [temp]
       ; let anns' = addAnns (EpAnn (spanAsAnchor loc) annsIn emptyComments) ann (cs1 Semi.<> cs2)
       ; return (L (noAnnSrcSpan loc) (SynDecl
                                { tcdSExt = anns'
                                , tcdLName = tc, tcdTyVars = tyvars
                                , tcdFixity = fixity
                                , tcdRhs = rhs })) }

mkStandaloneKindSig
  :: SrcSpan
  -> Located [LocatedN RdrName]   -- LHS
  -> LHsSigType GhcPs             -- RHS
  -> [AddEpAnn]
  -> P (LStandaloneKindSig GhcPs)
mkStandaloneKindSig loc lhs rhs anns =
  do { vs <- mapM check_lhs_name (unLoc lhs)
     ; v <- check_singular_lhs (reverse vs)
     ; cs <- getCommentsFor loc
     ; return $ L (noAnnSrcSpan loc)
       $ StandaloneKindSig (EpAnn (spanAsAnchor loc) anns cs) v rhs }
  where
    check_lhs_name v@(unLoc->name) =
      if isUnqual name && isTcOcc (rdrNameOcc name)
      then return v
      else addFatalError $ mkPlainErrorMsgEnvelope (getLocA v) $
             (PsErrUnexpectedQualifiedConstructor (unLoc v))
    check_singular_lhs vs =
      case vs of
        [] -> panic "mkStandaloneKindSig: empty left-hand side"
        [v] -> return v
        _ -> addFatalError $ mkPlainErrorMsgEnvelope (getLoc lhs) $
               (PsErrMultipleNamesInStandaloneKindSignature vs)

mkTyFamInstEqn :: SrcSpan
               -> HsOuterFamEqnTyVarBndrs GhcPs
               -> LHsType GhcPs
               -> LHsType GhcPs
               -> [AddEpAnn]
               -> P (LTyFamInstEqn GhcPs)
mkTyFamInstEqn loc bndrs lhs rhs anns
  = do { (tc, tparams, fixity, ann) <- checkTyClHdr False lhs
       ; cs <- getCommentsFor loc
       ; return (L (noAnnSrcSpan loc) $ FamEqn
                        { feqn_ext    = EpAnn (spanAsAnchor loc) (anns `mappend` ann) cs
                        , feqn_tycon  = tc
                        , feqn_bndrs  = bndrs
                        , feqn_pats   = tparams
                        , feqn_fixity = fixity
                        , feqn_rhs    = rhs })}

mkDataFamInst :: SrcSpan
              -> NewOrData
              -> Maybe (LocatedP CType)
              -> (Maybe ( LHsContext GhcPs), HsOuterFamEqnTyVarBndrs GhcPs
                        , LHsType GhcPs)
              -> Maybe (LHsKind GhcPs)
              -> [LConDecl GhcPs]
              -> Located (HsDeriving GhcPs)
              -> [AddEpAnn]
              -> P (LInstDecl GhcPs)
mkDataFamInst loc new_or_data cType (mcxt, bndrs, tycl_hdr)
              ksig data_cons (L _ maybe_deriv) anns
  = do { (tc, tparams, fixity, ann) <- checkTyClHdr False tycl_hdr
       ; cs <- getCommentsFor loc -- Add any API Annotations to the top SrcSpan
       ; let anns' = addAnns (EpAnn (spanAsAnchor loc) ann cs) anns emptyComments
       ; defn <- mkDataDefn new_or_data cType mcxt ksig data_cons maybe_deriv
       ; return (L (noAnnSrcSpan loc) (DataFamInstD anns' (DataFamInstDecl
                  (FamEqn { feqn_ext    = anns'
                          , feqn_tycon  = tc
                          , feqn_bndrs  = bndrs
                          , feqn_pats   = tparams
                          , feqn_fixity = fixity
                          , feqn_rhs    = defn })))) }

mkTyFamInst :: SrcSpan
            -> TyFamInstEqn GhcPs
            -> [AddEpAnn]
            -> P (LInstDecl GhcPs)
mkTyFamInst loc eqn anns = do
  cs <- getCommentsFor loc
  return (L (noAnnSrcSpan loc) (TyFamInstD noExtField
              (TyFamInstDecl (EpAnn (spanAsAnchor loc) anns cs) eqn)))

mkFamDecl :: SrcSpan
          -> FamilyInfo GhcPs
          -> TopLevelFlag
          -> LHsType GhcPs                   -- LHS
          -> LFamilyResultSig GhcPs          -- Optional result signature
          -> Maybe (LInjectivityAnn GhcPs)   -- Injectivity annotation
          -> [AddEpAnn]
          -> P (LTyClDecl GhcPs)
mkFamDecl loc info topLevel lhs ksig injAnn annsIn
  = do { (tc, tparams, fixity, ann) <- checkTyClHdr False lhs
       ; cs1 <- getCommentsFor loc -- Add any API Annotations to the top SrcSpan [temp]
       ; tyvars <- checkTyVars (ppr info) equals_or_where tc tparams
       ; cs2 <- getCommentsFor loc -- Add any API Annotations to the top SrcSpan [temp]
       ; let anns' = addAnns (EpAnn (spanAsAnchor loc) annsIn emptyComments) ann (cs1 Semi.<> cs2)
       ; return (L (noAnnSrcSpan loc) (FamDecl noExtField
                                         (FamilyDecl
                                           { fdExt       = anns'
                                           , fdTopLevel  = topLevel
                                           , fdInfo      = info, fdLName = tc
                                           , fdTyVars    = tyvars
                                           , fdFixity    = fixity
                                           , fdResultSig = ksig
                                           , fdInjectivityAnn = injAnn }))) }
  where
    equals_or_where = case info of
                        DataFamily          -> empty
                        OpenTypeFamily      -> empty
                        ClosedTypeFamily {} -> whereDots

mkSpliceDecl :: LHsExpr GhcPs -> P (LHsDecl GhcPs)
-- If the user wrote
--      [pads| ... ]   then return a QuasiQuoteD
--      $(e)           then return a SpliceD
-- but if they wrote, say,
--      f x            then behave as if they'd written $(f x)
--                     ie a SpliceD
--
-- Typed splices are not allowed at the top level, thus we do not represent them
-- as spliced declaration.  See #10945
mkSpliceDecl lexpr@(L loc expr)
  | HsSpliceE _ splice@(HsUntypedSplice {}) <- expr = do
    cs <- getCommentsFor (locA loc)
    return $ L (addCommentsToSrcAnn loc cs) $ SpliceD noExtField (SpliceDecl noExtField (L loc splice) ExplicitSplice)

  | HsSpliceE _ splice@(HsQuasiQuote {}) <- expr = do
    cs <- getCommentsFor (locA loc)
    return $ L (addCommentsToSrcAnn loc cs) $ SpliceD noExtField (SpliceDecl noExtField (L loc splice) ExplicitSplice)

  | otherwise = do
    cs <- getCommentsFor (locA loc)
    return $ L (addCommentsToSrcAnn loc cs) $ SpliceD noExtField (SpliceDecl noExtField
                                 (L loc (mkUntypedSplice noAnn BareSplice lexpr))
                                       ImplicitSplice)

mkRoleAnnotDecl :: SrcSpan
                -> LocatedN RdrName                -- type being annotated
                -> [Located (Maybe FastString)]    -- roles
                -> [AddEpAnn]
                -> P (LRoleAnnotDecl GhcPs)
mkRoleAnnotDecl loc tycon roles anns
  = do { roles' <- mapM parse_role roles
       ; cs <- getCommentsFor loc
       ; return $ L (noAnnSrcSpan loc)
         $ RoleAnnotDecl (EpAnn (spanAsAnchor loc) anns cs) tycon roles' }
  where
    role_data_type = dataTypeOf (undefined :: Role)
    all_roles = map fromConstr $ dataTypeConstrs role_data_type
    possible_roles = [(fsFromRole role, role) | role <- all_roles]

    parse_role (L loc_role Nothing) = return $ L (noAnnSrcSpan loc_role) Nothing
    parse_role (L loc_role (Just role))
      = case lookup role possible_roles of
          Just found_role -> return $ L (noAnnSrcSpan loc_role) $ Just found_role
          Nothing         ->
            let nearby = fuzzyLookup (unpackFS role)
                  (mapFst unpackFS possible_roles)
            in
            addFatalError $ mkPlainErrorMsgEnvelope loc_role $
              (PsErrIllegalRoleName role nearby)

-- | Converts a list of 'LHsTyVarBndr's annotated with their 'Specificity' to
-- binders without annotations. Only accepts specified variables, and errors if
-- any of the provided binders has an 'InferredSpec' annotation.
fromSpecTyVarBndrs :: [LHsTyVarBndr Specificity GhcPs] -> P [LHsTyVarBndr () GhcPs]
fromSpecTyVarBndrs = mapM fromSpecTyVarBndr

-- | Converts 'LHsTyVarBndr' annotated with its 'Specificity' to one without
-- annotations. Only accepts specified variables, and errors if the provided
-- binder has an 'InferredSpec' annotation.
fromSpecTyVarBndr :: LHsTyVarBndr Specificity GhcPs -> P (LHsTyVarBndr () GhcPs)
fromSpecTyVarBndr bndr = case bndr of
  (L loc (UserTyVar xtv flag idp))     -> (check_spec flag loc)
                                          >> return (L loc $ UserTyVar xtv () idp)
  (L loc (KindedTyVar xtv flag idp k)) -> (check_spec flag loc)
                                          >> return (L loc $ KindedTyVar xtv () idp k)
  where
    check_spec :: Specificity -> SrcSpanAnnA -> P ()
    check_spec SpecifiedSpec _   = return ()
    check_spec InferredSpec  loc = addFatalError $ mkPlainErrorMsgEnvelope (locA loc) $
                                     PsErrInferredTypeVarNotAllowed

-- | Add the annotation for a 'where' keyword to existing @HsLocalBinds@
annBinds :: AddEpAnn -> EpAnnComments -> HsLocalBinds GhcPs
  -> (HsLocalBinds GhcPs, Maybe EpAnnComments)
annBinds a cs (HsValBinds an bs)  = (HsValBinds (add_where a an cs) bs, Nothing)
annBinds a cs (HsIPBinds an bs)   = (HsIPBinds (add_where a an cs) bs, Nothing)
annBinds _ cs  (EmptyLocalBinds x) = (EmptyLocalBinds x, Just cs)

add_where :: AddEpAnn -> EpAnn AnnList -> EpAnnComments -> EpAnn AnnList
add_where an@(AddEpAnn _ (EpaSpan rs)) (EpAnn a (AnnList anc o c r t) cs) cs2
  | valid_anchor (anchor a)
  = EpAnn (widenAnchor a [an]) (AnnList anc o c (an:r) t) (cs Semi.<> cs2)
  | otherwise
  = EpAnn (patch_anchor rs a)
          (AnnList (fmap (patch_anchor rs) anc) o c (an:r) t) (cs Semi.<> cs2)
add_where an@(AddEpAnn _ (EpaSpan rs)) EpAnnNotUsed cs
  = EpAnn (Anchor rs UnchangedAnchor)
           (AnnList (Just $ Anchor rs UnchangedAnchor) Nothing Nothing [an] []) cs
add_where (AddEpAnn _ (EpaDelta _ _)) _ _ = panic "add_where"
 -- EpaDelta should only be used for transformations

valid_anchor :: RealSrcSpan -> Bool
valid_anchor r = srcSpanStartLine r >= 0

-- If the decl list for where binds is empty, the anchor ends up
-- invalid. In this case, use the parent one
patch_anchor :: RealSrcSpan -> Anchor -> Anchor
patch_anchor r1 (Anchor r0 op) = Anchor r op
  where
    r = if srcSpanStartLine r0 < 0 then r1 else r0

fixValbindsAnn :: EpAnn AnnList -> EpAnn AnnList
fixValbindsAnn EpAnnNotUsed = EpAnnNotUsed
fixValbindsAnn (EpAnn anchor (AnnList ma o c r t) cs)
  = (EpAnn (widenAnchor anchor (map trailingAnnToAddEpAnn t)) (AnnList ma o c r t) cs)

{- **********************************************************************

  #cvBinds-etc# Converting to @HsBinds@, etc.

  ********************************************************************* -}

-- | Function definitions are restructured here. Each is assumed to be recursive
-- initially, and non recursive definitions are discovered by the dependency
-- analyser.


--  | Groups together bindings for a single function
cvTopDecls :: OrdList (LHsDecl GhcPs) -> [LHsDecl GhcPs]
cvTopDecls decls = getMonoBindAll (fromOL decls)

-- Declaration list may only contain value bindings and signatures.
cvBindGroup :: OrdList (LHsDecl GhcPs) -> P (HsValBinds GhcPs)
cvBindGroup binding
  = do { (mbs, sigs, fam_ds, tfam_insts
         , dfam_insts, _) <- cvBindsAndSigs binding
       ; massert (null fam_ds && null tfam_insts && null dfam_insts)
       ; return $ ValBinds NoAnnSortKey mbs sigs }

cvBindsAndSigs :: OrdList (LHsDecl GhcPs)
  -> P (LHsBinds GhcPs, [LSig GhcPs], [LFamilyDecl GhcPs]
          , [LTyFamInstDecl GhcPs], [LDataFamInstDecl GhcPs], [LDocDecl GhcPs])
-- Input decls contain just value bindings and signatures
-- and in case of class or instance declarations also
-- associated type declarations. They might also contain Haddock comments.
cvBindsAndSigs fb = do
  fb' <- drop_bad_decls (fromOL fb)
  return (partitionBindsAndSigs (getMonoBindAll fb'))
  where
    -- cvBindsAndSigs is called in several places in the parser,
    -- and its items can be produced by various productions:
    --
    --    * decl       (when parsing a where clause or a let-expression)
    --    * decl_inst  (when parsing an instance declaration)
    --    * decl_cls   (when parsing a class declaration)
    --
    -- partitionBindsAndSigs can handle almost all declaration forms produced
    -- by the aforementioned productions, except for SpliceD, which we filter
    -- out here (in drop_bad_decls).
    --
    -- We're not concerned with every declaration form possible, such as those
    -- produced by the topdecl parser production, because cvBindsAndSigs is not
    -- called on top-level declarations.
    drop_bad_decls [] = return []
    drop_bad_decls (L l (SpliceD _ d) : ds) = do
      addError $ mkPlainErrorMsgEnvelope (locA l) $ PsErrDeclSpliceNotAtTopLevel d
      drop_bad_decls ds
    drop_bad_decls (d:ds) = (d:) <$> drop_bad_decls ds

-----------------------------------------------------------------------------
-- Group function bindings into equation groups

getMonoBind :: LHsBind GhcPs -> [LHsDecl GhcPs]
  -> (LHsBind GhcPs, [LHsDecl GhcPs])
-- Suppose      (b',ds') = getMonoBind b ds
--      ds is a list of parsed bindings
--      b is a MonoBinds that has just been read off the front

-- Then b' is the result of grouping more equations from ds that
-- belong with b into a single MonoBinds, and ds' is the depleted
-- list of parsed bindings.
--
-- All Haddock comments between equations inside the group are
-- discarded.
--
-- No AndMonoBinds or EmptyMonoBinds here; just single equations

getMonoBind (L loc1 (FunBind { fun_id = fun_id1@(L _ f1)
                             , fun_matches =
                               MG { mg_alts = (L _ m1@[L _ mtchs1]) } }))
            binds
  | has_args m1
  = go [L (removeCommentsA loc1) mtchs1] (commentsOnlyA loc1) binds []
  where
    go :: [LMatch GhcPs (LHsExpr GhcPs)] -> SrcSpanAnnA
       -> [LHsDecl GhcPs] -> [LHsDecl GhcPs]
       -> (LHsBind GhcPs,[LHsDecl GhcPs]) -- AZ
    go mtchs loc
       ((L loc2 (ValD _ (FunBind { fun_id = (L _ f2)
                                 , fun_matches =
                                    MG { mg_alts = (L _ [L lm2 mtchs2]) } })))
         : binds) _
        | f1 == f2 =
          let (loc2', lm2') = transferAnnsA loc2 lm2
          in go (L lm2' mtchs2 : mtchs)
                        (combineSrcSpansA loc loc2') binds []
    go mtchs loc (doc_decl@(L loc2 (DocD {})) : binds) doc_decls
        = let doc_decls' = doc_decl : doc_decls
          in go mtchs (combineSrcSpansA loc loc2) binds doc_decls'
    go mtchs loc binds doc_decls
        = ( L loc (makeFunBind fun_id1 (mkLocatedList $ reverse mtchs))
          , (reverse doc_decls) ++ binds)
        -- Reverse the final matches, to get it back in the right order
        -- Do the same thing with the trailing doc comments

getMonoBind bind binds = (bind, binds)

-- Group together adjacent FunBinds for every function.
getMonoBindAll :: [LHsDecl GhcPs] -> [LHsDecl GhcPs]
getMonoBindAll [] = []
getMonoBindAll (L l (ValD _ b) : ds) =
  let (L l' b', ds') = getMonoBind (L l b) ds
  in L l' (ValD noExtField b') : getMonoBindAll ds'
getMonoBindAll (d : ds) = d : getMonoBindAll ds

has_args :: [LMatch GhcPs (LHsExpr GhcPs)] -> Bool
has_args []                                  = panic "GHC.Parser.PostProcess.has_args"
has_args (L _ (Match { m_pats = args }) : _) = not (null args)
        -- Don't group together FunBinds if they have
        -- no arguments.  This is necessary now that variable bindings
        -- with no arguments are now treated as FunBinds rather
        -- than pattern bindings (tests/rename/should_fail/rnfail002).

{- **********************************************************************

  #PrefixToHS-utils# Utilities for conversion

  ********************************************************************* -}

{- Note [Parsing data constructors is hard]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The problem with parsing data constructors is that they look a lot like types.
Compare:

  (s1)   data T = C t1 t2
  (s2)   type T = C t1 t2

Syntactically, there's little difference between these declarations, except in
(s1) 'C' is a data constructor, but in (s2) 'C' is a type constructor.

This similarity would pose no problem if we knew ahead of time if we are
parsing a type or a constructor declaration. Looking at (s1) and (s2), a simple
(but wrong!) rule comes to mind: in 'data' declarations assume we are parsing
data constructors, and in other contexts (e.g. 'type' declarations) assume we
are parsing type constructors.

This simple rule does not work because of two problematic cases:

  (p1)   data T = C t1 t2 :+ t3
  (p2)   data T = C t1 t2 => t3

In (p1) we encounter (:+) and it turns out we are parsing an infix data
declaration, so (C t1 t2) is a type and 'C' is a type constructor.
In (p2) we encounter (=>) and it turns out we are parsing an existential
context, so (C t1 t2) is a constraint and 'C' is a type constructor.

As the result, in order to determine whether (C t1 t2) declares a data
constructor, a type, or a context, we would need unlimited lookahead which
'happy' is not so happy with.
-}

-- | Reinterpret a type constructor, including type operators, as a data
--   constructor.
-- See Note [Parsing data constructors is hard]
tyConToDataCon :: LocatedN RdrName -> Either (MsgEnvelope PsMessage) (LocatedN RdrName)
tyConToDataCon (L loc tc)
  | isTcOcc occ || isDataOcc occ
  , isLexCon (occNameFS occ)
  = return (L loc (setRdrNameSpace tc srcDataName))

  | otherwise
  = Left $ mkPlainErrorMsgEnvelope (locA loc) $ (PsErrNotADataCon tc)
  where
    occ = rdrNameOcc tc

mkPatSynMatchGroup :: LocatedN RdrName
                   -> LocatedL (OrdList (LHsDecl GhcPs))
                   -> P (MatchGroup GhcPs (LHsExpr GhcPs))
mkPatSynMatchGroup (L loc patsyn_name) (L ld decls) =
    do { matches <- mapM fromDecl (fromOL decls)
       ; when (null matches) (wrongNumberErr (locA loc))
       ; return $ mkMatchGroup FromSource (L ld matches) }
  where
    fromDecl (L loc decl@(ValD _ (PatBind _
                                 -- AZ: where should these anns come from?
                         pat@(L _ (ConPat noAnn ln@(L _ name) details))
                               rhs _))) =
        do { unless (name == patsyn_name) $
               wrongNameBindingErr (locA loc) decl
           ; match <- case details of
               PrefixCon _ pats -> return $ Match { m_ext = noAnn
                                                  , m_ctxt = ctxt, m_pats = pats
                                                  , m_grhss = rhs }
                   where
                     ctxt = FunRhs { mc_fun = ln
                                   , mc_fixity = Prefix
                                   , mc_strictness = NoSrcStrict }

               InfixCon p1 p2 -> return $ Match { m_ext = noAnn
                                                , m_ctxt = ctxt
                                                , m_pats = [p1, p2]
                                                , m_grhss = rhs }
                   where
                     ctxt = FunRhs { mc_fun = ln
                                   , mc_fixity = Infix
                                   , mc_strictness = NoSrcStrict }

               RecCon{} -> recordPatSynErr (locA loc) pat
           ; return $ L loc match }
    fromDecl (L loc decl) = extraDeclErr (locA loc) decl

    extraDeclErr loc decl =
        addFatalError $ mkPlainErrorMsgEnvelope loc $
          (PsErrNoSingleWhereBindInPatSynDecl patsyn_name decl)

    wrongNameBindingErr loc decl =
      addFatalError $ mkPlainErrorMsgEnvelope loc $
          (PsErrInvalidWhereBindInPatSynDecl patsyn_name decl)

    wrongNumberErr loc =
      addFatalError $ mkPlainErrorMsgEnvelope loc $
        (PsErrEmptyWhereInPatSynDecl patsyn_name)

recordPatSynErr :: SrcSpan -> LPat GhcPs -> P a
recordPatSynErr loc pat =
    addFatalError $ mkPlainErrorMsgEnvelope loc $
      (PsErrRecordSyntaxInPatSynDecl pat)

mkConDeclH98 :: EpAnn [AddEpAnn] -> LocatedN RdrName -> Maybe [LHsTyVarBndr Specificity GhcPs]
                -> Maybe (LHsContext GhcPs) -> HsConDeclH98Details GhcPs
                -> ConDecl GhcPs

mkConDeclH98 ann name mb_forall mb_cxt args
  = ConDeclH98 { con_ext    = ann
               , con_name   = name
               , con_forall = isJust mb_forall
               , con_ex_tvs = mb_forall `orElse` []
               , con_mb_cxt = mb_cxt
               , con_args   = args
               , con_doc    = Nothing }

-- | Construct a GADT-style data constructor from the constructor names and
-- their type. Some interesting aspects of this function:
--
-- * This splits up the constructor type into its quantified type variables (if
--   provided), context (if provided), argument types, and result type, and
--   records whether this is a prefix or record GADT constructor. See
--   Note [GADT abstract syntax] in "GHC.Hs.Decls" for more details.
mkGadtDecl :: SrcSpan
           -> [LocatedN RdrName]
           -> LHsSigType GhcPs
           -> [AddEpAnn]
           -> P (LConDecl GhcPs)
mkGadtDecl loc names ty annsIn = do
  cs <- getCommentsFor loc
  let l = noAnnSrcSpan loc

  (args, res_ty, annsa, csa) <-
    case body_ty of
     L ll (HsFunTy af hsArr (L loc' (HsRecTy an rf)) res_ty) -> do
       let an' = addCommentsToEpAnn (locA loc') an (comments af)
       arr <- case hsArr of
         HsUnrestrictedArrow arr -> return arr
         _ -> do addError $ mkPlainErrorMsgEnvelope (getLocA body_ty) $
                                 (PsErrIllegalGadtRecordMultiplicity hsArr)
                 return noHsUniTok

       return ( RecConGADT (L (SrcSpanAnn an' (locA loc')) rf) arr, res_ty
              , [], epAnnComments (ann ll))
     _ -> do
       let (anns, cs, arg_types, res_type) = splitHsFunType body_ty
       return (PrefixConGADT arg_types, res_type, anns, cs)

  let an = case outer_bndrs of
        _                -> EpAnn (spanAsAnchor loc) (annsIn ++ annsa) (cs Semi.<> csa)

  pure $ L l ConDeclGADT
                     { con_g_ext  = an
                     , con_names  = names
                     , con_bndrs  = L (getLoc ty) outer_bndrs
                     , con_mb_cxt = mcxt
                     , con_g_args = args
                     , con_res_ty = res_ty
                     , con_doc    = Nothing }
  where
    (outer_bndrs, mcxt, body_ty) = splitLHsGadtTy ty

setRdrNameSpace :: RdrName -> NameSpace -> RdrName
-- ^ This rather gruesome function is used mainly by the parser.
-- When parsing:
--
-- > data T a = T | T1 Int
--
-- we parse the data constructors as /types/ because of parser ambiguities,
-- so then we need to change the /type constr/ to a /data constr/
--
-- The exact-name case /can/ occur when parsing:
--
-- > data [] a = [] | a : [a]
--
-- For the exact-name case we return an original name.
setRdrNameSpace (Unqual occ) ns = Unqual (setOccNameSpace ns occ)
setRdrNameSpace (Qual m occ) ns = Qual m (setOccNameSpace ns occ)
setRdrNameSpace (Orig m occ) ns = Orig m (setOccNameSpace ns occ)
setRdrNameSpace (Exact n)    ns
  | Just thing <- wiredInNameTyThing_maybe n
  = setWiredInNameSpace thing ns
    -- Preserve Exact Names for wired-in things,
    -- notably tuples and lists

  | isExternalName n
  = Orig (nameModule n) occ

  | otherwise   -- This can happen when quoting and then
                -- splicing a fixity declaration for a type
  = Exact (mkSystemNameAt (nameUnique n) occ (nameSrcSpan n))
  where
    occ = setOccNameSpace ns (nameOccName n)

setWiredInNameSpace :: TyThing -> NameSpace -> RdrName
setWiredInNameSpace (ATyCon tc) ns
  | isDataConNameSpace ns
  = ty_con_data_con tc
  | isTcClsNameSpace ns
  = Exact (getName tc)      -- No-op

setWiredInNameSpace (AConLike (RealDataCon dc)) ns
  | isTcClsNameSpace ns
  = data_con_ty_con dc
  | isDataConNameSpace ns
  = Exact (getName dc)      -- No-op

setWiredInNameSpace thing ns
  = pprPanic "setWiredinNameSpace" (pprNameSpace ns <+> ppr thing)

ty_con_data_con :: TyCon -> RdrName
ty_con_data_con tc
  | isTupleTyCon tc
  , Just dc <- tyConSingleDataCon_maybe tc
  = Exact (getName dc)

  | tc `hasKey` listTyConKey
  = Exact nilDataConName

  | otherwise  -- See Note [setRdrNameSpace for wired-in names]
  = Unqual (setOccNameSpace srcDataName (getOccName tc))

data_con_ty_con :: DataCon -> RdrName
data_con_ty_con dc
  | let tc = dataConTyCon dc
  , isTupleTyCon tc
  = Exact (getName tc)

  | dc `hasKey` nilDataConKey
  = Exact listTyConName

  | otherwise  -- See Note [setRdrNameSpace for wired-in names]
  = Unqual (setOccNameSpace tcClsName (getOccName dc))



{- Note [setRdrNameSpace for wired-in names]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In GHC.Types, which declares (:), we have
  infixr 5 :
The ambiguity about which ":" is meant is resolved by parsing it as a
data constructor, but then using dataTcOccs to try the type constructor too;
and that in turn calls setRdrNameSpace to change the name-space of ":" to
tcClsName.  There isn't a corresponding ":" type constructor, but it's painful
to make setRdrNameSpace partial, so we just make an Unqual name instead. It
really doesn't matter!
-}

eitherToP :: MonadP m => Either (MsgEnvelope PsMessage) a -> m a
-- Adapts the Either monad to the P monad
eitherToP (Left err)    = addFatalError err
eitherToP (Right thing) = return thing

checkTyVars :: SDoc -> SDoc -> LocatedN RdrName -> [LHsTypeArg GhcPs]
            -> P (LHsQTyVars GhcPs)  -- the synthesized type variables
-- ^ Check whether the given list of type parameters are all type variables
-- (possibly with a kind signature).
checkTyVars pp_what equals_or_where tc tparms
  = do { tvs <- mapM check tparms
       ; return (mkHsQTvs tvs) }
  where
    check (HsTypeArg _ ki@(L loc _)) = addFatalError $ mkPlainErrorMsgEnvelope (locA loc) $
                                         (PsErrUnexpectedTypeAppInDecl ki pp_what (unLoc tc))
    check (HsValArg ty) = chkParens [] [] emptyComments ty
    check (HsArgPar sp) = addFatalError $ mkPlainErrorMsgEnvelope sp $
                            (PsErrMalformedDecl pp_what (unLoc tc))
        -- Keep around an action for adjusting the annotations of extra parens
    chkParens :: [AddEpAnn] -> [AddEpAnn] -> EpAnnComments -> LHsType GhcPs
              -> P (LHsTyVarBndr () GhcPs)
    chkParens ops cps cs (L l (HsParTy an ty))
      = let
          (o,c) = mkParensEpAnn (realSrcSpan $ locA l)
        in
          chkParens (o:ops) (c:cps) (cs Semi.<> epAnnComments an) ty
    chkParens ops cps cs ty = chk ops cps cs ty

        -- Check that the name space is correct!
    chk :: [AddEpAnn] -> [AddEpAnn] -> EpAnnComments -> LHsType GhcPs -> P (LHsTyVarBndr () GhcPs)
    chk ops cps cs (L l (HsKindSig annk (L annt (HsTyVar ann _ (L lv tv))) k))
        | isRdrTyVar tv
            = let
                an = (reverse ops) ++ cps
              in
                return (L (widenLocatedAn (l Semi.<> annt) an)
                       (KindedTyVar (addAnns (annk Semi.<> ann) an cs) () (L lv tv) k))
    chk ops cps cs (L l (HsTyVar ann _ (L ltv tv)))
        | isRdrTyVar tv
            = let
                an = (reverse ops) ++ cps
              in
                return (L (widenLocatedAn l an)
                                     (UserTyVar (addAnns ann an cs) () (L ltv tv)))
    chk _ _ _ t@(L loc _)
        = addFatalError $ mkPlainErrorMsgEnvelope (locA loc) $
            (PsErrUnexpectedTypeInDecl t pp_what (unLoc tc) tparms equals_or_where)


whereDots, equalsDots :: SDoc
-- Second argument to checkTyVars
whereDots  = text "where ..."
equalsDots = text "= ..."

checkDatatypeContext :: Maybe (LHsContext GhcPs) -> P ()
checkDatatypeContext Nothing = return ()
checkDatatypeContext (Just c)
    = do allowed <- getBit DatatypeContextsBit
         unless allowed $ addError $ mkPlainErrorMsgEnvelope (getLocA c) $
                                       (PsErrIllegalDataTypeContext c)

type LRuleTyTmVar = LocatedAn NoEpAnns RuleTyTmVar
data RuleTyTmVar = RuleTyTmVar (EpAnn [AddEpAnn]) (LocatedN RdrName) (Maybe (LHsType GhcPs))
-- ^ Essentially a wrapper for a @RuleBndr GhcPs@

-- turns RuleTyTmVars into RuleBnrs - this is straightforward
mkRuleBndrs :: [LRuleTyTmVar] -> [LRuleBndr GhcPs]
mkRuleBndrs = fmap (fmap cvt_one)
  where cvt_one (RuleTyTmVar ann v Nothing) = RuleBndr ann v
        cvt_one (RuleTyTmVar ann v (Just sig)) =
          RuleBndrSig ann v (mkHsPatSigType noAnn sig)

-- turns RuleTyTmVars into HsTyVarBndrs - this is more interesting
mkRuleTyVarBndrs :: [LRuleTyTmVar] -> [LHsTyVarBndr () GhcPs]
mkRuleTyVarBndrs = fmap cvt_one
  where cvt_one (L l (RuleTyTmVar ann v Nothing))
          = L (l2l l) (UserTyVar ann () (fmap tm_to_ty v))
        cvt_one (L l (RuleTyTmVar ann v (Just sig)))
          = L (l2l l) (KindedTyVar ann () (fmap tm_to_ty v) sig)
    -- takes something in namespace 'varName' to something in namespace 'tvName'
        tm_to_ty (Unqual occ) = Unqual (setOccNameSpace tvName occ)
        tm_to_ty _ = panic "mkRuleTyVarBndrs"

-- See Note [Parsing explicit foralls in Rules] in Parser.y
checkRuleTyVarBndrNames :: [LHsTyVarBndr flag GhcPs] -> P ()
checkRuleTyVarBndrNames = mapM_ (check . fmap hsTyVarName)
  where check (L loc (Unqual occ)) =
          -- TODO: don't use string here, OccName has a Unique/FastString
          when ((occNameString occ ==) `any` ["forall","family","role"])
            (addFatalError $ mkPlainErrorMsgEnvelope (locA loc) $
               (PsErrParseErrorOnInput occ))
        check _ = panic "checkRuleTyVarBndrNames"

checkRecordSyntax :: (MonadP m, Outputable a) => LocatedA a -> m (LocatedA a)
checkRecordSyntax lr@(L loc r)
    = do allowed <- getBit TraditionalRecordSyntaxBit
         unless allowed $ addError $ mkPlainErrorMsgEnvelope (locA loc) $
                                       (PsErrIllegalTraditionalRecordSyntax (ppr r))
         return lr

-- | Check if the gadt_constrlist is empty. Only raise parse error for
-- `data T where` to avoid affecting existing error message, see #8258.
checkEmptyGADTs :: Located ([AddEpAnn], [LConDecl GhcPs])
                -> P (Located ([AddEpAnn], [LConDecl GhcPs]))
checkEmptyGADTs gadts@(L span (_, []))           -- Empty GADT declaration.
    = do gadtSyntax <- getBit GadtSyntaxBit   -- GADTs implies GADTSyntax
         unless gadtSyntax $ addError $ mkPlainErrorMsgEnvelope span $
                                          PsErrIllegalWhereInDataDecl
         return gadts
checkEmptyGADTs gadts = return gadts              -- Ordinary GADT declaration.

checkTyClHdr :: Bool               -- True  <=> class header
                                   -- False <=> type header
             -> LHsType GhcPs
             -> P (LocatedN RdrName,     -- the head symbol (type or class name)
                   [LHsTypeArg GhcPs],   -- parameters of head symbol
                   LexicalFixity,        -- the declaration is in infix format
                   [AddEpAnn])           -- API Annotation for HsParTy
                                         -- when stripping parens
-- Well-formedness check and decomposition of type and class heads.
-- Decomposes   T ty1 .. tyn   into    (T, [ty1, ..., tyn])
--              Int :*: Bool   into    (:*:, [Int, Bool])
-- returning the pieces
checkTyClHdr is_cls ty
  = goL ty [] [] [] Prefix
  where
    goL (L l ty) acc ops cps fix = go (locA l) ty acc ops cps fix

    -- workaround to define '*' despite StarIsType
    go _ (HsParTy an (L l (HsStarTy _ isUni))) acc ops' cps' fix
      = do { addPsMessage (locA l) PsWarnStarBinder
           ; let name = mkOccName tcClsName (starSym isUni)
           ; let a' = newAnns l an
           ; return (L a' (Unqual name), acc, fix
                    , (reverse ops') ++ cps') }

    go _ (HsTyVar _ _ ltc@(L _ tc)) acc ops cps fix
      | isRdrTc tc               = return (ltc, acc, fix, (reverse ops) ++ cps)
    go _ (HsOpTy _ _ t1 ltc@(L _ tc) t2) acc ops cps _fix
      | isRdrTc tc               = return (ltc, HsValArg t1:HsValArg t2:acc, Infix, (reverse ops) ++ cps)
    go l (HsParTy _ ty)    acc ops cps fix = goL ty acc (o:ops) (c:cps) fix
      where
        (o,c) = mkParensEpAnn (realSrcSpan l)
    go _ (HsAppTy _ t1 t2) acc ops cps fix = goL t1 (HsValArg t2:acc) ops cps fix
    go _ (HsAppKindTy l ty ki) acc ops cps fix = goL ty (HsTypeArg l ki:acc) ops cps fix
    go l (HsTupleTy _ HsBoxedOrConstraintTuple ts) [] ops cps fix
      = return (L (noAnnSrcSpan l) (nameRdrName tup_name)
               , map HsValArg ts, fix, (reverse ops)++cps)
      where
        arity = length ts
        tup_name | is_cls    = cTupleTyConName arity
                 | otherwise = getName (tupleTyCon Boxed arity)
          -- See Note [Unit tuples] in GHC.Hs.Type  (TODO: is this still relevant?)
    go l _ _ _ _ _
      = addFatalError $ mkPlainErrorMsgEnvelope l $
          (PsErrMalformedTyOrClDecl ty)

    -- Combine the annotations from the HsParTy and HsStarTy into a
    -- new one for the LocatedN RdrName
    newAnns :: SrcSpanAnnA -> EpAnn AnnParen -> SrcSpanAnnN
    newAnns (SrcSpanAnn EpAnnNotUsed l) (EpAnn as (AnnParen _ o c) cs) =
      let
        lr = combineRealSrcSpans (realSrcSpan l) (anchor as)
        an = (EpAnn (Anchor lr UnchangedAnchor) (NameAnn NameParens o (EpaSpan $ realSrcSpan l) c []) cs)
      in SrcSpanAnn an (RealSrcSpan lr Strict.Nothing)
    newAnns _ EpAnnNotUsed = panic "missing AnnParen"
    newAnns (SrcSpanAnn (EpAnn ap (AnnListItem ta) csp) l) (EpAnn as (AnnParen _ o c) cs) =
      let
        lr = combineRealSrcSpans (anchor ap) (anchor as)
        an = (EpAnn (Anchor lr UnchangedAnchor) (NameAnn NameParens o (EpaSpan $ realSrcSpan l) c ta) (csp Semi.<> cs))
      in SrcSpanAnn an (RealSrcSpan lr Strict.Nothing)

-- | Yield a parse error if we have a function applied directly to a do block
-- etc. and BlockArguments is not enabled.
checkExpBlockArguments :: LHsExpr GhcPs -> PV ()
checkCmdBlockArguments :: LHsCmd GhcPs -> PV ()
(checkExpBlockArguments, checkCmdBlockArguments) = (checkExpr, checkCmd)
  where
    checkExpr :: LHsExpr GhcPs -> PV ()
    checkExpr expr = case unLoc expr of
      HsDo _ (DoExpr m) _  -> check (PsErrDoInFunAppExpr m)     expr
      HsDo _ (MDoExpr m) _ -> check (PsErrMDoInFunAppExpr m)    expr
      HsLam {}             -> check PsErrLambdaInFunAppExpr     expr
      HsCase {}            -> check PsErrCaseInFunAppExpr       expr
      HsLamCase {}         -> check PsErrLambdaCaseInFunAppExpr expr
      HsLet {}             -> check PsErrLetInFunAppExpr        expr
      HsIf {}              -> check PsErrIfInFunAppExpr         expr
      HsProc {}            -> check PsErrProcInFunAppExpr       expr
      _                    -> return ()

    checkCmd :: LHsCmd GhcPs -> PV ()
    checkCmd cmd = case unLoc cmd of
      HsCmdLam {}  -> check PsErrLambdaCmdInFunAppCmd cmd
      HsCmdCase {} -> check PsErrCaseCmdInFunAppCmd   cmd
      HsCmdIf {}   -> check PsErrIfCmdInFunAppCmd     cmd
      HsCmdLet {}  -> check PsErrLetCmdInFunAppCmd    cmd
      HsCmdDo {}   -> check PsErrDoCmdInFunAppCmd     cmd
      _            -> return ()

    check err a = do
      blockArguments <- getBit BlockArgumentsBit
      unless blockArguments $
        addError $ mkPlainErrorMsgEnvelope (getLocA a) $ (err a)

-- | Validate the context constraints and break up a context into a list
-- of predicates.
--
-- @
--     (Eq a, Ord b)        -->  [Eq a, Ord b]
--     Eq a                 -->  [Eq a]
--     (Eq a)               -->  [Eq a]
--     (((Eq a)))           -->  [Eq a]
-- @
checkContext :: LHsType GhcPs -> P (LHsContext GhcPs)
checkContext orig_t@(L (SrcSpanAnn _ l) _orig_t) =
  check ([],[],emptyComments) orig_t
 where
  check :: ([EpaLocation],[EpaLocation],EpAnnComments)
        -> LHsType GhcPs -> P (LHsContext GhcPs)
  check (oparens,cparens,cs) (L _l (HsTupleTy ann' HsBoxedOrConstraintTuple ts))
    -- (Eq a, Ord b) shows up as a tuple type. Only boxed tuples can
    -- be used as context constraints.
    -- Ditto ()
    = do
        let (op,cp,cs') = case ann' of
              EpAnnNotUsed -> ([],[],emptyComments)
              EpAnn _ (AnnParen _ o c) cs -> ([o],[c],cs)
        return (L (SrcSpanAnn (EpAnn (spanAsAnchor l)
                              -- Append parens so that the original order in the source is maintained
                               (AnnContext Nothing (oparens ++ op) (cp ++ cparens)) (cs Semi.<> cs')) l) ts)

  check (opi,cpi,csi) (L _lp1 (HsParTy ann' ty))
                                  -- to be sure HsParTy doesn't get into the way
    = do
        let (op,cp,cs') = case ann' of
                    EpAnnNotUsed -> ([],[],emptyComments)
                    EpAnn _ (AnnParen _ open close ) cs -> ([open],[close],cs)
        check (op++opi,cp++cpi,cs' Semi.<> csi) ty

  -- No need for anns, returning original
  check (_opi,_cpi,_csi) _t =
                 return (L (SrcSpanAnn (EpAnn (spanAsAnchor l) (AnnContext Nothing [] []) emptyComments) l) [orig_t])

checkImportDecl :: Maybe EpaLocation
                -> Maybe EpaLocation
                -> P ()
checkImportDecl mPre mPost = do
  let whenJust mg f = maybe (pure ()) f mg

  importQualifiedPostEnabled <- getBit ImportQualifiedPostBit

  -- Error if 'qualified' found in postpositive position and
  -- 'ImportQualifiedPost' is not in effect.
  whenJust mPost $ \post ->
    when (not importQualifiedPostEnabled) $
      failOpNotEnabledImportQualifiedPost (RealSrcSpan (epaLocationRealSrcSpan post) Strict.Nothing)

  -- Error if 'qualified' occurs in both pre and postpositive
  -- positions.
  whenJust mPost $ \post ->
    when (isJust mPre) $
      failOpImportQualifiedTwice (RealSrcSpan (epaLocationRealSrcSpan post) Strict.Nothing)

  -- Warn if 'qualified' found in prepositive position and
  -- 'Opt_WarnPrepositiveQualifiedModule' is enabled.
  whenJust mPre $ \pre ->
    warnPrepositiveQualifiedModule (RealSrcSpan (epaLocationRealSrcSpan pre) Strict.Nothing)

-- -------------------------------------------------------------------------
-- Checking Patterns.

-- We parse patterns as expressions and check for valid patterns below,
-- converting the expression into a pattern at the same time.

checkPattern :: LocatedA (PatBuilder GhcPs) -> P (LPat GhcPs)
checkPattern = runPV . checkLPat

checkPattern_details :: ParseContext -> PV (LocatedA (PatBuilder GhcPs)) -> P (LPat GhcPs)
checkPattern_details extraDetails pp = runPV_details extraDetails (pp >>= checkLPat)

checkLPat :: LocatedA (PatBuilder GhcPs) -> PV (LPat GhcPs)
checkLPat e@(L l _) = checkPat l e [] []

checkPat :: SrcSpanAnnA -> LocatedA (PatBuilder GhcPs) -> [HsPatSigType GhcPs] -> [LPat GhcPs]
         -> PV (LPat GhcPs)
checkPat loc (L l e@(PatBuilderVar (L ln c))) tyargs args
  | isRdrDataCon c = return . L loc $ ConPat
      { pat_con_ext = noAnn -- AZ: where should this come from?
      , pat_con = L ln c
      , pat_args = PrefixCon tyargs args
      }
  | not (null tyargs) =
      patFail (locA l) . PsErrInPat e $ PEIP_TypeArgs tyargs
  | (not (null args) && patIsRec c) = do
      ctx <- askParseContext
      patFail (locA l) . PsErrInPat e $ PEIP_RecPattern args YesPatIsRecursive ctx
checkPat loc (L _ (PatBuilderAppType f t)) tyargs args =
  checkPat loc f (t : tyargs) args
checkPat loc (L _ (PatBuilderApp f e)) [] args = do
  p <- checkLPat e
  checkPat loc f [] (p : args)
checkPat loc (L l e) [] [] = do
  p <- checkAPat loc e
  return (L l p)
checkPat loc e _ _ = do
  details <- fromParseContext <$> askParseContext
  patFail (locA loc) (PsErrInPat (unLoc e) details)

checkAPat :: SrcSpanAnnA -> PatBuilder GhcPs -> PV (Pat GhcPs)
checkAPat loc e0 = do
 nPlusKPatterns <- getBit NPlusKPatternsBit
 case e0 of
   PatBuilderPat p -> return p
   PatBuilderVar x -> return (VarPat noExtField x)

   -- Overloaded numeric patterns (e.g. f 0 x = x)
   -- Negation is recorded separately, so that the literal is zero or +ve
   -- NB. Negative *primitive* literals are already handled by the lexer
   PatBuilderOverLit pos_lit -> return (mkNPat (L (l2l loc) pos_lit) Nothing noAnn)

   -- n+k patterns
   PatBuilderOpApp
           (L _ (PatBuilderVar (L nloc n)))
           (L l plus)
           (L lloc (PatBuilderOverLit lit@(OverLit {ol_val = HsIntegral {}})))
           (EpAnn anc _ cs)
                     | nPlusKPatterns && (plus == plus_RDR)
                     -> return (mkNPlusKPat (L nloc n) (L (l2l lloc) lit)
                                (EpAnn anc (epaLocationFromSrcAnn l) cs))

   -- Improve error messages for the @-operator when the user meant an @-pattern
   PatBuilderOpApp _ op _ _ | opIsAt (unLoc op) -> do
     addError $ mkPlainErrorMsgEnvelope (getLocA op) PsErrAtInPatPos
     return (WildPat noExtField)

   PatBuilderOpApp l (L cl c) r anns
     | isRdrDataCon c -> do
         l <- checkLPat l
         r <- checkLPat r
         return $ ConPat
           { pat_con_ext = anns
           , pat_con = L cl c
           , pat_args = InfixCon l r
           }

   PatBuilderPar lpar e rpar -> do
     p <- checkLPat e
     return (ParPat (EpAnn (spanAsAnchor (locA loc)) NoEpAnns emptyComments) lpar p rpar)

   _           -> do
     details <- fromParseContext <$> askParseContext
     patFail (locA loc) (PsErrInPat e0 details)

placeHolderPunRhs :: DisambECP b => PV (LocatedA b)
-- The RHS of a punned record field will be filled in by the renamer
-- It's better not to make it an error, in case we want to print it when
-- debugging
placeHolderPunRhs = mkHsVarPV (noLocA pun_RDR)

plus_RDR, pun_RDR :: RdrName
plus_RDR = mkUnqual varName (fsLit "+") -- Hack
pun_RDR  = mkUnqual varName (fsLit "pun-right-hand-side")

checkPatField :: LHsRecField GhcPs (LocatedA (PatBuilder GhcPs))
              -> PV (LHsRecField GhcPs (LPat GhcPs))
checkPatField (L l fld) = do p <- checkLPat (hfbRHS fld)
                             return (L l (fld { hfbRHS = p }))

patFail :: SrcSpan -> PsMessage -> PV a
patFail loc msg = addFatalError $ mkPlainErrorMsgEnvelope loc $ msg

patIsRec :: RdrName -> Bool
patIsRec e = e == mkUnqual varName (fsLit "rec")

---------------------------------------------------------------------------
-- Check Equation Syntax

checkValDef :: SrcSpan
            -> LocatedA (PatBuilder GhcPs)
            -> Maybe (AddEpAnn, LHsType GhcPs)
            -> Located (GRHSs GhcPs (LHsExpr GhcPs))
            -> P (HsBind GhcPs)

checkValDef loc lhs (Just (sigAnn, sig)) grhss
        -- x :: ty = rhs  parses as a *pattern* binding
  = do lhs' <- runPV $ mkHsTySigPV (combineLocsA lhs sig) lhs sig [sigAnn]
                        >>= checkLPat
       checkPatBind loc [] lhs' grhss

checkValDef loc lhs Nothing g
  = do  { mb_fun <- isFunLhs lhs
        ; case mb_fun of
            Just (fun, is_infix, pats, ann) ->
              checkFunBind NoSrcStrict loc ann
                           fun is_infix pats g
            Nothing -> do
              lhs' <- checkPattern lhs
              checkPatBind loc [] lhs' g }

checkFunBind :: SrcStrictness
             -> SrcSpan
             -> [AddEpAnn]
             -> LocatedN RdrName
             -> LexicalFixity
             -> [LocatedA (PatBuilder GhcPs)]
             -> Located (GRHSs GhcPs (LHsExpr GhcPs))
             -> P (HsBind GhcPs)
checkFunBind strictness locF ann fun is_infix pats (L _ grhss)
  = do  ps <- runPV_details extraDetails (mapM checkLPat pats)
        let match_span = noAnnSrcSpan $ locF
        cs <- getCommentsFor locF
        return (makeFunBind fun (L (noAnnSrcSpan $ locA match_span)
                 [L match_span (Match { m_ext = EpAnn (spanAsAnchor locF) ann cs
                                      , m_ctxt = FunRhs
                                          { mc_fun    = fun
                                          , mc_fixity = is_infix
                                          , mc_strictness = strictness }
                                      , m_pats = ps
                                      , m_grhss = grhss })]))
        -- The span of the match covers the entire equation.
        -- That isn't quite right, but it'll do for now.
  where
    extraDetails
      | Infix <- is_infix = ParseContext (Just $ unLoc fun) NoIncompleteDoBlock
      | otherwise         = noParseContext

makeFunBind :: LocatedN RdrName -> LocatedL [LMatch GhcPs (LHsExpr GhcPs)]
            -> HsBind GhcPs
-- Like GHC.Hs.Utils.mkFunBind, but we need to be able to set the fixity too
makeFunBind fn ms
  = FunBind { fun_ext = noExtField,
              fun_id = fn,
              fun_matches = mkMatchGroup FromSource ms,
              fun_tick = [] }

-- See Note [FunBind vs PatBind]
checkPatBind :: SrcSpan
             -> [AddEpAnn]
             -> LPat GhcPs
             -> Located (GRHSs GhcPs (LHsExpr GhcPs))
             -> P (HsBind GhcPs)
checkPatBind loc annsIn (L _ (BangPat (EpAnn _ ans cs) (L _ (VarPat _ v))))
                        (L _match_span grhss)
      = return (makeFunBind v (L (noAnnSrcSpan loc)
                [L (noAnnSrcSpan loc) (m (EpAnn (spanAsAnchor loc) (ans++annsIn) cs) v)]))
  where
    m a v = Match { m_ext = a
                  , m_ctxt = FunRhs { mc_fun    = v
                                    , mc_fixity = Prefix
                                    , mc_strictness = SrcStrict }
                  , m_pats = []
                 , m_grhss = grhss }

checkPatBind loc annsIn lhs (L _ grhss) = do
  cs <- getCommentsFor loc
  return (PatBind (EpAnn (spanAsAnchor loc) annsIn cs) lhs grhss ([],[]))

checkValSigLhs :: LHsExpr GhcPs -> P (LocatedN RdrName)
checkValSigLhs (L _ (HsVar _ lrdr@(L _ v)))
  | isUnqual v
  , not (isDataOcc (rdrNameOcc v))
  = return lrdr

checkValSigLhs lhs@(L l _)
  = addFatalError $ mkPlainErrorMsgEnvelope (locA l) $ PsErrInvalidTypeSignature lhs

checkDoAndIfThenElse
  :: (Outputable a, Outputable b, Outputable c)
  => (a -> Bool -> b -> Bool -> c -> PsMessage)
  -> LocatedA a -> Bool -> LocatedA b -> Bool -> LocatedA c -> PV ()
checkDoAndIfThenElse err guardExpr semiThen thenExpr semiElse elseExpr
 | semiThen || semiElse = do
      doAndIfThenElse <- getBit DoAndIfThenElseBit
      let e   = err (unLoc guardExpr)
                    semiThen (unLoc thenExpr)
                    semiElse (unLoc elseExpr)
          loc = combineLocs (reLoc guardExpr) (reLoc elseExpr)

      unless doAndIfThenElse $ addError (mkPlainErrorMsgEnvelope loc e)
  | otherwise = return ()

isFunLhs :: LocatedA (PatBuilder GhcPs)
      -> P (Maybe (LocatedN RdrName, LexicalFixity,
                   [LocatedA (PatBuilder GhcPs)],[AddEpAnn]))
-- A variable binding is parsed as a FunBind.
-- Just (fun, is_infix, arg_pats) if e is a function LHS
isFunLhs e = go e [] [] []
 where
   go (L _ (PatBuilderVar (L loc f))) es ops cps
       | not (isRdrDataCon f)        = return (Just (L loc f, Prefix, es, (reverse ops) ++ cps))
   go (L _ (PatBuilderApp f e)) es       ops cps = go f (e:es) ops cps
   go (L l (PatBuilderPar _ e _)) es@(_:_) ops cps
                                      = let
                                          (o,c) = mkParensEpAnn (realSrcSpan $ locA l)
                                        in
                                          go e es (o:ops) (c:cps)
   go (L loc (PatBuilderOpApp l (L loc' op) r (EpAnn loca anns cs))) es ops cps
        | not (isRdrDataCon op)         -- We have found the function!
        = return (Just (L loc' op, Infix, (l:r:es), (anns ++ reverse ops ++ cps)))
        | otherwise                     -- Infix data con; keep going
        = do { mb_l <- go l es ops cps
             ; case mb_l of
                 Just (op', Infix, j : k : es', anns')
                   -> return (Just (op', Infix, j : op_app : es', anns'))
                   where
                     op_app = L loc (PatBuilderOpApp k
                               (L loc' op) r (EpAnn loca (reverse ops++cps) cs))
                 _ -> return Nothing }
   go _ _ _ _ = return Nothing

mkBangTy :: EpAnn [AddEpAnn] -> SrcStrictness -> LHsType GhcPs -> HsType GhcPs
mkBangTy anns strictness =
  HsBangTy anns (HsSrcBang NoSourceText NoSrcUnpack strictness)

-- | Result of parsing @{-\# UNPACK \#-}@ or @{-\# NOUNPACK \#-}@.
data UnpackednessPragma =
  UnpackednessPragma [AddEpAnn] SourceText SrcUnpackedness

-- | Annotate a type with either an @{-\# UNPACK \#-}@ or a @{-\# NOUNPACK \#-}@ pragma.
addUnpackednessP :: MonadP m => Located UnpackednessPragma -> LHsType GhcPs -> m (LHsType GhcPs)
addUnpackednessP (L lprag (UnpackednessPragma anns prag unpk)) ty = do
    let l' = combineSrcSpans lprag (getLocA ty)
    cs <- getCommentsFor l'
    let an = EpAnn (spanAsAnchor l') anns cs
        t' = addUnpackedness an ty
    return (L (noAnnSrcSpan l') t')
  where
    -- If we have a HsBangTy that only has a strictness annotation,
    -- such as ~T or !T, then add the pragma to the existing HsBangTy.
    --
    -- Otherwise, wrap the type in a new HsBangTy constructor.
    addUnpackedness an (L _ (HsBangTy x bang t))
      | HsSrcBang NoSourceText NoSrcUnpack strictness <- bang
      = HsBangTy (addAnns an (epAnnAnns x) (epAnnComments x)) (HsSrcBang prag unpk strictness) t
    addUnpackedness an t
      = HsBangTy an (HsSrcBang prag unpk NoSrcStrict) t

---------------------------------------------------------------------------
-- | Check for monad comprehensions
--
-- If the flag MonadComprehensions is set, return a 'MonadComp' context,
-- otherwise use the usual 'ListComp' context

checkMonadComp :: PV HsDoFlavour
checkMonadComp = do
    monadComprehensions <- getBit MonadComprehensionsBit
    return $ if monadComprehensions
                then MonadComp
                else ListComp

-- -------------------------------------------------------------------------
-- Expression/command/pattern ambiguity.
-- See Note [Ambiguous syntactic categories]
--

-- See Note [Ambiguous syntactic categories]
--
-- This newtype is required to avoid impredicative types in monadic
-- productions. That is, in a production that looks like
--
--    | ... {% return (ECP ...) }
--
-- we are dealing with
--    P ECP
-- whereas without a newtype we would be dealing with
--    P (forall b. DisambECP b => PV (Located b))
--
newtype ECP =
  ECP { unECP :: forall b. DisambECP b => PV (LocatedA b) }

ecpFromExp :: LHsExpr GhcPs -> ECP
ecpFromExp a = ECP (ecpFromExp' a)

ecpFromCmd :: LHsCmd GhcPs -> ECP
ecpFromCmd a = ECP (ecpFromCmd' a)

-- The 'fbinds' parser rule produces values of this type. See Note
-- [RecordDotSyntax field updates].
type Fbind b = Either (LHsRecField GhcPs (LocatedA b)) (LHsRecProj GhcPs (LocatedA b))

-- | Disambiguate infix operators.
-- See Note [Ambiguous syntactic categories]
class DisambInfixOp b where
  mkHsVarOpPV :: LocatedN RdrName -> PV (LocatedN b)
  mkHsConOpPV :: LocatedN RdrName -> PV (LocatedN b)
  mkHsInfixHolePV :: SrcSpan -> (EpAnnComments -> EpAnn EpAnnUnboundVar) -> PV (Located b)

instance DisambInfixOp (HsExpr GhcPs) where
  mkHsVarOpPV v = return $ L (getLoc v) (HsVar noExtField v)
  mkHsConOpPV v = return $ L (getLoc v) (HsVar noExtField v)
  mkHsInfixHolePV l ann = do
    cs <- getCommentsFor l
    return $ L l (hsHoleExpr (ann cs))

instance DisambInfixOp RdrName where
  mkHsConOpPV (L l v) = return $ L l v
  mkHsVarOpPV (L l v) = return $ L l v
  mkHsInfixHolePV l _ = addFatalError $ mkPlainErrorMsgEnvelope l $ PsErrInvalidInfixHole

type AnnoBody b
  = ( Anno (GRHS GhcPs (LocatedA (Body b GhcPs))) ~ SrcAnn NoEpAnns
    , Anno [LocatedA (Match GhcPs (LocatedA (Body b GhcPs)))] ~ SrcSpanAnnL
    , Anno (Match GhcPs (LocatedA (Body b GhcPs))) ~ SrcSpanAnnA
    , Anno (StmtLR GhcPs GhcPs (LocatedA (Body (Body b GhcPs) GhcPs))) ~ SrcSpanAnnA
    , Anno [LocatedA (StmtLR GhcPs GhcPs
                       (LocatedA (Body (Body (Body b GhcPs) GhcPs) GhcPs)))] ~ SrcSpanAnnL
    )

-- | Disambiguate constructs that may appear when we do not know ahead of time whether we are
-- parsing an expression, a command, or a pattern.
-- See Note [Ambiguous syntactic categories]
class (b ~ (Body b) GhcPs, AnnoBody b) => DisambECP b where
  -- | See Note [Body in DisambECP]
  type Body b :: Type -> Type
  -- | Return a command without ambiguity, or fail in a non-command context.
  ecpFromCmd' :: LHsCmd GhcPs -> PV (LocatedA b)
  -- | Return an expression without ambiguity, or fail in a non-expression context.
  ecpFromExp' :: LHsExpr GhcPs -> PV (LocatedA b)
  mkHsProjUpdatePV :: SrcSpan -> Located [LocatedAn NoEpAnns (DotFieldOcc GhcPs)]
    -> LocatedA b -> Bool -> [AddEpAnn] -> PV (LHsRecProj GhcPs (LocatedA b))
  -- | Disambiguate "\... -> ..." (lambda)
  mkHsLamPV
    :: SrcSpan -> (EpAnnComments -> MatchGroup GhcPs (LocatedA b)) -> PV (LocatedA b)
  -- | Disambiguate "let ... in ..."
  mkHsLetPV
    :: SrcSpan
    -> LHsToken "let" GhcPs
    -> HsLocalBinds GhcPs
    -> LHsToken "in" GhcPs
    -> LocatedA b
    -> PV (LocatedA b)
  -- | Infix operator representation
  type InfixOp b
  -- | Bring superclass constraints on InfixOp into scope.
  -- See Note [UndecidableSuperClasses for associated types]
  superInfixOp
    :: (DisambInfixOp (InfixOp b) => PV (LocatedA b )) -> PV (LocatedA b)
  -- | Disambiguate "f # x" (infix operator)
  mkHsOpAppPV :: SrcSpan -> LocatedA b -> LocatedN (InfixOp b) -> LocatedA b
              -> PV (LocatedA b)
  -- | Disambiguate "case ... of ..."
  mkHsCasePV :: SrcSpan -> LHsExpr GhcPs -> (LocatedL [LMatch GhcPs (LocatedA b)])
             -> EpAnnHsCase -> PV (LocatedA b)
  mkHsLamCasePV :: SrcSpan -> (LocatedL [LMatch GhcPs (LocatedA b)])
                -> [AddEpAnn]
                -> PV (LocatedA b)
  -- | Function argument representation
  type FunArg b
  -- | Bring superclass constraints on FunArg into scope.
  -- See Note [UndecidableSuperClasses for associated types]
  superFunArg :: (DisambECP (FunArg b) => PV (LocatedA b)) -> PV (LocatedA b)
  -- | Disambiguate "f x" (function application)
  mkHsAppPV :: SrcSpanAnnA -> LocatedA b -> LocatedA (FunArg b) -> PV (LocatedA b)
  -- | Disambiguate "f @t" (visible type application)
  mkHsAppTypePV :: SrcSpanAnnA -> LocatedA b -> SrcSpan -> LHsType GhcPs -> PV (LocatedA b)
  -- | Disambiguate "if ... then ... else ..."
  mkHsIfPV :: SrcSpan
         -> LHsExpr GhcPs
         -> Bool  -- semicolon?
         -> LocatedA b
         -> Bool  -- semicolon?
         -> LocatedA b
         -> AnnsIf
         -> PV (LocatedA b)
  -- | Disambiguate "do { ... }" (do notation)
  mkHsDoPV ::
    SrcSpan ->
    Maybe ModuleName ->
    LocatedL [LStmt GhcPs (LocatedA b)] ->
    AnnList ->
    PV (LocatedA b)
  -- | Disambiguate "( ... )" (parentheses)
  mkHsParPV :: SrcSpan -> LHsToken "(" GhcPs -> LocatedA b -> LHsToken ")" GhcPs -> PV (LocatedA b)
  -- | Disambiguate a variable "f" or a data constructor "MkF".
  mkHsVarPV :: LocatedN RdrName -> PV (LocatedA b)
  -- | Disambiguate a monomorphic literal
  mkHsLitPV :: Located (HsLit GhcPs) -> PV (Located b)
  -- | Disambiguate an overloaded literal
  mkHsOverLitPV :: LocatedAn a (HsOverLit GhcPs) -> PV (LocatedAn a b)
  -- | Disambiguate a wildcard
  mkHsWildCardPV :: SrcSpan -> PV (Located b)
  -- | Disambiguate "a :: t" (type annotation)
  mkHsTySigPV
    :: SrcSpanAnnA -> LocatedA b -> LHsType GhcPs -> [AddEpAnn] -> PV (LocatedA b)
  -- | Disambiguate "[a,b,c]" (list syntax)
  mkHsExplicitListPV :: SrcSpan -> [LocatedA b] -> AnnList -> PV (LocatedA b)
  -- | Disambiguate "$(...)" and "[quasi|...|]" (TH splices)
  mkHsSplicePV :: Located (HsSplice GhcPs) -> PV (Located b)
  -- | Disambiguate "f { a = b, ... }" syntax (record construction and record updates)
  mkHsRecordPV ::
    Bool -> -- Is OverloadedRecordUpdate in effect?
    SrcSpan ->
    SrcSpan ->
    LocatedA b ->
    ([Fbind b], Maybe SrcSpan) ->
    [AddEpAnn] ->
    PV (LocatedA b)
  -- | Disambiguate "-a" (negation)
  mkHsNegAppPV :: SrcSpan -> LocatedA b -> [AddEpAnn] -> PV (LocatedA b)
  -- | Disambiguate "(# a)" (right operator section)
  mkHsSectionR_PV
    :: SrcSpan -> LocatedA (InfixOp b) -> LocatedA b -> PV (Located b)
  -- | Disambiguate "(a -> b)" (view pattern)
  mkHsViewPatPV
    :: SrcSpan -> LHsExpr GhcPs -> LocatedA b -> [AddEpAnn] -> PV (LocatedA b)
  -- | Disambiguate "a@b" (as-pattern)
  mkHsAsPatPV
    :: SrcSpan -> LocatedN RdrName -> LocatedA b -> [AddEpAnn] -> PV (LocatedA b)
  -- | Disambiguate "~a" (lazy pattern)
  mkHsLazyPatPV :: SrcSpan -> LocatedA b -> [AddEpAnn] -> PV (LocatedA b)
  -- | Disambiguate "!a" (bang pattern)
  mkHsBangPatPV :: SrcSpan -> LocatedA b -> [AddEpAnn] -> PV (LocatedA b)
  -- | Disambiguate tuple sections and unboxed sums
  mkSumOrTuplePV
    :: SrcSpanAnnA -> Boxity -> SumOrTuple b -> [AddEpAnn] -> PV (LocatedA b)
  -- | Validate infixexp LHS to reject unwanted {-# SCC ... #-} pragmas
  rejectPragmaPV :: LocatedA b -> PV ()

{- Note [UndecidableSuperClasses for associated types]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
(This Note is about the code in GHC, not about the user code that we are parsing)

Assume we have a class C with an associated type T:

  class C a where
    type T a
    ...

If we want to add 'C (T a)' as a superclass, we need -XUndecidableSuperClasses:

  {-# LANGUAGE UndecidableSuperClasses #-}
  class C (T a) => C a where
    type T a
    ...

Unfortunately, -XUndecidableSuperClasses don't work all that well, sometimes
making GHC loop. The workaround is to bring this constraint into scope
manually with a helper method:

  class C a where
    type T a
    superT :: (C (T a) => r) -> r

In order to avoid ambiguous types, 'r' must mention 'a'.

For consistency, we use this approach for all constraints on associated types,
even when -XUndecidableSuperClasses are not required.
-}

{- Note [Body in DisambECP]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
There are helper functions (mkBodyStmt, mkBindStmt, unguardedRHS, etc) that
require their argument to take a form of (body GhcPs) for some (body :: Type ->
*). To satisfy this requirement, we say that (b ~ Body b GhcPs) in the
superclass constraints of DisambECP.

The alternative is to change mkBodyStmt, mkBindStmt, unguardedRHS, etc, to drop
this requirement. It is possible and would allow removing the type index of
PatBuilder, but leads to worse type inference, breaking some code in the
typechecker.
-}

instance DisambECP (HsCmd GhcPs) where
  type Body (HsCmd GhcPs) = HsCmd
  ecpFromCmd' = return
  ecpFromExp' (L l e) = cmdFail (locA l) (ppr e)
  mkHsProjUpdatePV l _ _ _ _ = addFatalError $ mkPlainErrorMsgEnvelope l $
                                                 PsErrOverloadedRecordDotInvalid
  mkHsLamPV l mg = do
    cs <- getCommentsFor l
    return $ L (noAnnSrcSpan l) (HsCmdLam NoExtField (mg cs))
  mkHsLetPV l tkLet bs tkIn e = do
    cs <- getCommentsFor l
    return $ L (noAnnSrcSpan l) (HsCmdLet (EpAnn (spanAsAnchor l) NoEpAnns cs) tkLet bs tkIn e)
  type InfixOp (HsCmd GhcPs) = HsExpr GhcPs
  superInfixOp m = m
  mkHsOpAppPV l c1 op c2 = do
    let cmdArg c = L (l2l $ getLoc c) $ HsCmdTop noExtField c
    cs <- getCommentsFor l
    return $ L (noAnnSrcSpan l) $ HsCmdArrForm (EpAnn (spanAsAnchor l) (AnnList Nothing Nothing Nothing [] []) cs) (reLocL op) Infix Nothing [cmdArg c1, cmdArg c2]
  mkHsCasePV l c (L lm m) anns = do
    cs <- getCommentsFor l
    let mg = mkMatchGroup FromSource (L lm m)
    return $ L (noAnnSrcSpan l) (HsCmdCase (EpAnn (spanAsAnchor l) anns cs) c mg)
  mkHsLamCasePV l (L lm m) anns = do
    cs <- getCommentsFor l
    let mg = mkMatchGroup FromSource (L lm m)
    return $ L (noAnnSrcSpan l) (HsCmdLamCase (EpAnn (spanAsAnchor l) anns cs) mg)
  type FunArg (HsCmd GhcPs) = HsExpr GhcPs
  superFunArg m = m
  mkHsAppPV l c e = do
    cs <- getCommentsFor (locA l)
    checkCmdBlockArguments c
    checkExpBlockArguments e
    return $ L l (HsCmdApp (comment (realSrcSpan $ locA l) cs) c e)
  mkHsAppTypePV l c _ t = cmdFail (locA l) (ppr c <+> text "@" <> ppr t)
  mkHsIfPV l c semi1 a semi2 b anns = do
    checkDoAndIfThenElse PsErrSemiColonsInCondCmd c semi1 a semi2 b
    cs <- getCommentsFor l
    return $ L (noAnnSrcSpan l) (mkHsCmdIf c a b (EpAnn (spanAsAnchor l) anns cs))
  mkHsDoPV l Nothing stmts anns = do
    cs <- getCommentsFor l
    return $ L (noAnnSrcSpan l) (HsCmdDo (EpAnn (spanAsAnchor l) anns cs) stmts)
  mkHsDoPV l (Just m)    _ _ = addFatalError $ mkPlainErrorMsgEnvelope l $ PsErrQualifiedDoInCmd m
  mkHsParPV l lpar c rpar = do
    cs <- getCommentsFor l
    return $ L (noAnnSrcSpan l) (HsCmdPar (EpAnn (spanAsAnchor l) NoEpAnns cs) lpar c rpar)
  mkHsVarPV (L l v) = cmdFail (locA l) (ppr v)
  mkHsLitPV (L l a) = cmdFail l (ppr a)
  mkHsOverLitPV (L l a) = cmdFail (locA l) (ppr a)
  mkHsWildCardPV l = cmdFail l (text "_")
  mkHsTySigPV l a sig _ = cmdFail (locA l) (ppr a <+> text "::" <+> ppr sig)
  mkHsExplicitListPV l xs _ = cmdFail l $
    brackets (fsep (punctuate comma (map ppr xs)))
  mkHsSplicePV (L l sp) = cmdFail l (ppr sp)
  mkHsRecordPV _ l _ a (fbinds, ddLoc) _ = do
    let (fs, ps) = partitionEithers fbinds
    if not (null ps)
      then addFatalError $ mkPlainErrorMsgEnvelope l $ PsErrOverloadedRecordDotInvalid
      else cmdFail l $ ppr a <+> ppr (mk_rec_fields fs ddLoc)
  mkHsNegAppPV l a _ = cmdFail l (text "-" <> ppr a)
  mkHsSectionR_PV l op c = cmdFail l $
    let pp_op = fromMaybe (panic "cannot print infix operator")
                          (ppr_infix_expr (unLoc op))
    in pp_op <> ppr c
  mkHsViewPatPV l a b _ = cmdFail l $
    ppr a <+> text "->" <+> ppr b
  mkHsAsPatPV l v c _ = cmdFail l $
    pprPrefixOcc (unLoc v) <> text "@" <> ppr c
  mkHsLazyPatPV l c _ = cmdFail l $
    text "~" <> ppr c
  mkHsBangPatPV l c _ = cmdFail l $
    text "!" <> ppr c
  mkSumOrTuplePV l boxity a _ = cmdFail (locA l) (pprSumOrTuple boxity a)
  rejectPragmaPV _ = return ()

cmdFail :: SrcSpan -> SDoc -> PV a
cmdFail loc e = addFatalError $ mkPlainErrorMsgEnvelope loc $ PsErrParseErrorInCmd e

checkLamMatchGroup :: SrcSpan -> MatchGroup GhcPs (LHsExpr GhcPs) -> PV ()
checkLamMatchGroup l (MG { mg_alts = (L _ (matches:_))}) = do
  when (null (hsLMatchPats matches)) $ addError $ mkPlainErrorMsgEnvelope l PsErrEmptyLambda
checkLamMatchGroup _ _ = return ()

instance DisambECP (HsExpr GhcPs) where
  type Body (HsExpr GhcPs) = HsExpr
  ecpFromCmd' (L l c) = do
    addError $ mkPlainErrorMsgEnvelope (locA l) $ PsErrArrowCmdInExpr c
    return (L l (hsHoleExpr noAnn))
  ecpFromExp' = return
  mkHsProjUpdatePV l fields arg isPun anns = do
    cs <- getCommentsFor l
    return $ mkRdrProjUpdate (noAnnSrcSpan l) fields arg isPun (EpAnn (spanAsAnchor l) anns cs)
  mkHsLamPV l mg = do
    cs <- getCommentsFor l
    let mg' = mg cs
    checkLamMatchGroup l mg'
    return $ L (noAnnSrcSpan l) (HsLam NoExtField mg')
  mkHsLetPV l tkLet bs tkIn c = do
    cs <- getCommentsFor l
    return $ L (noAnnSrcSpan l) (HsLet (EpAnn (spanAsAnchor l) NoEpAnns cs) tkLet bs tkIn c)
  type InfixOp (HsExpr GhcPs) = HsExpr GhcPs
  superInfixOp m = m
  mkHsOpAppPV l e1 op e2 = do
    cs <- getCommentsFor l
    return $ L (noAnnSrcSpan l) $ OpApp (EpAnn (spanAsAnchor l) [] cs) e1 (reLocL op) e2
  mkHsCasePV l e (L lm m) anns = do
    cs <- getCommentsFor l
    let mg = mkMatchGroup FromSource (L lm m)
    return $ L (noAnnSrcSpan l) (HsCase (EpAnn (spanAsAnchor l) anns cs) e mg)
  mkHsLamCasePV l (L lm m) anns = do
    cs <- getCommentsFor l
    let mg = mkMatchGroup FromSource (L lm m)
    return $ L (noAnnSrcSpan l) (HsLamCase (EpAnn (spanAsAnchor l) anns cs) mg)
  type FunArg (HsExpr GhcPs) = HsExpr GhcPs
  superFunArg m = m
  mkHsAppPV l e1 e2 = do
    cs <- getCommentsFor (locA l)
    checkExpBlockArguments e1
    checkExpBlockArguments e2
    return $ L l (HsApp (comment (realSrcSpan $ locA l) cs) e1 e2)
  mkHsAppTypePV l e la t = do
    checkExpBlockArguments e
    return $ L l (HsAppType la e (mkHsWildCardBndrs t))
  mkHsIfPV l c semi1 a semi2 b anns = do
    checkDoAndIfThenElse PsErrSemiColonsInCondExpr c semi1 a semi2 b
    cs <- getCommentsFor l
    return $ L (noAnnSrcSpan l) (mkHsIf c a b (EpAnn (spanAsAnchor l) anns cs))
  mkHsDoPV l mod stmts anns = do
    cs <- getCommentsFor l
    return $ L (noAnnSrcSpan l) (HsDo (EpAnn (spanAsAnchor l) anns cs) (DoExpr mod) stmts)
  mkHsParPV l lpar e rpar = do
    cs <- getCommentsFor l
    return $ L (noAnnSrcSpan l) (HsPar (EpAnn (spanAsAnchor l) NoEpAnns cs) lpar e rpar)
  mkHsVarPV v@(L l _) = return $ L (na2la l) (HsVar noExtField v)
  mkHsLitPV (L l a) = do
    cs <- getCommentsFor l
    return $ L l (HsLit (comment (realSrcSpan l) cs) a)
  mkHsOverLitPV (L l a) = do
    cs <- getCommentsFor (locA l)
    return $ L l (HsOverLit (comment (realSrcSpan (locA l)) cs) a)
  mkHsWildCardPV l = return $ L l (hsHoleExpr noAnn)
  mkHsTySigPV l a sig anns = do
    cs <- getCommentsFor (locA l)
    return $ L l (ExprWithTySig (EpAnn (spanAsAnchor $ locA l) anns cs) a (hsTypeToHsSigWcType sig))
  mkHsExplicitListPV l xs anns = do
    cs <- getCommentsFor l
    return $ L (noAnnSrcSpan l) (ExplicitList (EpAnn (spanAsAnchor l) anns cs) xs)
  mkHsSplicePV sp@(L l _) = do
    cs <- getCommentsFor l
    return $ mapLoc (HsSpliceE (EpAnn (spanAsAnchor l) NoEpAnns cs)) sp
  mkHsRecordPV opts l lrec a (fbinds, ddLoc) anns = do
    cs <- getCommentsFor l
    r <- mkRecConstrOrUpdate opts a lrec (fbinds, ddLoc) (EpAnn (spanAsAnchor l) anns cs)
    checkRecordSyntax (L (noAnnSrcSpan l) r)
  mkHsNegAppPV l a anns = do
    cs <- getCommentsFor l
    return $ L (noAnnSrcSpan l) (NegApp (EpAnn (spanAsAnchor l) anns cs) a noSyntaxExpr)
  mkHsSectionR_PV l op e = do
    cs <- getCommentsFor l
    return $ L l (SectionR (comment (realSrcSpan l) cs) op e)
  mkHsViewPatPV l a b _ = addError (mkPlainErrorMsgEnvelope l $ PsErrViewPatInExpr a b)
                          >> return (L (noAnnSrcSpan l) (hsHoleExpr noAnn))
  mkHsAsPatPV l v e   _ = addError (mkPlainErrorMsgEnvelope l $ PsErrTypeAppWithoutSpace (unLoc v) e)
                          >> return (L (noAnnSrcSpan l) (hsHoleExpr noAnn))
  mkHsLazyPatPV l e   _ = addError (mkPlainErrorMsgEnvelope l $ PsErrLazyPatWithoutSpace e)
                          >> return (L (noAnnSrcSpan l) (hsHoleExpr noAnn))
  mkHsBangPatPV l e   _ = addError (mkPlainErrorMsgEnvelope l $ PsErrBangPatWithoutSpace e)
                          >> return (L (noAnnSrcSpan l) (hsHoleExpr noAnn))
  mkSumOrTuplePV = mkSumOrTupleExpr
  rejectPragmaPV (L _ (OpApp _ _ _ e)) =
    -- assuming left-associative parsing of operators
    rejectPragmaPV e
  rejectPragmaPV (L l (HsPragE _ prag _)) = addError $ mkPlainErrorMsgEnvelope (locA l) $
                                                         (PsErrUnallowedPragma prag)
  rejectPragmaPV _                        = return ()

hsHoleExpr :: EpAnn EpAnnUnboundVar -> HsExpr GhcPs
hsHoleExpr anns = HsUnboundVar anns (mkVarOcc "_")

type instance Anno (GRHS GhcPs (LocatedA (PatBuilder GhcPs))) = SrcAnn NoEpAnns
type instance Anno [LocatedA (Match GhcPs (LocatedA (PatBuilder GhcPs)))] = SrcSpanAnnL
type instance Anno (Match GhcPs (LocatedA (PatBuilder GhcPs))) = SrcSpanAnnA
type instance Anno (StmtLR GhcPs GhcPs (LocatedA (PatBuilder GhcPs))) = SrcSpanAnnA

instance DisambECP (PatBuilder GhcPs) where
  type Body (PatBuilder GhcPs) = PatBuilder
  ecpFromCmd' (L l c)    = addFatalError $ mkPlainErrorMsgEnvelope (locA l) $ PsErrArrowCmdInPat c
  ecpFromExp' (L l e)    = addFatalError $ mkPlainErrorMsgEnvelope (locA l) $ PsErrArrowExprInPat e
  mkHsLamPV l _          = addFatalError $ mkPlainErrorMsgEnvelope l PsErrLambdaInPat
  mkHsLetPV l _ _ _ _    = addFatalError $ mkPlainErrorMsgEnvelope l PsErrLetInPat
  mkHsProjUpdatePV l _ _ _ _ = addFatalError $ mkPlainErrorMsgEnvelope l PsErrOverloadedRecordDotInvalid
  type InfixOp (PatBuilder GhcPs) = RdrName
  superInfixOp m = m
  mkHsOpAppPV l p1 op p2 = do
    cs <- getCommentsFor l
    let anns = EpAnn (spanAsAnchor l) [] cs
    return $ L (noAnnSrcSpan l) $ PatBuilderOpApp p1 op p2 anns
  mkHsCasePV l _ _ _     = addFatalError $ mkPlainErrorMsgEnvelope l PsErrCaseInPat
  mkHsLamCasePV l _ _    = addFatalError $ mkPlainErrorMsgEnvelope l PsErrLambdaCaseInPat
  type FunArg (PatBuilder GhcPs) = PatBuilder GhcPs
  superFunArg m = m
  mkHsAppPV l p1 p2      = return $ L l (PatBuilderApp p1 p2)
  mkHsAppTypePV l p la t = do
    cs <- getCommentsFor (locA l)
    let anns = EpAnn (spanAsAnchor (combineSrcSpans la (getLocA t))) (EpaSpan (realSrcSpan la)) cs
    return $ L l (PatBuilderAppType p (mkHsPatSigType anns t))
  mkHsIfPV l _ _ _ _ _ _ = addFatalError $ mkPlainErrorMsgEnvelope l PsErrIfThenElseInPat
  mkHsDoPV l _ _ _       = addFatalError $ mkPlainErrorMsgEnvelope l PsErrDoNotationInPat
  mkHsParPV l lpar p rpar   = return $ L (noAnnSrcSpan l) (PatBuilderPar lpar p rpar)
  mkHsVarPV v@(getLoc -> l) = return $ L (na2la l) (PatBuilderVar v)
  mkHsLitPV lit@(L l a) = do
    checkUnboxedLitPat lit
    return $ L l (PatBuilderPat (LitPat noExtField a))
  mkHsOverLitPV (L l a) = return $ L l (PatBuilderOverLit a)
  mkHsWildCardPV l = return $ L l (PatBuilderPat (WildPat noExtField))
  mkHsTySigPV l b sig anns = do
    p <- checkLPat b
    cs <- getCommentsFor (locA l)
    return $ L l (PatBuilderPat (SigPat (EpAnn (spanAsAnchor $ locA l) anns cs) p (mkHsPatSigType noAnn sig)))
  mkHsExplicitListPV l xs anns = do
    ps <- traverse checkLPat xs
    cs <- getCommentsFor l
    return (L (noAnnSrcSpan l) (PatBuilderPat (ListPat (EpAnn (spanAsAnchor l) anns cs) ps)))
  mkHsSplicePV (L l sp) = return $ L l (PatBuilderPat (SplicePat noExtField sp))
  mkHsRecordPV _ l _ a (fbinds, ddLoc) anns = do
    let (fs, ps) = partitionEithers fbinds
    if not (null ps)
     then addFatalError $ mkPlainErrorMsgEnvelope l PsErrOverloadedRecordDotInvalid
     else do
       cs <- getCommentsFor l
       r <- mkPatRec a (mk_rec_fields fs ddLoc) (EpAnn (spanAsAnchor l) anns cs)
       checkRecordSyntax (L (noAnnSrcSpan l) r)
  mkHsNegAppPV l (L lp p) anns = do
    lit <- case p of
      PatBuilderOverLit pos_lit -> return (L (l2l lp) pos_lit)
      _ -> patFail l $ PsErrInPat p PEIP_NegApp
    cs <- getCommentsFor l
    let an = EpAnn (spanAsAnchor l) anns cs
    return $ L (noAnnSrcSpan l) (PatBuilderPat (mkNPat lit (Just noSyntaxExpr) an))
  mkHsSectionR_PV l op p = patFail l (PsErrParseRightOpSectionInPat (unLoc op) (unLoc p))
  mkHsViewPatPV l a b anns = do
    p <- checkLPat b
    cs <- getCommentsFor l
    return $ L (noAnnSrcSpan l) (PatBuilderPat (ViewPat (EpAnn (spanAsAnchor l) anns cs) a p))
  mkHsAsPatPV l v e a = do
    p <- checkLPat e
    cs <- getCommentsFor l
    return $ L (noAnnSrcSpan l) (PatBuilderPat (AsPat (EpAnn (spanAsAnchor l) a cs) v p))
  mkHsLazyPatPV l e a = do
    p <- checkLPat e
    cs <- getCommentsFor l
    return $ L (noAnnSrcSpan l) (PatBuilderPat (LazyPat (EpAnn (spanAsAnchor l) a cs) p))
  mkHsBangPatPV l e an = do
    p <- checkLPat e
    cs <- getCommentsFor l
    let pb = BangPat (EpAnn (spanAsAnchor l) an cs) p
    hintBangPat l pb
    return $ L (noAnnSrcSpan l) (PatBuilderPat pb)
  mkSumOrTuplePV = mkSumOrTuplePat
  rejectPragmaPV _ = return ()

-- | Ensure that a literal pattern isn't of type Addr#, Float#, Double#.
checkUnboxedLitPat :: Located (HsLit GhcPs) -> PV ()
checkUnboxedLitPat (L loc lit) =
  case lit of
    -- Don't allow primitive string literal patterns.
    -- See #13260.
    HsStringPrim {}
      -> addError $ mkPlainErrorMsgEnvelope loc $
                           (PsErrIllegalUnboxedStringInPat lit)

   -- Don't allow Float#/Double# literal patterns.
   -- See #9238 and Note [Rules for floating-point comparisons]
   -- in GHC.Core.Opt.ConstantFold.
    _ | is_floating_lit lit
      -> addError $ mkPlainErrorMsgEnvelope loc $
                           (PsErrIllegalUnboxedFloatingLitInPat lit)

      | otherwise
      -> return ()

  where
    is_floating_lit :: HsLit GhcPs -> Bool
    is_floating_lit (HsFloatPrim  {}) = True
    is_floating_lit (HsDoublePrim {}) = True
    is_floating_lit _                 = False

mkPatRec ::
  LocatedA (PatBuilder GhcPs) ->
  HsRecFields GhcPs (LocatedA (PatBuilder GhcPs)) ->
  EpAnn [AddEpAnn] ->
  PV (PatBuilder GhcPs)
mkPatRec (unLoc -> PatBuilderVar c) (HsRecFields fs dd) anns
  | isRdrDataCon (unLoc c)
  = do fs <- mapM checkPatField fs
       return $ PatBuilderPat $ ConPat
         { pat_con_ext = anns
         , pat_con = c
         , pat_args = RecCon (HsRecFields fs dd)
         }
mkPatRec p _ _ =
  addFatalError $ mkPlainErrorMsgEnvelope (getLocA p) $
                    (PsErrInvalidRecordCon (unLoc p))

-- | Disambiguate constructs that may appear when we do not know
-- ahead of time whether we are parsing a type or a newtype/data constructor.
--
-- See Note [Ambiguous syntactic categories] for the general idea.
--
-- See Note [Parsing data constructors is hard] for the specific issue this
-- particular class is solving.
--
class DisambTD b where
  -- | Process the head of a type-level function/constructor application,
  -- i.e. the @H@ in @H a b c@.
  mkHsAppTyHeadPV :: LHsType GhcPs -> PV (LocatedA b)
  -- | Disambiguate @f x@ (function application or prefix data constructor).
  mkHsAppTyPV :: LocatedA b -> LHsType GhcPs -> PV (LocatedA b)
  -- | Disambiguate @f \@t@ (visible kind application)
  mkHsAppKindTyPV :: LocatedA b -> SrcSpan -> LHsType GhcPs -> PV (LocatedA b)
  -- | Disambiguate @f \# x@ (infix operator)
  mkHsOpTyPV :: PromotionFlag -> LHsType GhcPs -> LocatedN RdrName -> LHsType GhcPs -> PV (LocatedA b)
  -- | Disambiguate @{-\# UNPACK \#-} t@ (unpack/nounpack pragma)
  mkUnpackednessPV :: Located UnpackednessPragma -> LocatedA b -> PV (LocatedA b)

instance DisambTD (HsType GhcPs) where
  mkHsAppTyHeadPV = return
  mkHsAppTyPV t1 t2 = return (mkHsAppTy t1 t2)
  mkHsAppKindTyPV t l_at ki = return (mkHsAppKindTy l_at t ki)
  mkHsOpTyPV prom t1 op t2 = return (mkLHsOpTy prom t1 op t2)
  mkUnpackednessPV = addUnpackednessP

dataConBuilderCon :: DataConBuilder -> LocatedN RdrName
dataConBuilderCon (PrefixDataConBuilder _ dc) = dc
dataConBuilderCon (InfixDataConBuilder _ dc _) = dc

dataConBuilderDetails :: DataConBuilder -> HsConDeclH98Details GhcPs

-- Detect when the record syntax is used:
--   data T = MkT { ... }
dataConBuilderDetails (PrefixDataConBuilder flds _)
  | [L l_t (HsRecTy an fields)] <- toList flds
  = RecCon (L (SrcSpanAnn an (locA l_t)) fields)

-- Normal prefix constructor, e.g.  data T = MkT A B C
dataConBuilderDetails (PrefixDataConBuilder flds _)
  = PrefixCon noTypeArgs (map hsLinear (toList flds))

-- Infix constructor, e.g. data T = Int :! Bool
dataConBuilderDetails (InfixDataConBuilder lhs _ rhs)
  = InfixCon (hsLinear lhs) (hsLinear rhs)

instance DisambTD DataConBuilder where
  mkHsAppTyHeadPV = tyToDataConBuilder

  mkHsAppTyPV (L l (PrefixDataConBuilder flds fn)) t =
    return $
      L (noAnnSrcSpan $ combineSrcSpans (locA l) (getLocA t))
        (PrefixDataConBuilder (flds `snocOL` t) fn)
  mkHsAppTyPV (L _ InfixDataConBuilder{}) _ =
    -- This case is impossible because of the way
    -- the grammar in Parser.y is written (see infixtype/ftype).
    panic "mkHsAppTyPV: InfixDataConBuilder"

  mkHsAppKindTyPV lhs l_at ki =
    addFatalError $ mkPlainErrorMsgEnvelope l_at $
                      (PsErrUnexpectedKindAppInDataCon (unLoc lhs) (unLoc ki))

  mkHsOpTyPV _ lhs tc rhs = do
      check_no_ops (unLoc rhs)  -- check the RHS because parsing type operators is right-associative
      data_con <- eitherToP $ tyConToDataCon tc
      return $ L l (InfixDataConBuilder lhs data_con rhs)
    where
      l = combineLocsA lhs rhs
      check_no_ops (HsBangTy _ _ t) = check_no_ops (unLoc t)
      check_no_ops (HsOpTy{}) =
        addError $ mkPlainErrorMsgEnvelope (locA l) $
                     (PsErrInvalidInfixDataCon (unLoc lhs) (unLoc tc) (unLoc rhs))
      check_no_ops _ = return ()

  mkUnpackednessPV unpk constr_stuff
    | L _ (InfixDataConBuilder lhs data_con rhs) <- constr_stuff
    = -- When the user writes  data T = {-# UNPACK #-} Int :+ Bool
      --   we apply {-# UNPACK #-} to the LHS
      do lhs' <- addUnpackednessP unpk lhs
         let l = combineLocsA (reLocA unpk) constr_stuff
         return $ L l (InfixDataConBuilder lhs' data_con rhs)
    | otherwise =
      do addError $ mkPlainErrorMsgEnvelope (getLoc unpk) PsErrUnpackDataCon
         return constr_stuff

tyToDataConBuilder :: LHsType GhcPs -> PV (LocatedA DataConBuilder)
tyToDataConBuilder (L l (HsTyVar _ NotPromoted v)) = do
  data_con <- eitherToP $ tyConToDataCon v
  return $ L l (PrefixDataConBuilder nilOL data_con)
tyToDataConBuilder (L l (HsTupleTy _ HsBoxedOrConstraintTuple ts)) = do
  let data_con = L (l2l l) (getRdrName (tupleDataCon Boxed (length ts)))
  return $ L l (PrefixDataConBuilder (toOL ts) data_con)
tyToDataConBuilder t =
  addFatalError $ mkPlainErrorMsgEnvelope (getLocA t) $
                    (PsErrInvalidDataCon (unLoc t))

{- Note [Ambiguous syntactic categories]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
There are places in the grammar where we do not know whether we are parsing an
expression or a pattern without unlimited lookahead (which we do not have in
'happy'):

View patterns:

    f (Con a b     ) = ...  -- 'Con a b' is a pattern
    f (Con a b -> x) = ...  -- 'Con a b' is an expression

do-notation:

    do { Con a b <- x } -- 'Con a b' is a pattern
    do { Con a b }      -- 'Con a b' is an expression

Guards:

    x | True <- p && q = ...  -- 'True' is a pattern
    x | True           = ...  -- 'True' is an expression

Top-level value/function declarations (FunBind/PatBind):

    f ! a         -- TH splice
    f ! a = ...   -- function declaration

    Until we encounter the = sign, we don't know if it's a top-level
    TemplateHaskell splice where ! is used, or if it's a function declaration
    where ! is bound.

There are also places in the grammar where we do not know whether we are
parsing an expression or a command:

    proc x -> do { (stuff) -< x }   -- 'stuff' is an expression
    proc x -> do { (stuff) }        -- 'stuff' is a command

    Until we encounter arrow syntax (-<) we don't know whether to parse 'stuff'
    as an expression or a command.

In fact, do-notation is subject to both ambiguities:

    proc x -> do { (stuff) -< x }        -- 'stuff' is an expression
    proc x -> do { (stuff) <- f -< x }   -- 'stuff' is a pattern
    proc x -> do { (stuff) }             -- 'stuff' is a command

There are many possible solutions to this problem. For an overview of the ones
we decided against, see Note [Resolving parsing ambiguities: non-taken alternatives]

The solution that keeps basic definitions (such as HsExpr) clean, keeps the
concerns local to the parser, and does not require duplication of hsSyn types,
or an extra pass over the entire AST, is to parse into an overloaded
parser-validator (a so-called tagless final encoding):

    class DisambECP b where ...
    instance DisambECP (HsCmd GhcPs) where ...
    instance DisambECP (HsExp GhcPs) where ...
    instance DisambECP (PatBuilder GhcPs) where ...

The 'DisambECP' class contains functions to build and validate 'b'. For example,
to add parentheses we have:

  mkHsParPV :: DisambECP b => SrcSpan -> Located b -> PV (Located b)

'mkHsParPV' will wrap the inner value in HsCmdPar for commands, HsPar for
expressions, and 'PatBuilderPar' for patterns (later transformed into ParPat,
see Note [PatBuilder]).

Consider the 'alts' production used to parse case-of alternatives:

  alts :: { Located ([AddEpAnn],[LMatch GhcPs (LHsExpr GhcPs)]) }
    : alts1     { sL1 $1 (fst $ unLoc $1,snd $ unLoc $1) }
    | ';' alts  { sLL $1 $> ((mj AnnSemi $1:(fst $ unLoc $2)),snd $ unLoc $2) }

We abstract over LHsExpr GhcPs, and it becomes:

  alts :: { forall b. DisambECP b => PV (Located ([AddEpAnn],[LMatch GhcPs (Located b)])) }
    : alts1     { $1 >>= \ $1 ->
                  return $ sL1 $1 (fst $ unLoc $1,snd $ unLoc $1) }
    | ';' alts  { $2 >>= \ $2 ->
                  return $ sLL $1 $> ((mj AnnSemi $1:(fst $ unLoc $2)),snd $ unLoc $2) }

Compared to the initial definition, the added bits are:

    forall b. DisambECP b => PV ( ... ) -- in the type signature
    $1 >>= \ $1 -> return $             -- in one reduction rule
    $2 >>= \ $2 -> return $             -- in another reduction rule

The overhead is constant relative to the size of the rest of the reduction
rule, so this approach scales well to large parser productions.

Note that we write ($1 >>= \ $1 -> ...), so the second $1 is in a binding
position and shadows the previous $1. We can do this because internally
'happy' desugars $n to happy_var_n, and the rationale behind this idiom
is to be able to write (sLL $1 $>) later on. The alternative would be to
write this as ($1 >>= \ fresh_name -> ...), but then we couldn't refer
to the last fresh name as $>.

Finally, we instantiate the polymorphic type to a concrete one, and run the
parser-validator, for example:

    stmt   :: { forall b. DisambECP b => PV (LStmt GhcPs (Located b)) }
    e_stmt :: { LStmt GhcPs (LHsExpr GhcPs) }
            : stmt {% runPV $1 }

In e_stmt, three things happen:

  1. we instantiate: b ~ HsExpr GhcPs
  2. we embed the PV computation into P by using runPV
  3. we run validation by using a monadic production, {% ... }

At this point the ambiguity is resolved.
-}


{- Note [Resolving parsing ambiguities: non-taken alternatives]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Alternative I, extra constructors in GHC.Hs.Expr
------------------------------------------------
We could add extra constructors to HsExpr to represent command-specific and
pattern-specific syntactic constructs. Under this scheme, we parse patterns
and commands as expressions and rejig later.  This is what GHC used to do, and
it polluted 'HsExpr' with irrelevant constructors:

  * for commands: 'HsArrForm', 'HsArrApp'
  * for patterns: 'EWildPat', 'EAsPat', 'EViewPat', 'ELazyPat'

(As of now, we still do that for patterns, but we plan to fix it).

There are several issues with this:

  * The implementation details of parsing are leaking into hsSyn definitions.

  * Code that uses HsExpr has to panic on these impossible-after-parsing cases.

  * HsExpr is arbitrarily selected as the extension basis. Why not extend
    HsCmd or HsPat with extra constructors instead?

Alternative II, extra constructors in GHC.Hs.Expr for GhcPs
-----------------------------------------------------------
We could address some of the problems with Alternative I by using Trees That
Grow and extending HsExpr only in the GhcPs pass. However, GhcPs corresponds to
the output of parsing, not to its intermediate results, so we wouldn't want
them there either.

Alternative III, extra constructors in GHC.Hs.Expr for GhcPrePs
---------------------------------------------------------------
We could introduce a new pass, GhcPrePs, to keep GhcPs pristine.
Unfortunately, creating a new pass would significantly bloat conversion code
and slow down the compiler by adding another linear-time pass over the entire
AST. For example, in order to build HsExpr GhcPrePs, we would need to build
HsLocalBinds GhcPrePs (as part of HsLet), and we never want HsLocalBinds
GhcPrePs.


Alternative IV, sum type and bottom-up data flow
------------------------------------------------
Expressions and commands are disjoint. There are no user inputs that could be
interpreted as either an expression or a command depending on outer context:

  5        -- definitely an expression
  x -< y   -- definitely a command

Even though we have both 'HsLam' and 'HsCmdLam', we can look at
the body to disambiguate:

  \p -> 5        -- definitely an expression
  \p -> x -< y   -- definitely a command

This means we could use a bottom-up flow of information to determine
whether we are parsing an expression or a command, using a sum type
for intermediate results:

  Either (LHsExpr GhcPs) (LHsCmd GhcPs)

There are two problems with this:

  * We cannot handle the ambiguity between expressions and
    patterns, which are not disjoint.

  * Bottom-up flow of information leads to poor error messages. Consider

        if ... then 5 else (x -< y)

    Do we report that '5' is not a valid command or that (x -< y) is not a
    valid expression?  It depends on whether we want the entire node to be
    'HsIf' or 'HsCmdIf', and this information flows top-down, from the
    surrounding parsing context (are we in 'proc'?)

Alternative V, backtracking with parser combinators
---------------------------------------------------
One might think we could sidestep the issue entirely by using a backtracking
parser and doing something along the lines of (try pExpr <|> pPat).

Turns out, this wouldn't work very well, as there can be patterns inside
expressions (e.g. via 'case', 'let', 'do') and expressions inside patterns
(e.g. view patterns). To handle this, we would need to backtrack while
backtracking, and unbound levels of backtracking lead to very fragile
performance.

Alternative VI, an intermediate data type
-----------------------------------------
There are common syntactic elements of expressions, commands, and patterns
(e.g. all of them must have balanced parentheses), and we can capture this
common structure in an intermediate data type, Frame:

data Frame
  = FrameVar RdrName
    -- ^ Identifier: Just, map, BS.length
  | FrameTuple [LTupArgFrame] Boxity
    -- ^ Tuple (section): (a,b) (a,b,c) (a,,) (,a,)
  | FrameTySig LFrame (LHsSigWcType GhcPs)
    -- ^ Type signature: x :: ty
  | FramePar (SrcSpan, SrcSpan) LFrame
    -- ^ Parentheses
  | FrameIf LFrame LFrame LFrame
    -- ^ If-expression: if p then x else y
  | FrameCase LFrame [LFrameMatch]
    -- ^ Case-expression: case x of { p1 -> e1; p2 -> e2 }
  | FrameDo (HsStmtContext GhcRn) [LFrameStmt]
    -- ^ Do-expression: do { s1; a <- s2; s3 }
  ...
  | FrameExpr (HsExpr GhcPs)   -- unambiguously an expression
  | FramePat (HsPat GhcPs)     -- unambiguously a pattern
  | FrameCommand (HsCmd GhcPs) -- unambiguously a command

To determine which constructors 'Frame' needs to have, we take the union of
intersections between HsExpr, HsCmd, and HsPat.

The intersection between HsPat and HsExpr:

  HsPat  =  VarPat   | TuplePat      | SigPat        | ParPat   | ...
  HsExpr =  HsVar    | ExplicitTuple | ExprWithTySig | HsPar    | ...
  -------------------------------------------------------------------
  Frame  =  FrameVar | FrameTuple    | FrameTySig    | FramePar | ...

The intersection between HsCmd and HsExpr:

  HsCmd  = HsCmdIf | HsCmdCase | HsCmdDo | HsCmdPar
  HsExpr = HsIf    | HsCase    | HsDo    | HsPar
  ------------------------------------------------
  Frame = FrameIf  | FrameCase | FrameDo | FramePar

The intersection between HsCmd and HsPat:

  HsPat  = ParPat   | ...
  HsCmd  = HsCmdPar | ...
  -----------------------
  Frame  = FramePar | ...

Take the union of each intersection and this yields the final 'Frame' data
type. The problem with this approach is that we end up duplicating a good
portion of hsSyn:

    Frame         for  HsExpr, HsPat, HsCmd
    TupArgFrame   for  HsTupArg
    FrameMatch    for  Match
    FrameStmt     for  StmtLR
    FrameGRHS     for  GRHS
    FrameGRHSs    for  GRHSs
    ...

Alternative VII, a product type
-------------------------------
We could avoid the intermediate representation of Alternative VI by parsing
into a product of interpretations directly:

    type ExpCmdPat = ( PV (LHsExpr GhcPs)
                     , PV (LHsCmd GhcPs)
                     , PV (LHsPat GhcPs) )

This means that in positions where we do not know whether to produce
expression, a pattern, or a command, we instead produce a parser-validator for
each possible option.

Then, as soon as we have parsed far enough to resolve the ambiguity, we pick
the appropriate component of the product, discarding the rest:

    checkExpOf3 (e, _, _) = e  -- interpret as an expression
    checkCmdOf3 (_, c, _) = c  -- interpret as a command
    checkPatOf3 (_, _, p) = p  -- interpret as a pattern

We can easily define ambiguities between arbitrary subsets of interpretations.
For example, when we know ahead of type that only an expression or a command is
possible, but not a pattern, we can use a smaller type:

    type ExpCmd = (PV (LHsExpr GhcPs), PV (LHsCmd GhcPs))

    checkExpOf2 (e, _) = e  -- interpret as an expression
    checkCmdOf2 (_, c) = c  -- interpret as a command

However, there is a slight problem with this approach, namely code duplication
in parser productions. Consider the 'alts' production used to parse case-of
alternatives:

  alts :: { Located ([AddEpAnn],[LMatch GhcPs (LHsExpr GhcPs)]) }
    : alts1     { sL1 $1 (fst $ unLoc $1,snd $ unLoc $1) }
    | ';' alts  { sLL $1 $> ((mj AnnSemi $1:(fst $ unLoc $2)),snd $ unLoc $2) }

Under the new scheme, we have to completely duplicate its type signature and
each reduction rule:

  alts :: { ( PV (Located ([AddEpAnn],[LMatch GhcPs (LHsExpr GhcPs)])) -- as an expression
            , PV (Located ([AddEpAnn],[LMatch GhcPs (LHsCmd GhcPs)]))  -- as a command
            ) }
    : alts1
        { ( checkExpOf2 $1 >>= \ $1 ->
            return $ sL1 $1 (fst $ unLoc $1,snd $ unLoc $1)
          , checkCmdOf2 $1 >>= \ $1 ->
            return $ sL1 $1 (fst $ unLoc $1,snd $ unLoc $1)
          ) }
    | ';' alts
        { ( checkExpOf2 $2 >>= \ $2 ->
            return $ sLL $1 $> ((mj AnnSemi $1:(fst $ unLoc $2)),snd $ unLoc $2)
          , checkCmdOf2 $2 >>= \ $2 ->
            return $ sLL $1 $> ((mj AnnSemi $1:(fst $ unLoc $2)),snd $ unLoc $2)
          ) }

And the same goes for other productions: 'altslist', 'alts1', 'alt', 'alt_rhs',
'ralt', 'gdpats', 'gdpat', 'exp', ... and so on. That is a lot of code!

Alternative VIII, a function from a GADT
----------------------------------------
We could avoid code duplication of the Alternative VII by representing the product
as a function from a GADT:

    data ExpCmdG b where
      ExpG :: ExpCmdG HsExpr
      CmdG :: ExpCmdG HsCmd

    type ExpCmd = forall b. ExpCmdG b -> PV (Located (b GhcPs))

    checkExp :: ExpCmd -> PV (LHsExpr GhcPs)
    checkCmd :: ExpCmd -> PV (LHsCmd GhcPs)
    checkExp f = f ExpG  -- interpret as an expression
    checkCmd f = f CmdG  -- interpret as a command

Consider the 'alts' production used to parse case-of alternatives:

  alts :: { Located ([AddEpAnn],[LMatch GhcPs (LHsExpr GhcPs)]) }
    : alts1     { sL1 $1 (fst $ unLoc $1,snd $ unLoc $1) }
    | ';' alts  { sLL $1 $> ((mj AnnSemi $1:(fst $ unLoc $2)),snd $ unLoc $2) }

We abstract over LHsExpr, and it becomes:

  alts :: { forall b. ExpCmdG b -> PV (Located ([AddEpAnn],[LMatch GhcPs (Located (b GhcPs))])) }
    : alts1
        { \tag -> $1 tag >>= \ $1 ->
                  return $ sL1 $1 (fst $ unLoc $1,snd $ unLoc $1) }
    | ';' alts
        { \tag -> $2 tag >>= \ $2 ->
                  return $ sLL $1 $> ((mj AnnSemi $1:(fst $ unLoc $2)),snd $ unLoc $2) }

Note that 'ExpCmdG' is a singleton type, the value is completely
determined by the type:

  when (b~HsExpr),  tag = ExpG
  when (b~HsCmd),   tag = CmdG

This is a clear indication that we can use a class to pass this value behind
the scenes:

  class    ExpCmdI b      where expCmdG :: ExpCmdG b
  instance ExpCmdI HsExpr where expCmdG = ExpG
  instance ExpCmdI HsCmd  where expCmdG = CmdG

And now the 'alts' production is simplified, as we no longer need to
thread 'tag' explicitly:

  alts :: { forall b. ExpCmdI b => PV (Located ([AddEpAnn],[LMatch GhcPs (Located (b GhcPs))])) }
    : alts1     { $1 >>= \ $1 ->
                  return $ sL1 $1 (fst $ unLoc $1,snd $ unLoc $1) }
    | ';' alts  { $2 >>= \ $2 ->
                  return $ sLL $1 $> ((mj AnnSemi $1:(fst $ unLoc $2)),snd $ unLoc $2) }

This encoding works well enough, but introduces an extra GADT unlike the
tagless final encoding, and there's no need for this complexity.

-}

{- Note [PatBuilder]
~~~~~~~~~~~~~~~~~~~~
Unlike HsExpr or HsCmd, the Pat type cannot accommodate all intermediate forms,
so we introduce the notion of a PatBuilder.

Consider a pattern like this:

  Con a b c

We parse arguments to "Con" one at a time in the  fexp aexp  parser production,
building the result with mkHsAppPV, so the intermediate forms are:

  1. Con
  2. Con a
  3. Con a b
  4. Con a b c

In 'HsExpr', we have 'HsApp', so the intermediate forms are represented like
this (pseudocode):

  1. "Con"
  2. HsApp "Con" "a"
  3. HsApp (HsApp "Con" "a") "b"
  3. HsApp (HsApp (HsApp "Con" "a") "b") "c"

Similarly, in 'HsCmd' we have 'HsCmdApp'. In 'Pat', however, what we have
instead is 'ConPatIn', which is very awkward to modify and thus unsuitable for
the intermediate forms.

We also need an intermediate representation to postpone disambiguation between
FunBind and PatBind. Consider:

  a `Con` b = ...
  a `fun` b = ...

How do we know that (a `Con` b) is a PatBind but (a `fun` b) is a FunBind? We
learn this by inspecting an intermediate representation in 'isFunLhs' and
seeing that 'Con' is a data constructor but 'f' is not. We need an intermediate
representation capable of representing both a FunBind and a PatBind, so Pat is
insufficient.

PatBuilder is an extension of Pat that is capable of representing intermediate
parsing results for patterns and function bindings:

  data PatBuilder p
    = PatBuilderPat (Pat p)
    | PatBuilderApp (LocatedA (PatBuilder p)) (LocatedA (PatBuilder p))
    | PatBuilderOpApp (LocatedA (PatBuilder p)) (LocatedA RdrName) (LocatedA (PatBuilder p))
    ...

It can represent any pattern via 'PatBuilderPat', but it also has a variety of
other constructors which were added by following a simple principle: we never
pattern match on the pattern stored inside 'PatBuilderPat'.
-}

---------------------------------------------------------------------------
-- Miscellaneous utilities

-- | Check if a fixity is valid. We support bypassing the usual bound checks
-- for some special operators.
checkPrecP
        :: Located (SourceText,Int)              -- ^ precedence
        -> Located (OrdList (LocatedN RdrName))  -- ^ operators
        -> P ()
checkPrecP (L l (_,i)) (L _ ol)
 | 0 <= i, i <= maxPrecedence = pure ()
 | all specialOp ol = pure ()
 | otherwise = addFatalError $ mkPlainErrorMsgEnvelope l (PsErrPrecedenceOutOfRange i)
  where
    -- If you change this, consider updating Note [Fixity of (->)] in GHC/Types.hs
    specialOp op = unLoc op == getRdrName unrestrictedFunTyCon

mkRecConstrOrUpdate
        :: Bool
        -> LHsExpr GhcPs
        -> SrcSpan
        -> ([Fbind (HsExpr GhcPs)], Maybe SrcSpan)
        -> EpAnn [AddEpAnn]
        -> PV (HsExpr GhcPs)
mkRecConstrOrUpdate _ (L _ (HsVar _ (L l c))) _lrec (fbinds,dd) anns
  | isRdrDataCon c
  = do
      let (fs, ps) = partitionEithers fbinds
      if not (null ps)
        then addFatalError $ mkPlainErrorMsgEnvelope (getLocA (head ps)) $
                               PsErrOverloadedRecordDotInvalid
        else return (mkRdrRecordCon (L l c) (mk_rec_fields fs dd) anns)
mkRecConstrOrUpdate overloaded_update exp _ (fs,dd) anns
  | Just dd_loc <- dd = addFatalError $ mkPlainErrorMsgEnvelope dd_loc $
                                          PsErrDotsInRecordUpdate
  | otherwise = mkRdrRecordUpd overloaded_update exp fs anns

mkRdrRecordUpd :: Bool -> LHsExpr GhcPs -> [Fbind (HsExpr GhcPs)] -> EpAnn [AddEpAnn] -> PV (HsExpr GhcPs)
mkRdrRecordUpd overloaded_on exp@(L loc _) fbinds anns = do
  -- We do not need to know if OverloadedRecordDot is in effect. We do
  -- however need to know if OverloadedRecordUpdate (passed in
  -- overloaded_on) is in effect because it affects the Left/Right nature
  -- of the RecordUpd value we calculate.
  let (fs, ps) = partitionEithers fbinds
      fs' :: [LHsRecUpdField GhcPs]
      fs' = map (fmap mk_rec_upd_field) fs
  case overloaded_on of
    False | not $ null ps ->
      -- A '.' was found in an update and OverloadedRecordUpdate isn't on.
      addFatalError $ mkPlainErrorMsgEnvelope (locA loc) PsErrOverloadedRecordUpdateNotEnabled
    False ->
      -- This is just a regular record update.
      return RecordUpd {
        rupd_ext = anns
      , rupd_expr = exp
      , rupd_flds = Left fs' }
    True -> do
      let qualifiedFields =
            [ L l lbl | L _ (HsFieldBind _ (L l lbl) _ _) <- fs'
                      , isQual . rdrNameAmbiguousFieldOcc $ lbl
            ]
      if not $ null qualifiedFields
        then
          addFatalError $ mkPlainErrorMsgEnvelope (getLocA (head qualifiedFields)) $
            PsErrOverloadedRecordUpdateNoQualifiedFields
        else -- This is a RecordDotSyntax update.
          return RecordUpd {
            rupd_ext = anns
           , rupd_expr = exp
           , rupd_flds = Right (toProjUpdates fbinds) }
  where
    toProjUpdates :: [Fbind (HsExpr GhcPs)] -> [LHsRecUpdProj GhcPs]
    toProjUpdates = map (\case { Right p -> p; Left f -> recFieldToProjUpdate f })

    -- Convert a top-level field update like {foo=2} or {bar} (punned)
    -- to a projection update.
    recFieldToProjUpdate :: LHsRecField GhcPs  (LHsExpr GhcPs) -> LHsRecUpdProj GhcPs
    recFieldToProjUpdate (L l (HsFieldBind anns (L _ (FieldOcc _ (L loc rdr))) arg pun)) =
        -- The idea here is to convert the label to a singleton [FastString].
        let f = occNameFS . rdrNameOcc $ rdr
            fl = DotFieldOcc noAnn (L (l2l loc) f) -- AZ: what about the ann?
            lf = locA loc
        in mkRdrProjUpdate l (L lf [L (l2l loc) fl]) (punnedVar f) pun anns
        where
          -- If punning, compute HsVar "f" otherwise just arg. This
          -- has the effect that sentinel HsVar "pun-rhs" is replaced
          -- by HsVar "f" here, before the update is written to a
          -- setField expressions.
          punnedVar :: FastString -> LHsExpr GhcPs
          punnedVar f  = if not pun then arg else noLocA . HsVar noExtField . noLocA . mkRdrUnqual . mkVarOccFS $ f

mkRdrRecordCon
  :: LocatedN RdrName -> HsRecordBinds GhcPs -> EpAnn [AddEpAnn] -> HsExpr GhcPs
mkRdrRecordCon con flds anns
  = RecordCon { rcon_ext = anns, rcon_con = con, rcon_flds = flds }

mk_rec_fields :: [LocatedA (HsRecField (GhcPass p) arg)] -> Maybe SrcSpan -> HsRecFields (GhcPass p) arg
mk_rec_fields fs Nothing = HsRecFields { rec_flds = fs, rec_dotdot = Nothing }
mk_rec_fields fs (Just s)  = HsRecFields { rec_flds = fs
                                     , rec_dotdot = Just (L s (length fs)) }

mk_rec_upd_field :: HsRecField GhcPs (LHsExpr GhcPs) -> HsRecUpdField GhcPs
mk_rec_upd_field (HsFieldBind noAnn (L loc (FieldOcc _ rdr)) arg pun)
  = HsFieldBind noAnn (L loc (Unambiguous noExtField rdr)) arg pun

mkInlinePragma :: SourceText -> (InlineSpec, RuleMatchInfo) -> Maybe Activation
               -> InlinePragma
-- The (Maybe Activation) is because the user can omit
-- the activation spec (and usually does)
mkInlinePragma src (inl, match_info) mb_act
  = InlinePragma { inl_src = src -- Note [Pragma source text] in GHC.Types.SourceText
                 , inl_inline = inl
                 , inl_sat    = Nothing
                 , inl_act    = act
                 , inl_rule   = match_info }
  where
    act = case mb_act of
            Just act -> act
            Nothing  -> -- No phase specified
                        case inl of
                          NoInline _  -> NeverActive
                          Opaque _    -> NeverActive
                          _other      -> AlwaysActive

mkOpaquePragma :: SourceText -> InlinePragma
mkOpaquePragma src
  = InlinePragma { inl_src    = src
                 , inl_inline = Opaque src
                 , inl_sat    = Nothing
                 -- By marking the OPAQUE pragma NeverActive we stop
                 -- (constructor) specialisation on OPAQUE things.
                 --
                 -- See Note [OPAQUE pragma]
                 , inl_act    = NeverActive
                 , inl_rule   = FunLike
                 }

-----------------------------------------------------------------------------
-- utilities for foreign declarations

-- construct a foreign import declaration
--
mkImport :: Located CCallConv
         -> Located Safety
         -> (Located StringLiteral, LocatedN RdrName, LHsSigType GhcPs)
         -> P (EpAnn [AddEpAnn] -> HsDecl GhcPs)
mkImport cconv safety (L loc (StringLiteral esrc entity _), v, ty) =
    case unLoc cconv of
      CCallConv          -> returnSpec =<< mkCImport
      CApiConv           -> do
        imp <- mkCImport
        if isCWrapperImport imp
          then addFatalError $ mkPlainErrorMsgEnvelope loc PsErrInvalidCApiImport
          else returnSpec imp
      StdCallConv        -> returnSpec =<< mkCImport
      PrimCallConv       -> mkOtherImport
      JavaScriptCallConv -> mkOtherImport
  where
    -- Parse a C-like entity string of the following form:
    --   "[static] [chname] [&] [cid]" | "dynamic" | "wrapper"
    -- If 'cid' is missing, the function name 'v' is used instead as symbol
    -- name (cf section 8.5.1 in Haskell 2010 report).
    mkCImport = do
      let e = unpackFS entity
      case parseCImport cconv safety (mkExtName (unLoc v)) e (L loc esrc) of
        Nothing         -> addFatalError $ mkPlainErrorMsgEnvelope loc $
                             PsErrMalformedEntityString
        Just importSpec -> return importSpec

    isCWrapperImport (CImport _ _ _ CWrapper _) = True
    isCWrapperImport _ = False

    -- currently, all the other import conventions only support a symbol name in
    -- the entity string. If it is missing, we use the function name instead.
    mkOtherImport = returnSpec importSpec
      where
        entity'    = if nullFS entity
                        then mkExtName (unLoc v)
                        else entity
        funcTarget = CFunction (StaticTarget esrc entity' Nothing True)
        importSpec = CImport cconv safety Nothing funcTarget (L loc esrc)

    returnSpec spec = return $ \ann -> ForD noExtField $ ForeignImport
          { fd_i_ext  = ann
          , fd_name   = v
          , fd_sig_ty = ty
          , fd_fi     = spec
          }



-- the string "foo" is ambiguous: either a header or a C identifier.  The
-- C identifier case comes first in the alternatives below, so we pick
-- that one.
parseCImport :: Located CCallConv -> Located Safety -> FastString -> String
             -> Located SourceText
             -> Maybe ForeignImport
parseCImport cconv safety nm str sourceText =
 listToMaybe $ map fst $ filter (null.snd) $
     readP_to_S parse str
 where
   parse = do
       skipSpaces
       r <- choice [
          string "dynamic" >> return (mk Nothing (CFunction DynamicTarget)),
          string "wrapper" >> return (mk Nothing CWrapper),
          do optional (token "static" >> skipSpaces)
             ((mk Nothing <$> cimp nm) +++
              (do h <- munch1 hdr_char
                  skipSpaces
                  mk (Just (Header (SourceText h) (mkFastString h)))
                      <$> cimp nm))
         ]
       skipSpaces
       return r

   token str = do _ <- string str
                  toks <- look
                  case toks of
                      c : _
                       | id_char c -> pfail
                      _            -> return ()

   mk h n = CImport cconv safety h n sourceText

   hdr_char c = not (isSpace c)
   -- header files are filenames, which can contain
   -- pretty much any char (depending on the platform),
   -- so just accept any non-space character
   id_first_char c = isAlpha    c || c == '_'
   id_char       c = isAlphaNum c || c == '_'

   cimp nm = (ReadP.char '&' >> skipSpaces >> CLabel <$> cid)
             +++ (do isFun <- case unLoc cconv of
                               CApiConv ->
                                  option True
                                         (do token "value"
                                             skipSpaces
                                             return False)
                               _ -> return True
                     cid' <- cid
                     return (CFunction (StaticTarget NoSourceText cid'
                                        Nothing isFun)))
          where
            cid = return nm +++
                  (do c  <- satisfy id_first_char
                      cs <-  many (satisfy id_char)
                      return (mkFastString (c:cs)))


-- construct a foreign export declaration
--
mkExport :: Located CCallConv
         -> (Located StringLiteral, LocatedN RdrName, LHsSigType GhcPs)
         -> P (EpAnn [AddEpAnn] -> HsDecl GhcPs)
mkExport (L lc cconv) (L le (StringLiteral esrc entity _), v, ty)
 = return $ \ann -> ForD noExtField $
   ForeignExport { fd_e_ext = ann, fd_name = v, fd_sig_ty = ty
                 , fd_fe = CExport (L lc (CExportStatic esrc entity' cconv))
                                   (L le esrc) }
  where
    entity' | nullFS entity = mkExtName (unLoc v)
            | otherwise     = entity

-- Supplying the ext_name in a foreign decl is optional; if it
-- isn't there, the Haskell name is assumed. Note that no transformation
-- of the Haskell name is then performed, so if you foreign export (++),
-- it's external name will be "++". Too bad; it's important because we don't
-- want z-encoding (e.g. names with z's in them shouldn't be doubled)
--
mkExtName :: RdrName -> CLabelString
mkExtName rdrNm = mkFastString (occNameString (rdrNameOcc rdrNm))

--------------------------------------------------------------------------------
-- Help with module system imports/exports

data ImpExpSubSpec = ImpExpAbs
                   | ImpExpAll
                   | ImpExpList [LocatedA ImpExpQcSpec]
                   | ImpExpAllWith [LocatedA ImpExpQcSpec]

data ImpExpQcSpec = ImpExpQcName (LocatedN RdrName)
                  | ImpExpQcType EpaLocation (LocatedN RdrName)
                  | ImpExpQcWildcard

mkModuleImpExp :: [AddEpAnn] -> LocatedA ImpExpQcSpec -> ImpExpSubSpec -> P (IE GhcPs)
mkModuleImpExp anns (L l specname) subs = do
  cs <- getCommentsFor (locA l) -- AZ: IEVar can discard comments
  let ann = EpAnn (spanAsAnchor $ locA l) anns cs
  case subs of
    ImpExpAbs
      | isVarNameSpace (rdrNameSpace name)
                       -> return $ IEVar noExtField (L l (ieNameFromSpec specname))
      | otherwise      -> IEThingAbs ann . L l <$> nameT
    ImpExpAll          -> IEThingAll ann . L l <$> nameT
    ImpExpList xs      ->
      (\newName -> IEThingWith ann (L l newName)
        NoIEWildcard (wrapped xs)) <$> nameT
    ImpExpAllWith xs                       ->
      do allowed <- getBit PatternSynonymsBit
         if allowed
          then
            let withs = map unLoc xs
                pos   = maybe NoIEWildcard IEWildcard
                          (findIndex isImpExpQcWildcard withs)
                ies :: [LocatedA (IEWrappedName RdrName)]
                ies   = wrapped $ filter (not . isImpExpQcWildcard . unLoc) xs
            in (\newName
                        -> IEThingWith ann (L l newName) pos ies)
               <$> nameT
          else addFatalError $ mkPlainErrorMsgEnvelope (locA l) $
                 PsErrIllegalPatSynExport
  where
    name = ieNameVal specname
    nameT =
      if isVarNameSpace (rdrNameSpace name)
        then addFatalError $ mkPlainErrorMsgEnvelope (locA l) $
               (PsErrVarForTyCon name)
        else return $ ieNameFromSpec specname

    ieNameVal (ImpExpQcName ln)   = unLoc ln
    ieNameVal (ImpExpQcType _ ln) = unLoc ln
    ieNameVal (ImpExpQcWildcard)  = panic "ieNameVal got wildcard"

    ieNameFromSpec (ImpExpQcName   ln) = IEName   ln
    ieNameFromSpec (ImpExpQcType r ln) = IEType r ln
    ieNameFromSpec (ImpExpQcWildcard)  = panic "ieName got wildcard"

    wrapped = map (mapLoc ieNameFromSpec)

mkTypeImpExp :: LocatedN RdrName   -- TcCls or Var name space
             -> P (LocatedN RdrName)
mkTypeImpExp name =
  do allowed <- getBit ExplicitNamespacesBit
     unless allowed $ addError $ mkPlainErrorMsgEnvelope (getLocA name) $
                                   PsErrIllegalExplicitNamespace
     return (fmap (`setRdrNameSpace` tcClsName) name)

checkImportSpec :: LocatedL [LIE GhcPs] -> P (LocatedL [LIE GhcPs])
checkImportSpec ie@(L _ specs) =
    case [l | (L l (IEThingWith _ _ (IEWildcard _) _)) <- specs] of
      [] -> return ie
      (l:_) -> importSpecError (locA l)
  where
    importSpecError l =
      addFatalError $ mkPlainErrorMsgEnvelope l PsErrIllegalImportBundleForm

-- In the correct order
mkImpExpSubSpec :: [LocatedA ImpExpQcSpec] -> P ([AddEpAnn], ImpExpSubSpec)
mkImpExpSubSpec [] = return ([], ImpExpList [])
mkImpExpSubSpec [L la ImpExpQcWildcard] =
  return ([AddEpAnn AnnDotdot (EpaSpan $ la2r la)], ImpExpAll)
mkImpExpSubSpec xs =
  if (any (isImpExpQcWildcard . unLoc) xs)
    then return $ ([], ImpExpAllWith xs)
    else return $ ([], ImpExpList xs)

isImpExpQcWildcard :: ImpExpQcSpec -> Bool
isImpExpQcWildcard ImpExpQcWildcard = True
isImpExpQcWildcard _                = False

-----------------------------------------------------------------------------
-- Warnings and failures

warnPrepositiveQualifiedModule :: SrcSpan -> P ()
warnPrepositiveQualifiedModule span =
  addPsMessage span PsWarnImportPreQualified

failOpNotEnabledImportQualifiedPost :: SrcSpan -> P ()
failOpNotEnabledImportQualifiedPost loc =
  addError $ mkPlainErrorMsgEnvelope loc $ PsErrImportPostQualified

failOpImportQualifiedTwice :: SrcSpan -> P ()
failOpImportQualifiedTwice loc =
  addError $ mkPlainErrorMsgEnvelope loc $ PsErrImportQualifiedTwice

warnStarIsType :: SrcSpan -> P ()
warnStarIsType span = addPsMessage span PsWarnStarIsType

failOpFewArgs :: MonadP m => LocatedN RdrName -> m a
failOpFewArgs (L loc op) =
  do { star_is_type <- getBit StarIsTypeBit
     ; let is_star_type = if star_is_type then StarIsType else StarIsNotType
     ; addFatalError $ mkPlainErrorMsgEnvelope (locA loc) $
         (PsErrOpFewArgs is_star_type op) }

-----------------------------------------------------------------------------
-- Misc utils

data PV_Context =
  PV_Context
    { pv_options :: ParserOpts
    , pv_details :: ParseContext -- See Note [Parser-Validator Details]
    }

data PV_Accum =
  PV_Accum
    { pv_warnings        :: Messages PsMessage
    , pv_errors          :: Messages PsMessage
    , pv_header_comments :: Strict.Maybe [LEpaComment]
    , pv_comment_q       :: [LEpaComment]
    }

data PV_Result a = PV_Ok PV_Accum a | PV_Failed PV_Accum

-- During parsing, we make use of several monadic effects: reporting parse errors,
-- accumulating warnings, adding API annotations, and checking for extensions. These
-- effects are captured by the 'MonadP' type class.
--
-- Sometimes we need to postpone some of these effects to a later stage due to
-- ambiguities described in Note [Ambiguous syntactic categories].
-- We could use two layers of the P monad, one for each stage:
--
--   abParser :: forall x. DisambAB x => P (P x)
--
-- The outer layer of P consumes the input and builds the inner layer, which
-- validates the input. But this type is not particularly helpful, as it obscures
-- the fact that the inner layer of P never consumes any input.
--
-- For clarity, we introduce the notion of a parser-validator: a parser that does
-- not consume any input, but may fail or use other effects. Thus we have:
--
--   abParser :: forall x. DisambAB x => P (PV x)
--
newtype PV a = PV { unPV :: PV_Context -> PV_Accum -> PV_Result a }

instance Functor PV where
  fmap = liftM

instance Applicative PV where
  pure a = a `seq` PV (\_ acc -> PV_Ok acc a)
  (<*>) = ap

instance Monad PV where
  m >>= f = PV $ \ctx acc ->
    case unPV m ctx acc of
      PV_Ok acc' a -> unPV (f a) ctx acc'
      PV_Failed acc' -> PV_Failed acc'

runPV :: PV a -> P a
runPV = runPV_details noParseContext

askParseContext :: PV ParseContext
askParseContext = PV $ \(PV_Context _ details) acc -> PV_Ok acc details

runPV_details :: ParseContext -> PV a -> P a
runPV_details details m =
  P $ \s ->
    let
      pv_ctx = PV_Context
        { pv_options = options s
        , pv_details = details }
      pv_acc = PV_Accum
        { pv_warnings = warnings s
        , pv_errors   = errors s
        , pv_header_comments = header_comments s
        , pv_comment_q = comment_q s }
      mkPState acc' =
        s { warnings = pv_warnings acc'
          , errors   = pv_errors acc'
          , comment_q = pv_comment_q acc' }
    in
      case unPV m pv_ctx pv_acc of
        PV_Ok acc' a -> POk (mkPState acc') a
        PV_Failed acc' -> PFailed (mkPState acc')

instance MonadP PV where
  addError err =
    PV $ \_ctx acc -> PV_Ok acc{pv_errors = err `addMessage` pv_errors acc} ()
  addWarning w =
    PV $ \_ctx acc ->
      -- No need to check for the warning flag to be set, GHC will correctly discard suppressed
      -- diagnostics.
      PV_Ok acc{pv_warnings= w `addMessage` pv_warnings acc} ()
  addFatalError err =
    addError err >> PV (const PV_Failed)
  getBit ext =
    PV $ \ctx acc ->
      let b = ext `xtest` pExtsBitmap (pv_options ctx) in
      PV_Ok acc $! b
  allocateCommentsP ss = PV $ \_ s ->
    let (comment_q', newAnns) = allocateComments ss (pv_comment_q s) in
      PV_Ok s {
         pv_comment_q = comment_q'
       } (EpaComments newAnns)
  allocatePriorCommentsP ss = PV $ \_ s ->
    let (header_comments', comment_q', newAnns)
          = allocatePriorComments ss (pv_comment_q s) (pv_header_comments s) in
      PV_Ok s {
         pv_header_comments = header_comments',
         pv_comment_q = comment_q'
       } (EpaComments newAnns)
  allocateFinalCommentsP ss = PV $ \_ s ->
    let (header_comments', comment_q', newAnns)
          = allocateFinalComments ss (pv_comment_q s) (pv_header_comments s) in
      PV_Ok s {
         pv_header_comments = header_comments',
         pv_comment_q = comment_q'
       } (EpaCommentsBalanced (Strict.fromMaybe [] header_comments') (reverse newAnns))

{- Note [Parser-Validator Details]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A PV computation is parametrized by some 'ParseContext' for diagnostic messages, which can be set
depending on validation context. We use this in checkPattern to fix #984.

Consider this example, where the user has forgotten a 'do':

  f _ = do
    x <- computation
    case () of
      _ ->
        result <- computation
        case () of () -> undefined

GHC parses it as follows:

  f _ = do
    x <- computation
    (case () of
      _ ->
        result) <- computation
        case () of () -> undefined

Note that this fragment is parsed as a pattern:

  case () of
    _ ->
      result

We attempt to detect such cases and add a hint to the diagnostic messages:

  T984.hs:6:9:
    Parse error in pattern: case () of { _ -> result }
    Possibly caused by a missing 'do'?

The "Possibly caused by a missing 'do'?" suggestion is the hint that is computed
out of the 'ParseContext', which are read by functions like 'patFail' when
constructing the 'PsParseErrorInPatDetails' data structure. When validating in a
context other than 'bindpat' (a pattern to the left of <-), we set the
details to 'noParseContext' and it has no effect on the diagnostic messages.

-}

-- | Hint about bang patterns, assuming @BangPatterns@ is off.
hintBangPat :: SrcSpan -> Pat GhcPs -> PV ()
hintBangPat span e = do
    bang_on <- getBit BangPatBit
    unless bang_on $
      addError $ mkPlainErrorMsgEnvelope span $ PsErrIllegalBangPattern e

mkSumOrTupleExpr :: SrcSpanAnnA -> Boxity -> SumOrTuple (HsExpr GhcPs)
                 -> [AddEpAnn]
                 -> PV (LHsExpr GhcPs)

-- Tuple
mkSumOrTupleExpr l boxity (Tuple es) anns = do
    cs <- getCommentsFor (locA l)
    return $ L l (ExplicitTuple (EpAnn (spanAsAnchor $ locA l) anns cs) (map toTupArg es) boxity)
  where
    toTupArg :: Either (EpAnn EpaLocation) (LHsExpr GhcPs) -> HsTupArg GhcPs
    toTupArg (Left ann) = missingTupArg ann
    toTupArg (Right a)  = Present noAnn a

-- Sum
-- mkSumOrTupleExpr l Unboxed (Sum alt arity e) =
--     return $ L l (ExplicitSum noExtField alt arity e)
mkSumOrTupleExpr l Unboxed (Sum alt arity e barsp barsa) anns = do
    let an = case anns of
               [AddEpAnn AnnOpenPH o, AddEpAnn AnnClosePH c] ->
                 AnnExplicitSum o barsp barsa c
               _ -> panic "mkSumOrTupleExpr"
    cs <- getCommentsFor (locA l)
    return $ L l (ExplicitSum (EpAnn (spanAsAnchor $ locA l) an cs) alt arity e)
mkSumOrTupleExpr l Boxed a@Sum{} _ =
    addFatalError $ mkPlainErrorMsgEnvelope (locA l) $ PsErrUnsupportedBoxedSumExpr a

mkSumOrTuplePat
  :: SrcSpanAnnA -> Boxity -> SumOrTuple (PatBuilder GhcPs) -> [AddEpAnn]
  -> PV (LocatedA (PatBuilder GhcPs))

-- Tuple
mkSumOrTuplePat l boxity (Tuple ps) anns = do
  ps' <- traverse toTupPat ps
  cs <- getCommentsFor (locA l)
  return $ L l (PatBuilderPat (TuplePat (EpAnn (spanAsAnchor $ locA l) anns cs) ps' boxity))
  where
    toTupPat :: Either (EpAnn EpaLocation) (LocatedA (PatBuilder GhcPs)) -> PV (LPat GhcPs)
    -- Ignore the element location so that the error message refers to the
    -- entire tuple. See #19504 (and the discussion) for details.
    toTupPat p = case p of
      Left _ -> addFatalError $
                  mkPlainErrorMsgEnvelope (locA l) PsErrTupleSectionInPat
      Right p' -> checkLPat p'

-- Sum
mkSumOrTuplePat l Unboxed (Sum alt arity p barsb barsa) anns = do
   p' <- checkLPat p
   cs <- getCommentsFor (locA l)
   let an = EpAnn (spanAsAnchor $ locA l) (EpAnnSumPat anns barsb barsa) cs
   return $ L l (PatBuilderPat (SumPat an p' alt arity))
mkSumOrTuplePat l Boxed a@Sum{} _ =
    addFatalError $
      mkPlainErrorMsgEnvelope (locA l) $ PsErrUnsupportedBoxedSumPat a

mkLHsOpTy :: PromotionFlag -> LHsType GhcPs -> LocatedN RdrName -> LHsType GhcPs -> LHsType GhcPs
mkLHsOpTy prom x op y =
  let loc = getLoc x `combineSrcSpansA` (noAnnSrcSpan $ getLocA op) `combineSrcSpansA` getLoc y
  in L loc (mkHsOpTy prom x op y)

mkMultTy :: LHsToken "%" GhcPs -> LHsType GhcPs -> LHsUniToken "->" "→" GhcPs -> HsArrow GhcPs
mkMultTy pct t@(L _ (HsTyLit _ (HsNumTy (SourceText "1") 1))) arr
  -- See #18888 for the use of (SourceText "1") above
  = HsLinearArrow (HsPct1 (L locOfPct1 HsTok) arr)
  where
    -- The location of "%" combined with the location of "1".
    locOfPct1 :: TokenLocation
    locOfPct1 = token_location_widenR (getLoc pct) (locA (getLoc t))
mkMultTy pct t arr = HsExplicitMult pct t arr

mkTokenLocation :: SrcSpan -> TokenLocation
mkTokenLocation (UnhelpfulSpan _) = NoTokenLoc
mkTokenLocation (RealSrcSpan r _)  = TokenLoc (EpaSpan r)

-- Precondition: the TokenLocation has EpaSpan, never EpaDelta.
token_location_widenR :: TokenLocation -> SrcSpan -> TokenLocation
token_location_widenR NoTokenLoc _ = NoTokenLoc
token_location_widenR tl (UnhelpfulSpan _) = tl
token_location_widenR (TokenLoc (EpaSpan r1)) (RealSrcSpan r2 _) =
                      (TokenLoc (EpaSpan (combineRealSrcSpans r1 r2)))
token_location_widenR (TokenLoc (EpaDelta _ _)) _ =
  -- Never happens because the parser does not produce EpaDelta.
  panic "token_location_widenR: EpaDelta"


-----------------------------------------------------------------------------
-- Token symbols

starSym :: Bool -> String
starSym True = "★"
starSym False = "*"

-----------------------------------------
-- Bits and pieces for RecordDotSyntax.

mkRdrGetField :: SrcSpanAnnA -> LHsExpr GhcPs -> LocatedAn NoEpAnns (DotFieldOcc GhcPs)
  -> EpAnnCO -> LHsExpr GhcPs
mkRdrGetField loc arg field anns =
  L loc HsGetField {
      gf_ext = anns
    , gf_expr = arg
    , gf_field = field
    }

mkRdrProjection :: NonEmpty (LocatedAn NoEpAnns (DotFieldOcc GhcPs)) -> EpAnn AnnProjection -> HsExpr GhcPs
mkRdrProjection flds anns =
  HsProjection {
      proj_ext = anns
    , proj_flds = flds
    }

mkRdrProjUpdate :: SrcSpanAnnA -> Located [LocatedAn NoEpAnns (DotFieldOcc GhcPs)]
                -> LHsExpr GhcPs -> Bool -> EpAnn [AddEpAnn]
                -> LHsRecProj GhcPs (LHsExpr GhcPs)
mkRdrProjUpdate _ (L _ []) _ _ _ = panic "mkRdrProjUpdate: The impossible has happened!"
mkRdrProjUpdate loc (L l flds) arg isPun anns =
  L loc HsFieldBind {
      hfbAnn = anns
    , hfbLHS = L (noAnnSrcSpan l) (FieldLabelStrings flds)
    , hfbRHS = arg
    , hfbPun = isPun
  }