path: root/compiler/Language/Haskell/Syntax/Expr.hs

{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilyDependencies #-}
{-# LANGUAGE UndecidableInstances #-} -- Wrinkle in Note [Trees That Grow]
                                      -- in module Language.Haskell.Syntax.Extension

{-
(c) The University of Glasgow 2006
(c) The GRASP/AQUA Project, Glasgow University, 1992-1998
-}

-- See Note [Language.Haskell.Syntax.* Hierarchy] for why not GHC.Hs.*

-- | Abstract Haskell syntax for expressions.
module Language.Haskell.Syntax.Expr where

import Language.Haskell.Syntax.Basic
import Language.Haskell.Syntax.Decls
import Language.Haskell.Syntax.Pat
import Language.Haskell.Syntax.Lit
import Language.Haskell.Syntax.Concrete
import Language.Haskell.Syntax.Extension
import Language.Haskell.Syntax.Type
import Language.Haskell.Syntax.Binds

-- others:
import GHC.Types.Fixity (LexicalFixity(Infix), Fixity)
import GHC.Types.SourceText (StringLiteral)

import GHC.Unit.Module (ModuleName)
import GHC.Data.FastString (FastString)

-- libraries:
import Data.Data hiding (Fixity(..))
import Data.Bool
import Data.Either
import Data.Eq
import Data.Maybe
import Data.List.NonEmpty ( NonEmpty )
import GHC.Types.Name.Reader

{- Note [RecordDotSyntax field updates]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The extensions @OverloadedRecordDot@ @OverloadedRecordUpdate@ together
enable record updates like @a{foo.bar.baz = 1}@. Introducing this
syntax slightly complicates parsing. This note explains how it's done.

In the event a record is being constructed or updated, it's this
production that's in play:
@
aexp1 -> aexp1 '{' fbinds '}' {
  ...
  mkHsRecordPV ... $1 (snd $3)
}
@
@fbinds@ is a list of field bindings. @mkHsRecordPV@ is a function of
the @DisambECP b@ typeclass, see Note [Ambiguous syntactic
categories].

The "normal" rules for an @fbind@ are:
@
fbind
        : qvar '=' texp
        | qvar
@
These rules compute values of @LHsRecField GhcPs (Located b)@. They
apply in the context of record construction, record updates, record
patterns and record expressions. That is, @b@ ranges over @HsExpr
GhcPs@, @HsPat GhcPs@ and @HsCmd GhcPs@.

When @OverloadedRecordDot@ and @OverloadedRecordUpdate@ are both
enabled, two additional @fbind@ rules are admitted:
@
        | field TIGHT_INFIX_PROJ fieldToUpdate '=' texp
        | field TIGHT_INFIX_PROJ fieldToUpdate
@

These rules only make sense when parsing record update expressions
(that is, patterns and commands cannot be parsed by these rules and
neither record constructions).

The results of these new rules cannot be represented by @LHsRecField
GhcPs (LHsExpr GhcPs)@ values as the type is defined today. We
minimize modifying existing code by having these new rules calculate
@LHsRecProj GhcPs (LHsExpr GhcPs)@ ("record projection") values
instead:
@
newtype FieldLabelStrings = FieldLabelStrings [XRec p (DotFieldOcc p)]
type RecProj arg = HsFieldBind FieldLabelStrings arg
type LHsRecProj p arg = XRec p (RecProj arg)
@

The @fbind@ rule is then given the type @fbind :: { forall b.
DisambECP b => PV (Fbind b) }@ accommodating both alternatives:
@
type Fbind b = Either
                  (LHsRecField GhcPs (LocatedA b))
                  ( LHsRecProj GhcPs (LocatedA b))
@

In @data HsExpr p@, the @RecordUpd@ constuctor indicates regular
updates vs. projection updates by means of the @rupd_flds@ member
type, an @Either@ instance:
@
  | RecordUpd
      { rupd_ext  :: XRecordUpd p
      , rupd_expr :: LHsExpr p
      , rupd_flds :: Either [LHsRecUpdField p] [LHsRecUpdProj p]
      }
@
Here,
@
type RecUpdProj p = RecProj p (LHsExpr p)
type LHsRecUpdProj p = XRec p (RecUpdProj p)
@
and @Left@ values indicating regular record update, @Right@ values
updates desugared to @setField@s.

If @OverloadedRecordUpdate@ is enabled, any updates parsed as
@LHsRecField GhcPs@ values are converted to @LHsRecUpdProj GhcPs@
values (see function @mkRdrRecordUpd@ in 'GHC.Parser.PostProcess').
-}

-- | RecordDotSyntax field updates

type LFieldLabelStrings p = XRec p (FieldLabelStrings p)

newtype FieldLabelStrings p =
  FieldLabelStrings [XRec p (DotFieldOcc p)]

-- Field projection updates (e.g. @foo.bar.baz = 1@). See Note
-- [RecordDotSyntax field updates].
type RecProj p arg = HsFieldBind (LFieldLabelStrings p) arg

-- The phantom type parameter @p@ is for symmetry with @LHsRecField p
-- arg@ in the definition of @data Fbind@ (see GHC.Parser.Process).
type LHsRecProj p arg = XRec p (RecProj p arg)

-- These two synonyms are used in the definition of syntax @RecordUpd@
-- below.
type RecUpdProj p = RecProj p (LHsExpr p)
type LHsRecUpdProj p = XRec p (RecUpdProj p)

{-
************************************************************************
*                                                                      *
\subsection{Expressions proper}
*                                                                      *
************************************************************************
-}

-- * Expressions proper

-- | Located Haskell Expression
type LHsExpr p = XRec p (HsExpr p)
  -- ^ May have 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnComma' when
  --   in a list

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation

-------------------------
{- Note [NoSyntaxExpr]
~~~~~~~~~~~~~~~~~~~~~~
Syntax expressions can be missing (NoSyntaxExprRn or NoSyntaxExprTc)
for several reasons:

 1. As described in Note [Rebindable if]

 2. In order to suppress "not in scope: xyz" messages when a bit of
    rebindable syntax does not apply. For example, when using an irrefutable
    pattern in a BindStmt, we don't need a `fail` operator.

 3. Rebindable syntax might just not make sense. For example, a BodyStmt
    contains the syntax for `guard`, but that's used only in monad comprehensions.
    If we had more of a whiz-bang type system, we might be able to rule this
    case out statically.
-}

-- | Syntax Expression
--
-- SyntaxExpr is represents the function used in interpreting rebindable
-- syntax. In the parser, we have no information to supply; in the renamer,
-- we have the name of the function (but see
-- Note [Monad fail : Rebindable syntax, overloaded strings] for a wrinkle)
-- and in the type-checker we have a more elaborate structure 'SyntaxExprTc'.
--
-- In some contexts, rebindable syntax is not implemented, and so we have
-- constructors to represent that possibility in both the renamer and
-- typechecker instantiations.
--
-- E.g. @(>>=)@ is filled in before the renamer by the appropriate 'Name' for
--      @(>>=)@, and then instantiated by the type checker with its type args
--      etc
type family SyntaxExpr p

{-
Note [Record selectors in the AST]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Here is how record selectors are expressed in GHC's AST:

Example data type
  data T = MkT { size :: Int }

Record selectors:
                      |    GhcPs     |   GhcRn              |    GhcTc            |
----------------------------------------------------------------------------------|
size (assuming one    | HsVar        | HsRecSel             | HsRecSel            |
     'size' in scope) |              |                      |                     |
----------------------|--------------|----------------------|---------------------|
.size (assuming       | HsProjection | getField @"size"     | getField @"size"    |
 OverloadedRecordDot) |              |                      |                     |
----------------------|--------------|----------------------|---------------------|
e.size (assuming      | HsGetField   | getField @"size" e   | getField @"size" e  |
 OverloadedRecordDot) |              |                      |                     |

NB 1: DuplicateRecordFields makes no difference to the first row of
this table, except that if 'size' is a field of more than one data
type, then a naked use of the record selector 'size' may well be
ambiguous. You have to use a qualified name. And there is no way to do
this if both data types are declared in the same module.

NB 2: The notation getField @"size" e is short for
HsApp (HsAppType (HsVar "getField") (HsWC (HsTyLit (HsStrTy "size")) [])) e.
We track the original parsed syntax via HsExpanded.

-}

{-
Note [Non-overloaded record field selectors]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
    data T = MkT { x,y :: Int }
    f r x = x + y r

This parses with HsVar for x, y, r on the RHS of f. Later, the renamer
recognises that y in the RHS of f is really a record selector, and
changes it to a HsRecSel. In contrast x is locally bound, shadowing
the record selector, and stays as an HsVar.

The renamer adds the Name of the record selector into the XCFieldOcc
extension field, The typechecker keeps HsRecSel as HsRecSel, and
transforms the record-selector Name to an Id.
-}

-- | A Haskell expression.
data HsExpr p
  = HsVar     (XVar p)
              (LIdP p) -- ^ Variable
                       -- See Note [Located RdrNames]

  | HsUnboundVar (XUnboundVar p)
                 RdrName     -- ^ Unbound variable; also used for "holes"
                             --   (_ or _x).
                             -- Turned from HsVar to HsUnboundVar by the
                             --   renamer, when it finds an out-of-scope
                             --   variable or hole.
                             -- The (XUnboundVar p) field becomes an HoleExprRef
                             --   after typechecking; this is where the
                             --   erroring expression will be written after
                             --   solving. See Note [Holes] in GHC.Tc.Types.Constraint.


  | HsRecSel  (XRecSel p)
              (FieldOcc p) -- ^ Variable pointing to record selector
                           -- See Note [Non-overloaded record field selectors] and
                           -- Note [Record selectors in the AST]

  | HsOverLabel (XOverLabel p) FastString
     -- ^ Overloaded label (Note [Overloaded labels] in GHC.OverloadedLabels)

  | HsIPVar   (XIPVar p)
              HsIPName   -- ^ Implicit parameter (not in use after typechecking)
  | HsOverLit (XOverLitE p)
              (HsOverLit p)  -- ^ Overloaded literals

  | HsLit     (XLitE p)
              (HsLit p)      -- ^ Simple (non-overloaded) literals

  | HsLam     (XLam p)
              (MatchGroup p (LHsExpr p))
                       -- ^ Lambda abstraction. Currently always a single match
       --
       -- - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnLam',
       --       'GHC.Parser.Annotation.AnnRarrow',

       -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation

  -- | Lambda-case
  --
  -- - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnLam',
  --           'GHC.Parser.Annotation.AnnCase','GHC.Parser.Annotation.AnnOpen',
  --           'GHC.Parser.Annotation.AnnClose'
  -- - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnLam',
  --           'GHC.Parser.Annotation.AnnCases','GHC.Parser.Annotation.AnnOpen',
  --           'GHC.Parser.Annotation.AnnClose'

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
  | HsLamCase (XLamCase p) LamCaseVariant (MatchGroup p (LHsExpr p))

  | HsApp     (XApp p) (LHsExpr p) (LHsExpr p) -- ^ Application

  | HsAppType (XAppTypeE p) -- After typechecking: the type argument
              (LHsExpr p)
             !(LHsToken "@" p)
              (LHsWcType (NoGhcTc p))  -- ^ Visible type application
       --
       -- Explicit type argument; e.g  f @Int x y
       -- NB: Has wildcards, but no implicit quantification
       --
       -- - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnAt',

  -- | Operator applications:
  -- NB Bracketed ops such as (+) come out as Vars.

  -- NB Sadly, we need an expr for the operator in an OpApp/Section since
  -- the renamer may turn a HsVar into HsRecSel or HsUnboundVar

  | OpApp       (XOpApp p)
                (LHsExpr p)       -- left operand
                (LHsExpr p)       -- operator
                (LHsExpr p)       -- right operand

  -- | Negation operator. Contains the negated expression and the name
  -- of 'negate'
  --
  --  - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnMinus'

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
  | NegApp      (XNegApp p)
                (LHsExpr p)
                (SyntaxExpr p)

  -- | - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnOpen' @'('@,
  --             'GHC.Parser.Annotation.AnnClose' @')'@

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
  | HsPar       (XPar p)
               !(LHsToken "(" p)
                (LHsExpr p)  -- ^ Parenthesised expr; see Note [Parens in HsSyn]
               !(LHsToken ")" p)

  | SectionL    (XSectionL p)
                (LHsExpr p)    -- operand; see Note [Sections in HsSyn]
                (LHsExpr p)    -- operator
  | SectionR    (XSectionR p)
                (LHsExpr p)    -- operator; see Note [Sections in HsSyn]
                (LHsExpr p)    -- operand

  -- | Used for explicit tuples and sections thereof
  --
  --  - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnOpen',
  --         'GHC.Parser.Annotation.AnnClose'

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
  -- Note [ExplicitTuple]
  | ExplicitTuple
        (XExplicitTuple p)
        [HsTupArg p]
        Boxity

  -- | Used for unboxed sum types
  --
  --  - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnOpen' @'(#'@,
  --          'GHC.Parser.Annotation.AnnVbar', 'GHC.Parser.Annotation.AnnClose' @'#)'@,
  --
  --  There will be multiple 'GHC.Parser.Annotation.AnnVbar', (1 - alternative) before
  --  the expression, (arity - alternative) after it
  | ExplicitSum
          (XExplicitSum p)
          ConTag   --  Alternative (one-based)
          SumWidth --  Sum arity
          (LHsExpr p)

  -- | - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnCase',
  --       'GHC.Parser.Annotation.AnnOf','GHC.Parser.Annotation.AnnOpen' @'{'@,
  --       'GHC.Parser.Annotation.AnnClose' @'}'@

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
  | HsCase      (XCase p)
                (LHsExpr p)
                (MatchGroup p (LHsExpr p))

  -- | - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnIf',
  --       'GHC.Parser.Annotation.AnnSemi',
  --       'GHC.Parser.Annotation.AnnThen','GHC.Parser.Annotation.AnnSemi',
  --       'GHC.Parser.Annotation.AnnElse',

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
  | HsIf        (XIf p)        -- GhcPs: this is a Bool; False <=> do not use
                               --  rebindable syntax
                (LHsExpr p)    --  predicate
                (LHsExpr p)    --  then part
                (LHsExpr p)    --  else part

  -- | Multi-way if
  --
  -- - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnIf'
  --       'GHC.Parser.Annotation.AnnOpen','GHC.Parser.Annotation.AnnClose',

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
  | HsMultiIf   (XMultiIf p) [LGRHS p (LHsExpr p)]

  -- | let(rec)
  --
  -- - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnLet',
  --       'GHC.Parser.Annotation.AnnOpen' @'{'@,
  --       'GHC.Parser.Annotation.AnnClose' @'}'@,'GHC.Parser.Annotation.AnnIn'

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
  | HsLet       (XLet p)
               !(LHsToken "let" p)
                (HsLocalBinds p)
               !(LHsToken "in" p)
                (LHsExpr  p)

  -- | - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnDo',
  --             'GHC.Parser.Annotation.AnnOpen', 'GHC.Parser.Annotation.AnnSemi',
  --             'GHC.Parser.Annotation.AnnVbar',
  --             'GHC.Parser.Annotation.AnnClose'

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
  | HsDo        (XDo p)                  -- Type of the whole expression
                HsDoFlavour
                (XRec p [ExprLStmt p])   -- "do":one or more stmts

  -- | Syntactic list: [a,b,c,...]
  --
  --  - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnOpen' @'['@,
  --              'GHC.Parser.Annotation.AnnClose' @']'@

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
  -- See Note [Empty lists]
  | ExplicitList
                (XExplicitList p)  -- Gives type of components of list
                [LHsExpr p]

  -- | Record construction
  --
  --  - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnOpen' @'{'@,
  --         'GHC.Parser.Annotation.AnnDotdot','GHC.Parser.Annotation.AnnClose' @'}'@

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
  | RecordCon
      { rcon_ext  :: XRecordCon p
      , rcon_con  :: XRec p (ConLikeP p)  -- The constructor
      , rcon_flds :: HsRecordBinds p }    -- The fields

  -- | Record update
  --
  --  - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnOpen' @'{'@,
  --         'GHC.Parser.Annotation.AnnDotdot','GHC.Parser.Annotation.AnnClose' @'}'@
  --         'GHC.Parser.Annotation.AnnComma, 'GHC.Parser.Annotation.AnnDot',
  --         'GHC.Parser.Annotation.AnnClose' @'}'@

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
  | RecordUpd
      { rupd_ext  :: XRecordUpd p
      , rupd_expr :: LHsExpr p
      , rupd_flds :: Either [LHsRecUpdField p] [LHsRecUpdProj p]
      }
  -- For a type family, the arg types are of the *instance* tycon,
  -- not the family tycon

  -- | Record field selection e.g @z.x@.
  --
  --  - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnDot'

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation

  -- This case only arises when the OverloadedRecordDot langauge
  -- extension is enabled. See Note [Record selectors in the AST].
  | HsGetField {
        gf_ext :: XGetField p
      , gf_expr :: LHsExpr p
      , gf_field :: XRec p (DotFieldOcc p)
      }

  -- | Record field selector. e.g. @(.x)@ or @(.x.y)@
  --
  -- This case only arises when the OverloadedRecordDot langauge
  -- extensions is enabled. See Note [Record selectors in the AST].

  --  - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnOpenP'
  --         'GHC.Parser.Annotation.AnnDot', 'GHC.Parser.Annotation.AnnCloseP'

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
  | HsProjection {
        proj_ext :: XProjection p
      , proj_flds :: NonEmpty (XRec p (DotFieldOcc p))
      }

  -- | Expression with an explicit type signature. @e :: type@
  --
  --  - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnDcolon'

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
  | ExprWithTySig
                (XExprWithTySig p)

                (LHsExpr p)
                (LHsSigWcType (NoGhcTc p))

  -- | Arithmetic sequence
  --
  --  - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnOpen' @'['@,
  --              'GHC.Parser.Annotation.AnnComma','GHC.Parser.Annotation.AnnDotdot',
  --              'GHC.Parser.Annotation.AnnClose' @']'@

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
  | ArithSeq
                (XArithSeq p)
                (Maybe (SyntaxExpr p))
                                  -- For OverloadedLists, the fromList witness
                (ArithSeqInfo p)

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation

  -----------------------------------------------------------
  -- MetaHaskell Extensions

  -- | - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnOpen',
  --         'GHC.Parser.Annotation.AnnOpenE','GHC.Parser.Annotation.AnnOpenEQ',
  --         'GHC.Parser.Annotation.AnnClose','GHC.Parser.Annotation.AnnCloseQ'

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
  | HsTypedBracket   (XTypedBracket p)   (LHsExpr p)
  | HsUntypedBracket (XUntypedBracket p) (HsQuote p)

  -- | - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnOpen',
  --         'GHC.Parser.Annotation.AnnClose'

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
  | HsTypedSplice    (XTypedSplice p)   (LHsExpr p) -- `$$z` or `$$(f 4)`
  | HsUntypedSplice  (XUntypedSplice p) (HsUntypedSplice p)

  -----------------------------------------------------------
  -- Arrow notation extension

  -- | @proc@ notation for Arrows
  --
  --  - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnProc',
  --          'GHC.Parser.Annotation.AnnRarrow'

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
  | HsProc      (XProc p)
                (LPat p)               -- arrow abstraction, proc
                (LHsCmdTop p)          -- body of the abstraction
                                       -- always has an empty stack

  ---------------------------------------
  -- static pointers extension
  -- | - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnStatic',

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
  | HsStatic (XStatic p) -- Free variables of the body, and type after typechecking
             (LHsExpr p)        -- Body

  ---------------------------------------
  -- Expressions annotated with pragmas, written as {-# ... #-}
  | HsPragE (XPragE p) (HsPragE p) (LHsExpr p)

  | XExpr       !(XXExpr p)
  -- Note [Trees That Grow] in Language.Haskell.Syntax.Extension for the
  -- general idea, and Note [Rebindable syntax and HsExpansion] in GHC.Hs.Expr
  -- for an example of how we use it.

-- ---------------------------------------------------------------------

data DotFieldOcc p
  = DotFieldOcc
    { dfoExt   :: XCDotFieldOcc p
    , dfoLabel :: XRec p FieldLabelString
    }
  | XDotFieldOcc !(XXDotFieldOcc p)

-- ---------------------------------------------------------------------

-- | A pragma, written as {-# ... #-}, that may appear within an expression.
data HsPragE p
  = HsPragSCC   (XSCC p)
                StringLiteral         -- "set cost centre" SCC pragma

  -- | - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnOpen',
  --       'GHC.Parser.Annotation.AnnOpen' @'{-\# GENERATED'@,
  --       'GHC.Parser.Annotation.AnnVal','GHC.Parser.Annotation.AnnVal',
  --       'GHC.Parser.Annotation.AnnColon','GHC.Parser.Annotation.AnnVal',
  --       'GHC.Parser.Annotation.AnnMinus',
  --       'GHC.Parser.Annotation.AnnVal','GHC.Parser.Annotation.AnnColon',
  --       'GHC.Parser.Annotation.AnnVal',
  --       'GHC.Parser.Annotation.AnnClose' @'\#-}'@

  | XHsPragE !(XXPragE p)

-- | Located Haskell Tuple Argument
--
-- 'HsTupArg' is used for tuple sections
-- @(,a,)@ is represented by
-- @ExplicitTuple [Missing ty1, Present a, Missing ty3]@
-- Which in turn stands for @(\x:ty1 \y:ty2. (x,a,y))@
type LHsTupArg id = XRec id (HsTupArg id)
-- | - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnComma'

-- For details on above see Note [exact print annotations] in GHC.Parser.Annotation

-- | Haskell Tuple Argument
data HsTupArg id
  = Present (XPresent id) (LHsExpr id)     -- ^ The argument
  | Missing (XMissing id)    -- ^ The argument is missing, but this is its type
  | XTupArg !(XXTupArg id)   -- ^ Extension point; see Note [Trees That Grow]
                             -- in Language.Haskell.Syntax.Extension

-- | Which kind of lambda case are we dealing with?
data LamCaseVariant
  = LamCase -- ^ `\case`
  | LamCases -- ^ `\cases`
  deriving (Data, Eq)

{-
Note [Parens in HsSyn]
~~~~~~~~~~~~~~~~~~~~~~
HsPar (and ParPat in patterns, HsParTy in types) is used as follows

  * HsPar is required; the pretty printer does not add parens.

  * HsPars are respected when rearranging operator fixities.
    So   a * (b + c)  means what it says (where the parens are an HsPar)

  * For ParPat and HsParTy the pretty printer does add parens but this should be
    a no-op for ParsedSource, based on the pretty printer round trip feature
    introduced in
    https://phabricator.haskell.org/rGHC499e43824bda967546ebf95ee33ec1f84a114a7c

  * ParPat and HsParTy are pretty printed as '( .. )' regardless of whether or
    not they are strictly necessary. This should be addressed when #13238 is
    completed, to be treated the same as HsPar.


Note [Sections in HsSyn]
~~~~~~~~~~~~~~~~~~~~~~~~
Sections should always appear wrapped in an HsPar, thus
         HsPar (SectionR ...)
The parser parses sections in a wider variety of situations
(See Note [Parsing sections]), but the renamer checks for those
parens.  This invariant makes pretty-printing easier; we don't need
a special case for adding the parens round sections.

Note [Rebindable if]
~~~~~~~~~~~~~~~~~~~~
The rebindable syntax for 'if' is a bit special, because when
rebindable syntax is *off* we do not want to treat
   (if c then t else e)
as if it was an application (ifThenElse c t e).  Why not?
Because we allow an 'if' to return *unboxed* results, thus
  if blah then 3# else 4#
whereas that would not be possible using a all to a polymorphic function
(because you can't call a polymorphic function at an unboxed type).

So we use NoSyntaxExpr to mean "use the old built-in typing rule".

A further complication is that, in the `deriving` code, we never want
to use rebindable syntax. So, even in GhcPs, we want to denote whether
to use rebindable syntax or not. This is done via the type instance
for XIf GhcPs.

Note [Record Update HsWrapper]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
There is a wrapper in RecordUpd which is used for the *required*
constraints for pattern synonyms. This wrapper is created in the
typechecking and is then directly used in the desugaring without
modification.

For example, if we have the record pattern synonym P,
  pattern P :: (Show a) => a -> Maybe a
  pattern P{x} = Just x

  foo = (Just True) { x = False }
then `foo` desugars to something like
  foo = case Just True of
          P x -> P False
hence we need to provide the correct dictionaries to P's matcher on
the RHS so that we can build the expression.

Note [Located RdrNames]
~~~~~~~~~~~~~~~~~~~~~~~
A number of syntax elements have seemingly redundant locations
attached to them.  This is deliberate, to allow transformations making
use of the exact print annotations to easily correlate a Located Name
in the RenamedSource with a Located RdrName in the ParsedSource.

There are unfortunately enough differences between the ParsedSource
and the RenamedSource that the exact print annotations cannot be used
directly with RenamedSource, so this allows a simple mapping to be
used based on the location.

Note [ExplicitTuple]
~~~~~~~~~~~~~~~~~~~~
An ExplicitTuple is never just a data constructor like (,,,).
That is, the `[LHsTupArg p]` argument of `ExplicitTuple` has at least
one `Present` member (and is thus never empty).

A tuple data constructor like () or (,,,) is parsed as an `HsVar`, not an
`ExplicitTuple`, and stays that way. This is important for two reasons:

  1. We don't need -XTupleSections for (,,,)
  2. The type variables in (,,,) can be instantiated with visible type application.
     That is,

       (,,)     :: forall a b c. a -> b -> c -> (a,b,c)
       (True,,) :: forall {b} {c}. b -> c -> (Bool,b,c)

     Note that the tuple section has *inferred* arguments, while the data
     constructor has *specified* ones.
     (See Note [Required, Specified, and Inferred for types] in GHC.Tc.TyCl
     for background.)

Sadly, the grammar for this is actually ambiguous, and it's only thanks to the
preference of a shift in a shift/reduce conflict that the parser works as this
Note details. Search for a reference to this Note in GHC.Parser for further
explanation.

Note [Empty lists]
~~~~~~~~~~~~~~~~~~
An empty list could be considered either a data constructor (stored with
HsVar) or an ExplicitList. This Note describes how empty lists flow through the
various phases and why.

Parsing
-------
An empty list is parsed by the sysdcon nonterminal. It thus comes to life via
HsVar nilDataCon (defined in GHC.Builtin.Types). A freshly-parsed (HsExpr GhcPs) empty list
is never a ExplicitList.

Renaming
--------
If -XOverloadedLists is enabled, we must type-check the empty list as if it
were a call to fromListN. (This is true regardless of the setting of
-XRebindableSyntax.) This is very easy if the empty list is an ExplicitList,
but an annoying special case if it's an HsVar. So the renamer changes a
HsVar nilDataCon to an ExplicitList [], but only if -XOverloadedLists is on.
(Why not always? Read on, dear friend.) This happens in the HsVar case of rnExpr.

Type-checking
-------------
We want to accept an expression like [] @Int. To do this, we must infer that
[] :: forall a. [a]. This is easy if [] is a HsVar with the right DataCon inside.
However, the type-checking for explicit lists works differently: [x,y,z] is never
polymorphic. Instead, we unify the types of x, y, and z together, and use the
unified type as the argument to the cons and nil constructors. Thus, treating
[] as an empty ExplicitList in the type-checker would prevent [] @Int from working.

However, if -XOverloadedLists is on, then [] @Int really shouldn't be allowed:
it's just like fromListN 0 [] @Int. Since
  fromListN :: forall list. IsList list => Int -> [Item list] -> list
that expression really should be rejected. Thus, the renamer's behaviour is
exactly what we want: treat [] as a datacon when -XNoOverloadedLists, and as
an empty ExplicitList when -XOverloadedLists.

See also #13680, which requested [] @Int to work.
-}


{-
HsSyn records exactly where the user put parens, with HsPar.
So generally speaking we print without adding any parens.
However, some code is internally generated, and in some places
parens are absolutely required; so for these places we use
pprParendLExpr (but don't print double parens of course).

For operator applications we don't add parens, because the operator
fixities should do the job, except in debug mode (-dppr-debug) so we
can see the structure of the parse tree.
-}

{-
************************************************************************
*                                                                      *
\subsection{Commands (in arrow abstractions)}
*                                                                      *
************************************************************************

We re-use HsExpr to represent these.
-}

-- | Located Haskell Command (for arrow syntax)
type LHsCmd id = XRec id (HsCmd id)

-- | Haskell Command (e.g. a "statement" in an Arrow proc block)
data HsCmd id
  -- | - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.Annlarrowtail',
  --          'GHC.Parser.Annotation.Annrarrowtail','GHC.Parser.Annotation.AnnLarrowtail',
  --          'GHC.Parser.Annotation.AnnRarrowtail'

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
  = HsCmdArrApp          -- Arrow tail, or arrow application (f -< arg)
        (XCmdArrApp id)  -- type of the arrow expressions f,
                         -- of the form a t t', where arg :: t
        (LHsExpr id)     -- arrow expression, f
        (LHsExpr id)     -- input expression, arg
        HsArrAppType     -- higher-order (-<<) or first-order (-<)
        Bool             -- True => right-to-left (f -< arg)
                         -- False => left-to-right (arg >- f)

  -- | - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnOpenB' @'(|'@,
  --         'GHC.Parser.Annotation.AnnCloseB' @'|)'@

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
  | HsCmdArrForm         -- Command formation,  (| e cmd1 .. cmdn |)
        (XCmdArrForm id)
        (LHsExpr id)     -- The operator.
                         -- After type-checking, a type abstraction to be
                         -- applied to the type of the local environment tuple
        LexicalFixity    -- Whether the operator appeared prefix or infix when
                         -- parsed.
        (Maybe Fixity)   -- fixity (filled in by the renamer), for forms that
                         -- were converted from OpApp's by the renamer
        [LHsCmdTop id]   -- argument commands

  | HsCmdApp    (XCmdApp id)
                (LHsCmd id)
                (LHsExpr id)

  | HsCmdLam    (XCmdLam id)
                (MatchGroup id (LHsCmd id))     -- kappa
       -- ^ - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnLam',
       --       'GHC.Parser.Annotation.AnnRarrow',

       -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation

  | HsCmdPar    (XCmdPar id)
               !(LHsToken "(" id)
                (LHsCmd id)                     -- parenthesised command
               !(LHsToken ")" id)
    -- ^ - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnOpen' @'('@,
    --             'GHC.Parser.Annotation.AnnClose' @')'@

    -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation

  | HsCmdCase   (XCmdCase id)
                (LHsExpr id)
                (MatchGroup id (LHsCmd id))     -- bodies are HsCmd's
    -- ^ - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnCase',
    --       'GHC.Parser.Annotation.AnnOf','GHC.Parser.Annotation.AnnOpen' @'{'@,
    --       'GHC.Parser.Annotation.AnnClose' @'}'@

    -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation

  -- | Lambda-case
  --
  -- - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnLam',
  --     'GHC.Parser.Annotation.AnnCase','GHC.Parser.Annotation.AnnOpen' @'{'@,
  --     'GHC.Parser.Annotation.AnnClose' @'}'@
  -- - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnLam',
  --     'GHC.Parser.Annotation.AnnCases','GHC.Parser.Annotation.AnnOpen' @'{'@,
  --     'GHC.Parser.Annotation.AnnClose' @'}'@

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
  | HsCmdLamCase (XCmdLamCase id) LamCaseVariant
                 (MatchGroup id (LHsCmd id)) -- bodies are HsCmd's

  | HsCmdIf     (XCmdIf id)
                (SyntaxExpr id)         -- cond function
                (LHsExpr id)            -- predicate
                (LHsCmd id)             -- then part
                (LHsCmd id)             -- else part
    -- ^ - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnIf',
    --       'GHC.Parser.Annotation.AnnSemi',
    --       'GHC.Parser.Annotation.AnnThen','GHC.Parser.Annotation.AnnSemi',
    --       'GHC.Parser.Annotation.AnnElse',

    -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation

  | HsCmdLet    (XCmdLet id)
               !(LHsToken "let" id)
                (HsLocalBinds id)      -- let(rec)
               !(LHsToken "in" id)
                (LHsCmd  id)
    -- ^ - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnLet',
    --       'GHC.Parser.Annotation.AnnOpen' @'{'@,
    --       'GHC.Parser.Annotation.AnnClose' @'}'@,'GHC.Parser.Annotation.AnnIn'

    -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation

  | HsCmdDo     (XCmdDo id)                     -- Type of the whole expression
                (XRec id [CmdLStmt id])
    -- ^ - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnDo',
    --             'GHC.Parser.Annotation.AnnOpen', 'GHC.Parser.Annotation.AnnSemi',
    --             'GHC.Parser.Annotation.AnnVbar',
    --             'GHC.Parser.Annotation.AnnClose'

    -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation

  | XCmd        !(XXCmd id)     -- Extension point; see Note [Trees That Grow]
                                -- in Language.Haskell.Syntax.Extension


-- | Haskell arrow application type.
data HsArrAppType
  -- | First order arrow application '-<'
  = HsHigherOrderApp
  -- | Higher order arrow application '-<<'
  | HsFirstOrderApp
    deriving Data

{- | Top-level command, introducing a new arrow.
This may occur inside a proc (where the stack is empty) or as an
argument of a command-forming operator.
-}

-- | Located Haskell Top-level Command
type LHsCmdTop p = XRec p (HsCmdTop p)

-- | Haskell Top-level Command
data HsCmdTop p
  = HsCmdTop (XCmdTop p)
             (LHsCmd p)
  | XCmdTop !(XXCmdTop p)        -- Extension point; see Note [Trees That Grow]
                                 -- in Language.Haskell.Syntax.Extension

-----------------------

{-
************************************************************************
*                                                                      *
\subsection{Record binds}
*                                                                      *
************************************************************************
-}

-- | Haskell Record Bindings
type HsRecordBinds p = HsRecFields p (LHsExpr p)

{-
************************************************************************
*                                                                      *
\subsection{@Match@, @GRHSs@, and @GRHS@ datatypes}
*                                                                      *
************************************************************************

@Match@es are sets of pattern bindings and right hand sides for
functions, patterns or case branches. For example, if a function @g@
is defined as:
\begin{verbatim}
g (x,y) = y
g ((x:ys),y) = y+1,
\end{verbatim}
then \tr{g} has two @Match@es: @(x,y) = y@ and @((x:ys),y) = y+1@.

It is always the case that each element of an @[Match]@ list has the
same number of @pats@s inside it.  This corresponds to saying that
a function defined by pattern matching must have the same number of
patterns in each equation.
-}

data MatchGroup p body
  = MG { mg_ext     :: XMG p body -- Post-typechecker, types of args and result, and origin
       , mg_alts    :: XRec p [LMatch p body] } -- The alternatives
     -- The type is the type of the entire group
     --      t1 -> ... -> tn -> tr
     -- where there are n patterns
  | XMatchGroup !(XXMatchGroup p body)

-- | Located Match
type LMatch id body = XRec id (Match id body)
-- ^ May have 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnSemi' when in a
--   list

-- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
data Match p body
  = Match {
        m_ext :: XCMatch p body,
        m_ctxt :: HsMatchContext p,
          -- See Note [m_ctxt in Match]
        m_pats :: [LPat p], -- The patterns
        m_grhss :: (GRHSs p body)
  }
  | XMatch !(XXMatch p body)

{-
Note [m_ctxt in Match]
~~~~~~~~~~~~~~~~~~~~~~

A Match can occur in a number of contexts, such as a FunBind, HsCase, HsLam and
so on.

In order to simplify tooling processing and pretty print output, the provenance
is captured in an HsMatchContext.

This is particularly important for the exact print annotations for a
multi-equation FunBind.

The parser initially creates a FunBind with a single Match in it for
every function definition it sees.

These are then grouped together by getMonoBind into a single FunBind,
where all the Matches are combined.

In the process, all the original FunBind fun_id's bar one are
discarded, including the locations.

This causes a problem for source to source conversions via exact print
annotations, so the original fun_ids and infix flags are preserved in
the Match, when it originates from a FunBind.

Example infix function definition requiring individual exact print
annotations

    (&&&  ) [] [] =  []
    xs    &&&   [] =  xs
    (  &&&  ) [] ys =  ys



-}


isInfixMatch :: Match id body -> Bool
isInfixMatch match = case m_ctxt match of
  FunRhs {mc_fixity = Infix} -> True
  _                          -> False

-- | Guarded Right-Hand Sides
--
-- GRHSs are used both for pattern bindings and for Matches
--
--  - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnVbar',
--        'GHC.Parser.Annotation.AnnEqual','GHC.Parser.Annotation.AnnWhere',
--        'GHC.Parser.Annotation.AnnOpen','GHC.Parser.Annotation.AnnClose'
--        'GHC.Parser.Annotation.AnnRarrow','GHC.Parser.Annotation.AnnSemi'

-- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
data GRHSs p body
  = GRHSs {
      grhssExt :: XCGRHSs p body,
      grhssGRHSs :: [LGRHS p body],     -- ^ Guarded RHSs
      grhssLocalBinds :: HsLocalBinds p -- ^ The where clause
    }
  | XGRHSs !(XXGRHSs p body)

-- | Located Guarded Right-Hand Side
type LGRHS id body = XRec id (GRHS id body)

-- | Guarded Right Hand Side.
data GRHS p body = GRHS (XCGRHS p body)
                        [GuardLStmt p] -- Guards
                        body           -- Right hand side
                  | XGRHS !(XXGRHS p body)

-- We know the list must have at least one @Match@ in it.

{-
************************************************************************
*                                                                      *
\subsection{Do stmts and list comprehensions}
*                                                                      *
************************************************************************
-}

-- | Located @do@ block Statement
type LStmt id body = XRec id (StmtLR id id body)

-- | Located Statement with separate Left and Right id's
type LStmtLR idL idR body = XRec idL (StmtLR idL idR body)

-- | @do@ block Statement
type Stmt id body = StmtLR id id body

-- | Command Located Statement
type CmdLStmt   id = LStmt id (LHsCmd  id)

-- | Command Statement
type CmdStmt    id = Stmt  id (LHsCmd  id)

-- | Expression Located Statement
type ExprLStmt  id = LStmt id (LHsExpr id)

-- | Expression Statement
type ExprStmt   id = Stmt  id (LHsExpr id)

-- | Guard Located Statement
type GuardLStmt id = LStmt id (LHsExpr id)

-- | Guard Statement
type GuardStmt  id = Stmt  id (LHsExpr id)

-- | Ghci Located Statement
type GhciLStmt  id = LStmt id (LHsExpr id)

-- | Ghci Statement
type GhciStmt   id = Stmt  id (LHsExpr id)

-- The SyntaxExprs in here are used *only* for do-notation and monad
-- comprehensions, which have rebindable syntax. Otherwise they are unused.
-- | Exact print annotations when in qualifier lists or guards
--  - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnVbar',
--         'GHC.Parser.Annotation.AnnComma','GHC.Parser.Annotation.AnnThen',
--         'GHC.Parser.Annotation.AnnBy','GHC.Parser.Annotation.AnnBy',
--         'GHC.Parser.Annotation.AnnGroup','GHC.Parser.Annotation.AnnUsing'

-- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
data StmtLR idL idR body -- body should always be (LHs**** idR)
  = LastStmt  -- Always the last Stmt in ListComp, MonadComp,
              -- and (after the renamer, see GHC.Rename.Expr.checkLastStmt) DoExpr, MDoExpr
              -- Not used for GhciStmtCtxt, PatGuard, which scope over other stuff
          (XLastStmt idL idR body)
          body
          (Maybe Bool)  -- Whether return was stripped
            -- Just True <=> return with a dollar was stripped by ApplicativeDo
            -- Just False <=> return without a dollar was stripped by ApplicativeDo
            -- Nothing <=> Nothing was stripped
          (SyntaxExpr idR)   -- The return operator
            -- The return operator is used only for MonadComp
            -- For ListComp we use the baked-in 'return'
            -- For DoExpr, MDoExpr, we don't apply a 'return' at all
            -- See Note [Monad Comprehensions]
            -- - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnLarrow'

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
  | BindStmt (XBindStmt idL idR body)
             -- ^ Post renaming has optional fail and bind / (>>=) operator.
             -- Post typechecking, also has multiplicity of the argument
             -- and the result type of the function passed to bind;
             -- that is, (P, S) in (>>=) :: Q -> (R % P -> S) -> T
             -- See Note [The type of bind in Stmts]
             (LPat idL)
             body

  -- | 'ApplicativeStmt' represents an applicative expression built with
  -- '<$>' and '<*>'.  It is generated by the renamer, and is desugared into the
  -- appropriate applicative expression by the desugarer, but it is intended
  -- to be invisible in error messages.
  --
  -- For full details, see Note [ApplicativeDo] in "GHC.Rename.Expr"
  --
  | ApplicativeStmt
             (XApplicativeStmt idL idR body) -- Post typecheck, Type of the body
             [ ( SyntaxExpr idR
               , ApplicativeArg idL) ]
                      -- [(<$>, e1), (<*>, e2), ..., (<*>, en)]
             (Maybe (SyntaxExpr idR))  -- 'join', if necessary

  | BodyStmt (XBodyStmt idL idR body) -- Post typecheck, element type
                                      -- of the RHS (used for arrows)
             body              -- See Note [BodyStmt]
             (SyntaxExpr idR)  -- The (>>) operator
             (SyntaxExpr idR)  -- The `guard` operator; used only in MonadComp
                               -- See notes [Monad Comprehensions]

  -- | - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnLet'
  --          'GHC.Parser.Annotation.AnnOpen' @'{'@,'GHC.Parser.Annotation.AnnClose' @'}'@,

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
  | LetStmt  (XLetStmt idL idR body) (HsLocalBindsLR idL idR)

  -- ParStmts only occur in a list/monad comprehension
  | ParStmt  (XParStmt idL idR body)    -- Post typecheck,
                                        -- S in (>>=) :: Q -> (R -> S) -> T
             [ParStmtBlock idL idR]
             (HsExpr idR)               -- Polymorphic `mzip` for monad comprehensions
             (SyntaxExpr idR)           -- The `>>=` operator
                                        -- See notes [Monad Comprehensions]
            -- After renaming, the ids are the binders
            -- bound by the stmts and used after them

  | TransStmt {
      trS_ext   :: XTransStmt idL idR body, -- Post typecheck,
                                            -- R in (>>=) :: Q -> (R -> S) -> T
      trS_form  :: TransForm,
      trS_stmts :: [ExprLStmt idL],   -- Stmts to the *left* of the 'group'
                                      -- which generates the tuples to be grouped

      trS_bndrs :: [(IdP idR, IdP idR)], -- See Note [TransStmt binder map]

      trS_using :: LHsExpr idR,
      trS_by :: Maybe (LHsExpr idR),  -- "by e" (optional)
        -- Invariant: if trS_form = GroupBy, then grp_by = Just e

      trS_ret :: SyntaxExpr idR,      -- The monomorphic 'return' function for
                                      -- the inner monad comprehensions
      trS_bind :: SyntaxExpr idR,     -- The '(>>=)' operator
      trS_fmap :: HsExpr idR          -- The polymorphic 'fmap' function for desugaring
                                      -- Only for 'group' forms
                                      -- Just a simple HsExpr, because it's
                                      -- too polymorphic for tcSyntaxOp
    }                                 -- See Note [Monad Comprehensions]

  -- Recursive statement (see Note [How RecStmt works] below)
  -- | - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnRec'

  -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
  | RecStmt
     { recS_ext :: XRecStmt idL idR body
     , recS_stmts :: XRec idR [LStmtLR idL idR body]
     -- Assume XRec is the same for idL and idR, pick one arbitrarily

        -- The next two fields are only valid after renaming
     , recS_later_ids :: [IdP idR]
                         -- The ids are a subset of the variables bound by the
                         -- stmts that are used in stmts that follow the RecStmt

     , recS_rec_ids :: [IdP idR]
                         -- Ditto, but these variables are the "recursive" ones,
                         -- that are used before they are bound in the stmts of
                         -- the RecStmt.
        -- An Id can be in both groups
        -- Both sets of Ids are (now) treated monomorphically
        -- See Note [How RecStmt works] for why they are separate

        -- Rebindable syntax
     , recS_bind_fn :: SyntaxExpr idR -- The bind function
     , recS_ret_fn  :: SyntaxExpr idR -- The return function
     , recS_mfix_fn :: SyntaxExpr idR -- The mfix function
      }
  | XStmtLR !(XXStmtLR idL idR body)

data TransForm   -- The 'f' below is the 'using' function, 'e' is the by function
  = ThenForm     -- then f               or    then f by e             (depending on trS_by)
  | GroupForm    -- then group using f   or    then group by e using f (depending on trS_by)
  deriving Data

-- | Parenthesised Statement Block
data ParStmtBlock idL idR
  = ParStmtBlock
        (XParStmtBlock idL idR)
        [ExprLStmt idL]
        [IdP idR]          -- The variables to be returned
        (SyntaxExpr idR)   -- The return operator
  | XParStmtBlock !(XXParStmtBlock idL idR)

-- | The fail operator
--
-- This is used for `.. <-` "bind statements" in do notation, including
-- non-monadic "binds" in applicative.
--
-- The fail operator is 'Just expr' if it potentially fail monadically. if the
-- pattern match cannot fail, or shouldn't fail monadically (regular incomplete
-- pattern exception), it is 'Nothing'.
--
-- See Note [Monad fail : Rebindable syntax, overloaded strings] for the type of
-- expression in the 'Just' case, and why it is so.
--
-- See Note [Failing pattern matches in Stmts] for which contexts for
-- '@BindStmt@'s should use the monadic fail and which shouldn't.
type FailOperator id = Maybe (SyntaxExpr id)

-- | Applicative Argument
data ApplicativeArg idL
  = ApplicativeArgOne      -- A single statement (BindStmt or BodyStmt)
    { xarg_app_arg_one  :: XApplicativeArgOne idL
      -- ^ The fail operator, after renaming
      --
      -- The fail operator is needed if this is a BindStmt
      -- where the pattern can fail. E.g.:
      -- (Just a) <- stmt
      -- The fail operator will be invoked if the pattern
      -- match fails.
      -- It is also used for guards in MonadComprehensions.
      -- The fail operator is Nothing
      -- if the pattern match can't fail
    , app_arg_pattern   :: LPat idL -- WildPat if it was a BodyStmt (see below)
    , arg_expr          :: LHsExpr idL
    , is_body_stmt      :: Bool
      -- ^ True <=> was a BodyStmt,
      -- False <=> was a BindStmt.
      -- See Note [Applicative BodyStmt]
    }
  | ApplicativeArgMany     -- do { stmts; return vars }
    { xarg_app_arg_many :: XApplicativeArgMany idL
    , app_stmts         :: [ExprLStmt idL] -- stmts
    , final_expr        :: HsExpr idL    -- return (v1,..,vn), or just (v1,..,vn)
    , bv_pattern        :: LPat idL      -- (v1,...,vn)
    , stmt_context      :: HsDoFlavour
      -- ^ context of the do expression, used in pprArg
    }
  | XApplicativeArg !(XXApplicativeArg idL)

{-
Note [The type of bind in Stmts]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Some Stmts, notably BindStmt, keep the (>>=) bind operator.
We do NOT assume that it has type
    (>>=) :: m a -> (a -> m b) -> m b
In some cases (see #303, #1537) it might have a more
exotic type, such as
    (>>=) :: m i j a -> (a -> m j k b) -> m i k b
So we must be careful not to make assumptions about the type.
In particular, the monad may not be uniform throughout.

Note [TransStmt binder map]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
The [(idR,idR)] in a TransStmt behaves as follows:

  * Before renaming: []

  * After renaming:
          [ (x27,x27), ..., (z35,z35) ]
    These are the variables
       bound by the stmts to the left of the 'group'
       and used either in the 'by' clause,
                or     in the stmts following the 'group'
    Each item is a pair of identical variables.

  * After typechecking:
          [ (x27:Int, x27:[Int]), ..., (z35:Bool, z35:[Bool]) ]
    Each pair has the same unique, but different *types*.

Note [BodyStmt]
~~~~~~~~~~~~~~~
BodyStmts are a bit tricky, because what they mean
depends on the context.  Consider the following contexts:

        A do expression of type (m res_ty)
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        * BodyStmt E any_ty:   do { ....; E; ... }
                E :: m any_ty
          Translation: E >> ...

        A list comprehensions of type [elt_ty]
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        * BodyStmt E Bool:   [ .. | .... E ]
                        [ .. | ..., E, ... ]
                        [ .. | .... | ..., E | ... ]
                E :: Bool
          Translation: if E then fail else ...

        A guard list, guarding a RHS of type rhs_ty
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        * BodyStmt E BooParStmtBlockl:   f x | ..., E, ... = ...rhs...
                E :: Bool
          Translation: if E then fail else ...

        A monad comprehension of type (m res_ty)
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        * BodyStmt E Bool:   [ .. | .... E ]
                E :: Bool
          Translation: guard E >> ...

Array comprehensions are handled like list comprehensions.

Note [How RecStmt works]
~~~~~~~~~~~~~~~~~~~~~~~~
Example:
   HsDo [ BindStmt x ex

        , RecStmt { recS_rec_ids   = [a, c]
                  , recS_stmts     = [ BindStmt b (return (a,c))
                                     , LetStmt a = ...b...
                                     , BindStmt c ec ]
                  , recS_later_ids = [a, b]

        , return (a b) ]

Here, the RecStmt binds a,b,c; but
  - Only a,b are used in the stmts *following* the RecStmt,
  - Only a,c are used in the stmts *inside* the RecStmt
        *before* their bindings

Why do we need *both* rec_ids and later_ids?  For monads they could be
combined into a single set of variables, but not for arrows.  That
follows from the types of the respective feedback operators:

        mfix :: MonadFix m => (a -> m a) -> m a
        loop :: ArrowLoop a => a (b,d) (c,d) -> a b c

* For mfix, the 'a' covers the union of the later_ids and the rec_ids
* For 'loop', 'c' is the later_ids and 'd' is the rec_ids

Note [Typing a RecStmt]
~~~~~~~~~~~~~~~~~~~~~~~
A (RecStmt stmts) types as if you had written

  (v1,..,vn, _, ..., _) <- mfix (\~(_, ..., _, r1, ..., rm) ->
                                 do { stmts
                                    ; return (v1,..vn, r1, ..., rm) })

where v1..vn are the later_ids
      r1..rm are the rec_ids

Note [Monad Comprehensions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
Monad comprehensions require separate functions like 'return' and
'>>=' for desugaring. These functions are stored in the statements
used in monad comprehensions. For example, the 'return' of the 'LastStmt'
expression is used to lift the body of the monad comprehension:

  [ body | stmts ]
   =>
  stmts >>= \bndrs -> return body

In transform and grouping statements ('then ..' and 'then group ..') the
'return' function is required for nested monad comprehensions, for example:

  [ body | stmts, then f, rest ]
   =>
  f [ env | stmts ] >>= \bndrs -> [ body | rest ]

BodyStmts require the 'Control.Monad.guard' function for boolean
expressions:

  [ body | exp, stmts ]
   =>
  guard exp >> [ body | stmts ]

Parallel statements require the 'Control.Monad.Zip.mzip' function:

  [ body | stmts1 | stmts2 | .. ]
   =>
  mzip stmts1 (mzip stmts2 (..)) >>= \(bndrs1, (bndrs2, ..)) -> return body

In any other context than 'MonadComp', the fields for most of these
'SyntaxExpr's stay bottom.


Note [Applicative BodyStmt]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
(#12143) For the purposes of ApplicativeDo, we treat any BodyStmt
as if it was a BindStmt with a wildcard pattern.  For example,

  do
    x <- A
    B
    return x

is transformed as if it were

  do
    x <- A
    _ <- B
    return x

so it transforms to

  (\(x,_) -> x) <$> A <*> B

But we have to remember when we treat a BodyStmt like a BindStmt,
because in error messages we want to emit the original syntax the user
wrote, not our internal representation.  So ApplicativeArgOne has a
Bool flag that is True when the original statement was a BodyStmt, so
that we can pretty-print it correctly.
-}


{-
************************************************************************
*                                                                      *
                Template Haskell quotation brackets
*                                                                      *
************************************************************************
-}

{-
Note [Quasi-quote overview]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
The "quasi-quote" extension is described by Geoff Mainland's paper
"Why it's nice to be quoted: quasiquoting for Haskell" (Haskell
Workshop 2007).

Briefly, one writes
        [p| stuff |]
and the arbitrary string "stuff" gets parsed by the parser 'p', whose type
should be Language.Haskell.TH.Quote.QuasiQuoter.  'p' must be defined in
another module, because we are going to run it here.  It's a bit like an
/untyped/ TH splice where the parser is applied the "stuff" as a string, thus:
     $(p "stuff")

Notice that it's an /untyped/ TH splice: it can occur in patterns and types, as well
as in expressions; and it runs in the renamer.
-}

-- | Haskell Splice
data HsUntypedSplice id
   = HsUntypedSpliceExpr --  $z  or $(f 4)
        (XUntypedSpliceExpr id)
        (LHsExpr id)

   | HsQuasiQuote            -- See Note [Quasi-quote overview]
        (XQuasiQuote id)
        (IdP id)             -- The quoter (the bit between `[` and `|`)
        (XRec id FastString) -- The enclosed string

   | XUntypedSplice !(XXUntypedSplice id) -- Extension point; see Note [Trees That Grow]
                                          -- in Language.Haskell.Syntax.Extension

-- | Haskell (Untyped) Quote = Expr + Pat + Type + Var
data HsQuote p
  = ExpBr  (XExpBr p)   (LHsExpr p)   -- [|  expr  |]
  | PatBr  (XPatBr p)   (LPat p)      -- [p| pat   |]
  | DecBrL (XDecBrL p)  [LHsDecl p]   -- [d| decls |]; result of parser
  | DecBrG (XDecBrG p)  (HsGroup p)   -- [d| decls |]; result of renamer
  | TypBr  (XTypBr p)   (LHsType p)   -- [t| type  |]
  | VarBr  (XVarBr p)   Bool (LIdP p) -- True: 'x, False: ''T
  | XQuote !(XXQuote p) -- Extension point; see Note [Trees That Grow]
                        -- in Language.Haskell.Syntax.Extension

{-
************************************************************************
*                                                                      *
\subsection{Enumerations and list comprehensions}
*                                                                      *
************************************************************************
-}

-- | Arithmetic Sequence Information
data ArithSeqInfo id
  = From            (LHsExpr id)
  | FromThen        (LHsExpr id)
                    (LHsExpr id)
  | FromTo          (LHsExpr id)
                    (LHsExpr id)
  | FromThenTo      (LHsExpr id)
                    (LHsExpr id)
                    (LHsExpr id)
-- AZ: Should ArithSeqInfo have a TTG extension?

{-
************************************************************************
*                                                                      *
\subsection{HsMatchCtxt}
*                                                                      *
************************************************************************
-}

-- | Haskell Match Context
--
-- Context of a pattern match. This is more subtle than it would seem. See
-- Note [FunBind vs PatBind].
data HsMatchContext p
  = FunRhs
    -- ^ A pattern matching on an argument of a
    -- function binding
      { mc_fun        :: LIdP (NoGhcTc p)    -- ^ function binder of @f@
                                             -- See Note [mc_fun field of FunRhs]
                                             -- See #20415 for a long discussion about
                                             -- this field and why it uses NoGhcTc.
      , mc_fixity     :: LexicalFixity -- ^ fixing of @f@
      , mc_strictness :: SrcStrictness -- ^ was @f@ banged?
                                       -- See Note [FunBind vs PatBind]
      }
  | LambdaExpr                  -- ^Patterns of a lambda
  | CaseAlt                     -- ^Patterns and guards in a case alternative
  | LamCaseAlt LamCaseVariant   -- ^Patterns and guards in @\case@ and @\cases@
  | IfAlt                       -- ^Guards of a multi-way if alternative
  | ArrowMatchCtxt              -- ^A pattern match inside arrow notation
      HsArrowMatchContext
  | PatBindRhs                  -- ^A pattern binding  eg [y] <- e = e
  | PatBindGuards               -- ^Guards of pattern bindings, e.g.,
                                --    (Just b) | Just _ <- x = e
                                --             | otherwise   = e'

  | RecUpd                      -- ^Record update [used only in GHC.HsToCore.Expr to
                                --    tell matchWrapper what sort of
                                --    runtime error message to generate]

  | StmtCtxt (HsStmtContext p)  -- ^Pattern of a do-stmt, list comprehension,
                                -- pattern guard, etc

  | ThPatSplice            -- ^A Template Haskell pattern splice
  | ThPatQuote             -- ^A Template Haskell pattern quotation [p| (a,b) |]
  | PatSyn                 -- ^A pattern synonym declaration

{-
Note [mc_fun field of FunRhs]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The mc_fun field of FunRhs has type `LIdP (NoGhcTc p)`, which means it will be
a `RdrName` in pass `GhcPs`, a `Name` in `GhcRn`, and (importantly) still a
`Name` in `GhcTc` -- not an `Id`.  See Note [NoGhcTc] in GHC.Hs.Extension.

Why a `Name` in the typechecker phase?  Because:
* A `Name` is all we need, as it turns out.
* Using an `Id` involves knot-tying in the monad, which led to #22695.

See #20415 for a long discussion.

-}

isPatSynCtxt :: HsMatchContext p -> Bool
isPatSynCtxt ctxt =
  case ctxt of
    PatSyn -> True
    _      -> False

-- | Haskell Statement Context.
data HsStmtContext p
  = HsDoStmt HsDoFlavour             -- ^Context for HsDo (do-notation and comprehensions)
  | PatGuard (HsMatchContext p)      -- ^Pattern guard for specified thing
  | ParStmtCtxt (HsStmtContext p)    -- ^A branch of a parallel stmt
  | TransStmtCtxt (HsStmtContext p)  -- ^A branch of a transform stmt
  | ArrowExpr                        -- ^do-notation in an arrow-command context

-- | Haskell arrow match context.
data HsArrowMatchContext
  = ProcExpr                       -- ^ A proc expression
  | ArrowCaseAlt                   -- ^ A case alternative inside arrow notation
  | ArrowLamCaseAlt LamCaseVariant -- ^ A \case or \cases alternative inside arrow notation
  | KappaExpr                      -- ^ An arrow kappa abstraction

data HsDoFlavour
  = DoExpr (Maybe ModuleName)        -- ^[ModuleName.]do { ... }
  | MDoExpr (Maybe ModuleName)       -- ^[ModuleName.]mdo { ... }  ie recursive do-expression
  | GhciStmtCtxt                     -- ^A command-line Stmt in GHCi pat <- rhs
  | ListComp
  | MonadComp

qualifiedDoModuleName_maybe :: HsStmtContext p -> Maybe ModuleName
qualifiedDoModuleName_maybe ctxt = case ctxt of
  HsDoStmt (DoExpr m) -> m
  HsDoStmt (MDoExpr m) -> m
  _ -> Nothing

isComprehensionContext :: HsStmtContext id -> Bool
-- Uses comprehension syntax [ e | quals ]
isComprehensionContext (ParStmtCtxt c)   = isComprehensionContext c
isComprehensionContext (TransStmtCtxt c) = isComprehensionContext c
isComprehensionContext ArrowExpr = False
isComprehensionContext (PatGuard _) = False
isComprehensionContext (HsDoStmt flavour) = isDoComprehensionContext flavour

isDoComprehensionContext :: HsDoFlavour -> Bool
isDoComprehensionContext GhciStmtCtxt = False
isDoComprehensionContext (DoExpr _) = False
isDoComprehensionContext (MDoExpr _) = False
isDoComprehensionContext ListComp = True
isDoComprehensionContext MonadComp = True

-- | Is this a monadic context?
isMonadStmtContext :: HsStmtContext id -> Bool
isMonadStmtContext (ParStmtCtxt ctxt)   = isMonadStmtContext ctxt
isMonadStmtContext (TransStmtCtxt ctxt) = isMonadStmtContext ctxt
isMonadStmtContext (HsDoStmt flavour) = isMonadDoStmtContext flavour
isMonadStmtContext (PatGuard _) = False
isMonadStmtContext ArrowExpr = False

isMonadDoStmtContext :: HsDoFlavour -> Bool
isMonadDoStmtContext ListComp     = False
isMonadDoStmtContext MonadComp    = True
isMonadDoStmtContext DoExpr{}     = True
isMonadDoStmtContext MDoExpr{}    = True
isMonadDoStmtContext GhciStmtCtxt = True

isMonadCompContext :: HsStmtContext id -> Bool
isMonadCompContext (HsDoStmt flavour)   = isMonadDoCompContext flavour
isMonadCompContext (ParStmtCtxt _)   = False
isMonadCompContext (TransStmtCtxt _) = False
isMonadCompContext (PatGuard _)      = False
isMonadCompContext ArrowExpr         = False

isMonadDoCompContext :: HsDoFlavour -> Bool
isMonadDoCompContext MonadComp    = True
isMonadDoCompContext ListComp     = False
isMonadDoCompContext GhciStmtCtxt = False
isMonadDoCompContext (DoExpr _)   = False
isMonadDoCompContext (MDoExpr _)  = False