path: root/compiler/GHC/Tc/TyCl/Build.hs

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



{-# OPTIONS_GHC -Wno-incomplete-uni-patterns #-}

module GHC.Tc.TyCl.Build (
        buildDataCon,
        buildPatSyn,
        TcMethInfo, MethInfo, buildClass,
        mkNewTyConRhs,
        newImplicitBinder, newTyConRepName
    ) where

import GHC.Prelude

import GHC.Iface.Env
import GHC.Builtin.Types( isCTupleTyConName, unboxedUnitTy )

import GHC.Tc.Utils.TcType
import GHC.Tc.Utils.Monad

import GHC.Core.DataCon
import GHC.Core.PatSyn
import GHC.Core.Class
import GHC.Core.TyCon
import GHC.Core.Type
import GHC.Core.Multiplicity
import GHC.Core.FamInstEnv( FamInstEnvs, mkNewTypeCoAxiom )

import GHC.Types.SrcLoc( SrcSpan, noSrcSpan )
import GHC.Types.Var
import GHC.Types.Var.Set
import GHC.Types.Basic
import GHC.Types.Name
import GHC.Types.Name.Env
import GHC.Types.Id.Make
import GHC.Types.SourceText
import GHC.Types.Unique.Supply

import GHC.Utils.Misc
import GHC.Utils.Outputable
import GHC.Utils.Panic


mkNewTyConRhs :: Name -> TyCon -> DataCon -> TcRnIf m n AlgTyConRhs
-- ^ Monadic because it makes a Name for the coercion TyCon
--   We pass the Name of the parent TyCon, as well as the TyCon itself,
--   because the latter is part of a knot, whereas the former is not.
mkNewTyConRhs tycon_name tycon con
  = do  { co_tycon_name <- newImplicitBinder tycon_name mkNewTyCoOcc
        ; let nt_ax = mkNewTypeCoAxiom co_tycon_name tycon etad_tvs etad_roles etad_rhs
        ; traceIf (text "mkNewTyConRhs" <+> ppr nt_ax)
        ; return (NewTyCon { data_con     = con,
                             nt_rhs       = rhs_ty,
                             nt_etad_rhs  = (etad_tvs, etad_rhs),
                             nt_co        = nt_ax,
                             nt_fixed_rep = isFixedRuntimeRepKind res_kind } ) }
                             -- Coreview looks through newtypes with a Nothing
                             -- for nt_co, or uses explicit coercions otherwise
  where
    tvs      = tyConTyVars tycon
    roles    = tyConRoles tycon
    res_kind = tyConResKind tycon
    con_arg_ty = case dataConRepArgTys con of
                   [arg_ty] -> scaledThing arg_ty
                   tys -> pprPanic "mkNewTyConRhs" (ppr con <+> ppr tys)
    rhs_ty = substTyWith (dataConUnivTyVars con)
                         (mkTyVarTys tvs) con_arg_ty
        -- Instantiate the newtype's RHS with the
        -- type variables from the tycon
        -- NB: a newtype DataCon has a type that must look like
        --        forall tvs.  <arg-ty> -> T tvs
        -- Note that we *can't* use dataConInstOrigArgTys here because
        -- the newtype arising from   class Foo a => Bar a where {}
        -- has a single argument (Foo a) that is a *type class*, so
        -- dataConInstOrigArgTys returns [].

    -- Eta-reduce the newtype
    -- See Note [Newtype eta] in GHC.Core.TyCon
    etad_tvs   :: [TyVar]  -- Matched lazily, so that mkNewTypeCo can
    etad_roles :: [Role]   -- return a TyCon without pulling on rhs_ty
    etad_rhs   :: Type     -- See Note [Tricky iface loop] in GHC.Iface.Load
    (etad_tvs, etad_roles, etad_rhs) = eta_reduce (reverse tvs) (reverse roles) rhs_ty

    eta_reduce :: [TyVar]       -- Reversed
               -> [Role]        -- also reversed
               -> Type          -- Rhs type
               -> ([TyVar], [Role], Type)  -- Eta-reduced version
                                           -- (tyvars in normal order)
    eta_reduce (a:as) (_:rs) ty
      | Just (fun, arg) <- splitAppTy_maybe ty
      , Just tv <- getTyVar_maybe arg
      , tv == a
      , not (a `elemVarSet` tyCoVarsOfType fun)
      , typeKind fun `eqType` typeKind (mkTyConApp tycon (mkTyVarTys $ reverse as))
        -- Why this kind-check?  See Note [Newtype eta and homogeneous axioms]
      = eta_reduce as rs fun
    eta_reduce as rs ty = (reverse as, reverse rs, ty)

{- Note [Newtype eta and homogeneous axioms]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When eta-reducing a newtype, we must be careful to make sure the axiom
is homogeneous.  (That is, the two types related by the axiom must
have the same kind.)  All known proofs of type safety for Core rely on
the homogeneity of axioms, so let's not monkey with that.

It is easy to mistakenly make an inhomogeneous axiom (#19739):
  type T :: forall (a :: Type) -> Type
  newtype T a = MkT (Proxy a)

Can we eta-reduce, thus?
  axT :: T ~ Proxy

No!  Because
   T     :: forall a -> Type
   Proxy :: Type     -> Type

This is inhomogeneous. Hence, we have an extra kind-check in eta_reduce,
to make sure the reducts have the same kind. This is simple, although
perhaps quadratic in complexity, if we eta-reduce many arguments (which
seems vanishingly unlikely in practice).  But NB that the free-variable
check, which immediately precedes the kind check, is also potentially
quadratic.

There are other ways we could do the check (discussion on #19739):

* We could look at the sequence of binders on the newtype and on the
  head of the representation type, and make sure the visibilities on
  the binders match up. This is quite a bit more code, and the reasoning
  is subtler.

* We could, say, do the kind-check at the end and then backtrack until the
  kinds match up. Hard to know whether that's more performant or not.
-}

------------------------------------------------------
buildDataCon :: FamInstEnvs
            -> DataConBangOpts
            -> Name
            -> Bool                     -- Declared infix
            -> TyConRepName
            -> [HsSrcBang]
           -> [FieldLabel]             -- Field labels
           -> [TyVar]                  -- Universals
           -> [TyCoVar]                -- Existentials
           -> [InvisTVBinder]          -- User-written 'TyVarBinder's
           -> [EqSpec]                 -- Equality spec
           -> KnotTied ThetaType       -- Does not include the "stupid theta"
                                       -- or the GADT equalities
           -> [KnotTied (Scaled Type)] -- Arguments
           -> KnotTied Type            -- Result types
           -> KnotTied TyCon           -- Rep tycon
           -> NameEnv ConTag           -- Maps the Name of each DataCon to its
                                       -- ConTag
           -> TcRnIf m n DataCon
-- A wrapper for DataCon.mkDataCon that
--   a) makes the worker Id
--   b) makes the wrapper Id if necessary, including
--      allocating its unique (hence monadic)
buildDataCon fam_envs dc_bang_opts src_name declared_infix prom_info src_bangs
             field_lbls univ_tvs ex_tvs user_tvbs eq_spec ctxt arg_tys res_ty
             rep_tycon tag_map
  = do  { wrap_name <- newImplicitBinder src_name mkDataConWrapperOcc
        ; work_name <- newImplicitBinder src_name mkDataConWorkerOcc
        -- This last one takes the name of the data constructor in the source
        -- code, which (for Haskell source anyway) will be in the DataName name
        -- space, and puts it into the VarName name space

        ; traceIf (text "buildDataCon 1" <+> ppr src_name)
        ; us <- newUniqueSupply
        ; let stupid_ctxt = mkDataConStupidTheta rep_tycon (map scaledThing arg_tys) univ_tvs
              tag = lookupNameEnv_NF tag_map src_name
              -- See Note [Constructor tag allocation], fixes #14657
              data_con = mkDataCon src_name declared_infix prom_info
                                   src_bangs field_lbls
                                   univ_tvs ex_tvs user_tvbs eq_spec ctxt
                                   arg_tys res_ty NoPromInfo rep_tycon tag
                                   stupid_ctxt dc_wrk dc_rep
              dc_wrk = mkDataConWorkId work_name data_con
              dc_rep = initUs_ us (mkDataConRep dc_bang_opts fam_envs wrap_name data_con)

        ; traceIf (text "buildDataCon 2" <+> ppr src_name)
        ; return data_con }


-- The stupid context for a data constructor should be limited to
-- the type variables mentioned in the arg_tys
-- ToDo: Or functionally dependent on?
--       This whole stupid theta thing is, well, stupid.
--
-- See Note [The stupid context] in GHC.Core.DataCon.
mkDataConStupidTheta :: TyCon -> [Type] -> [TyVar] -> [PredType]
mkDataConStupidTheta tycon arg_tys univ_tvs
  | null stupid_theta = []      -- The common case
  | otherwise         = filter in_arg_tys stupid_theta
  where
    tc_subst     = zipTvSubst (tyConTyVars tycon)
                              (mkTyVarTys univ_tvs)
    stupid_theta = substTheta tc_subst (tyConStupidTheta tycon)
        -- Start by instantiating the master copy of the
        -- stupid theta, taken from the TyCon

    arg_tyvars      = tyCoVarsOfTypes arg_tys
    in_arg_tys pred = tyCoVarsOfType pred `intersectsVarSet` arg_tyvars


------------------------------------------------------
buildPatSyn :: Name -> Bool
            -> PatSynMatcher -> PatSynBuilder
            -> ([InvisTVBinder], ThetaType) -- ^ Univ and req
            -> ([InvisTVBinder], ThetaType) -- ^ Ex and prov
            -> [FRRType]                    -- ^ Argument types
            -> Type                         -- ^ Result type
            -> [FieldLabel]                 -- ^ Field labels for
                                            --   a record pattern synonym
            -> PatSyn
buildPatSyn src_name declared_infix matcher@(_, matcher_ty,_) builder
            (univ_tvs, req_theta) (ex_tvs, prov_theta) arg_tys
            pat_ty field_labels
  = -- The assertion checks that the matcher is
    -- compatible with the pattern synonym
    assertPpr (and [ univ_tvs `equalLength` univ_tvs1
                   , ex_tvs `equalLength` ex_tvs1
                   , pat_ty `eqType` substTy subst (scaledThing pat_ty1)
                   , prov_theta `eqTypes` substTys subst prov_theta1
                   , req_theta `eqTypes` substTys subst req_theta1
                   , compareArgTys arg_tys (substTys subst (map scaledThing arg_tys1))
                   ])
              (vcat [ ppr univ_tvs <+> twiddle <+> ppr univ_tvs1
                    , ppr ex_tvs <+> twiddle <+> ppr ex_tvs1
                    , ppr pat_ty <+> twiddle <+> ppr pat_ty1
                    , ppr prov_theta <+> twiddle <+> ppr prov_theta1
                    , ppr req_theta <+> twiddle <+> ppr req_theta1
                    , ppr arg_tys <+> twiddle <+> ppr arg_tys1]) $
    mkPatSyn src_name declared_infix
             (univ_tvs, req_theta) (ex_tvs, prov_theta)
             arg_tys pat_ty
             matcher builder field_labels
  where
    ((_:_:univ_tvs1), req_theta1, tau) = tcSplitSigmaTy $ matcher_ty
    ([pat_ty1, cont_sigma, _], _)      = tcSplitFunTys tau
    (ex_tvs1, prov_theta1, cont_tau)   = tcSplitSigmaTy (scaledThing cont_sigma)
    (arg_tys1, _) = (tcSplitFunTys cont_tau)
    twiddle = char '~'
    subst = zipTvSubst (univ_tvs1 ++ ex_tvs1)
                       (mkTyVarTys (binderVars (univ_tvs ++ ex_tvs)))

    -- For a nullary pattern synonym we add a single (# #) argument to the
    -- matcher to preserve laziness in the case of unlifted types.
    -- See #12746
    compareArgTys :: [Type] -> [Type] -> Bool
    compareArgTys [] [x] = x `eqType` unboxedUnitTy
    compareArgTys arg_tys matcher_arg_tys = arg_tys `eqTypes` matcher_arg_tys


------------------------------------------------------
type TcMethInfo = MethInfo  -- this variant needs zonking
type MethInfo       -- A temporary intermediate, to communicate
                    -- between tcClassSigs and buildClass.
  = ( Name   -- Name of the class op
    , Type   -- Type of the class op
    , Maybe (DefMethSpec (SrcSpan, Type)))
         -- Nothing                    => no default method
         --
         -- Just VanillaDM             => There is an ordinary
         --                               polymorphic default method
         --
         -- Just (GenericDM (loc, ty)) => There is a generic default metho
         --                               Here is its type, and the location
         --                               of the type signature
         --    We need that location /only/ to attach it to the
         --    generic default method's Name; and we need /that/
         --    only to give the right location of an ambiguity error
         --    for the generic default method, spat out by checkValidClass

buildClass :: Name  -- Name of the class/tycon (they have the same Name)
           -> [TyConBinder]                -- Of the tycon
           -> [Role]
           -> [FunDep TyVar]               -- Functional dependencies
           -- Super classes, associated types, method info, minimal complete def.
           -- This is Nothing if the class is abstract.
           -> Maybe (KnotTied ThetaType, [ClassATItem], [KnotTied MethInfo], ClassMinimalDef)
           -> TcRnIf m n Class

buildClass tycon_name binders roles fds Nothing
  = fixM  $ \ rec_clas ->       -- Only name generation inside loop
    do  { traceIf (text "buildClass")

        ; tc_rep_name  <- newTyConRepName tycon_name
        ; let univ_tvs = binderVars binders
              tycon = mkClassTyCon tycon_name binders roles
                                   AbstractTyCon
                                   rec_clas tc_rep_name
              result = mkAbstractClass tycon_name univ_tvs fds tycon
        ; traceIf (text "buildClass" <+> ppr tycon)
        ; return result }

buildClass tycon_name binders roles fds
           (Just (sc_theta, at_items, sig_stuff, mindef))
  = fixM  $ \ rec_clas ->       -- Only name generation inside loop
    do  { traceIf (text "buildClass")

        ; datacon_name <- newImplicitBinder tycon_name mkClassDataConOcc
        ; tc_rep_name  <- newTyConRepName tycon_name

        ; op_items <- mapM (mk_op_item rec_clas) sig_stuff
                        -- Build the selector id and default method id

              -- Make selectors for the superclasses
        ; sc_sel_names <- mapM  (newImplicitBinder tycon_name . mkSuperDictSelOcc)
                                (takeList sc_theta [fIRST_TAG..])
        ; let sc_sel_ids = [ mkDictSelId sc_name rec_clas
                           | sc_name <- sc_sel_names]
              -- We number off the Dict superclass selectors, 1, 2, 3 etc so that we
              -- can construct names for the selectors. Thus
              --      class (C a, C b) => D a b where ...
              -- gives superclass selectors
              --      D_sc1, D_sc2
              -- (We used to call them D_C, but now we can have two different
              --  superclasses both called C!)

        ; let use_newtype = isSingleton (sc_theta ++ op_tys)
                -- Use a newtype if the data constructor
                --   (a) has exactly one value field
                --       i.e. exactly one operation or superclass taken together
                --   (b) that value is of lifted type (which they always are, because
                --       we box equality superclasses)
                -- See Note [Class newtypes and equality predicates]
                --
                -- In the case of
                --     class C a => D a
                -- we use a newtype, but with one superclass and no arguments
              args       = sc_sel_names ++ op_names
              op_tys     = [ty | (_,ty,_) <- sig_stuff]
              op_names   = [op | (op,_,_) <- sig_stuff]
              rec_tycon  = classTyCon rec_clas
              univ_bndrs = tyConInvisTVBinders binders
              univ_tvs   = binderVars univ_bndrs
              bang_opts  = FixedBangOpts (map (const HsLazy) args)

        ; rep_nm   <- newTyConRepName datacon_name
        ; dict_con <- buildDataCon (panic "buildClass: FamInstEnvs")
                                   bang_opts
                                   datacon_name
                                   False        -- Not declared infix
                                   rep_nm
                                   (map (const no_bang) args)
                                   [{- No fields -}]
                                   univ_tvs
                                   [{- no existentials -}]
                                   univ_bndrs
                                   [{- No GADT equalities -}]
                                   sc_theta
                                   (map unrestricted op_tys) -- type classes are unrestricted
                                   (mkTyConApp rec_tycon (mkTyVarTys univ_tvs))
                                   rec_tycon
                                   (mkTyConTagMap rec_tycon)

        ; rhs <- case () of
                  _ | use_newtype
                    -> mkNewTyConRhs tycon_name rec_tycon dict_con
                    | isCTupleTyConName tycon_name
                    -> return (TupleTyCon { data_con = dict_con
                                          , tup_sort = ConstraintTuple })
                    | otherwise
                    -> return (mkDataTyConRhs [dict_con])

        ; let { tycon = mkClassTyCon tycon_name binders roles
                                     rhs rec_clas tc_rep_name
                -- A class can be recursive, and in the case of newtypes
                -- this matters.  For example
                --      class C a where { op :: C b => a -> b -> Int }
                -- Because C has only one operation, it is represented by
                -- a newtype, and it should be a *recursive* newtype.
                -- [If we don't make it a recursive newtype, we'll expand the
                -- newtype like a synonym, but that will lead to an infinite
                -- type]

              ; result = mkClass tycon_name univ_tvs fds
                                 sc_theta sc_sel_ids at_items
                                 op_items mindef tycon
              }
        ; traceIf (text "buildClass" <+> ppr tycon)
        ; return result }
  where
    no_bang = HsSrcBang NoSourceText NoSrcUnpack NoSrcStrict

    mk_op_item :: Class -> TcMethInfo -> TcRnIf n m ClassOpItem
    mk_op_item rec_clas (op_name, _, dm_spec)
      = do { dm_info <- mk_dm_info op_name dm_spec
           ; return (mkDictSelId op_name rec_clas, dm_info) }

    mk_dm_info :: Name -> Maybe (DefMethSpec (SrcSpan, Type))
               -> TcRnIf n m (Maybe (Name, DefMethSpec Type))
    mk_dm_info _ Nothing
      = return Nothing
    mk_dm_info op_name (Just VanillaDM)
      = do { dm_name <- newImplicitBinder op_name mkDefaultMethodOcc
           ; return (Just (dm_name, VanillaDM)) }
    mk_dm_info op_name (Just (GenericDM (loc, dm_ty)))
      = do { dm_name <- newImplicitBinderLoc op_name mkDefaultMethodOcc loc
           ; return (Just (dm_name, GenericDM dm_ty)) }

{-
Note [Class newtypes and equality predicates]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
        class (a ~ F b) => C a b where
          op :: a -> b

We cannot represent this by a newtype, even though it's not
existential, because there are two value fields (the equality
predicate and op. See #2238

Moreover,
          class (a ~ F b) => C a b where {}
Here we can't use a newtype either, even though there is only
one field, because equality predicates are unboxed, and classes
are boxed.
-}

newImplicitBinder :: Name                       -- Base name
                  -> (OccName -> OccName)       -- Occurrence name modifier
                  -> TcRnIf m n Name            -- Implicit name
-- Called in GHC.Tc.TyCl.Build to allocate the implicit binders of type/class decls
-- For source type/class decls, this is the first occurrence
-- For iface ones, GHC.Iface.Load has already allocated a suitable name in the cache
newImplicitBinder base_name mk_sys_occ
  = newImplicitBinderLoc base_name mk_sys_occ (nameSrcSpan base_name)

newImplicitBinderLoc :: Name                       -- Base name
                     -> (OccName -> OccName)       -- Occurrence name modifier
                     -> SrcSpan
                     -> TcRnIf m n Name            -- Implicit name
-- Just the same, but lets you specify the SrcSpan
newImplicitBinderLoc base_name mk_sys_occ loc
  | Just mod <- nameModule_maybe base_name
  = newGlobalBinder mod occ loc
  | otherwise           -- When typechecking a [d| decl bracket |],
                        -- TH generates types, classes etc with Internal names,
                        -- so we follow suit for the implicit binders
  = do  { uniq <- newUnique
        ; return (mkInternalName uniq occ loc) }
  where
    occ = mk_sys_occ (nameOccName base_name)

-- | Make the 'TyConRepName' for this 'TyCon'
newTyConRepName :: Name -> TcRnIf gbl lcl TyConRepName
newTyConRepName tc_name
  | Just mod <- nameModule_maybe tc_name
  , (mod, occ) <- tyConRepModOcc mod (nameOccName tc_name)
  = newGlobalBinder mod occ noSrcSpan
  | otherwise
  = newImplicitBinder tc_name mkTyConRepOcc