% % (c) The University of Glasgow 2006 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 % \section[TcType]{Types used in the typechecker} This module provides the Type interface for front-end parts of the compiler. These parts * treat "source types" as opaque: newtypes, and predicates are meaningful. * look through usage types The "tc" prefix is for "TypeChecker", because the type checker is the principal client. \begin{code}

    module TcType (
      --------------------------------
      -- Types
      TcType, TcSigmaType, TcRhoType, TcTauType, TcPredType, TcThetaType,
      TcTyVar, TcTyVarSet, TcKind, TcCoVar,

      --------------------------------
      -- MetaDetails
      UserTypeCtxt(..), pprUserTypeCtxt,
      TcTyVarDetails(..), pprTcTyVarDetails, vanillaSkolemTv, superSkolemTv,
      MetaDetails(Flexi, Indirect), MetaInfo(..),
      isImmutableTyVar, isSkolemTyVar, isMetaTyVar,  isMetaTyVarTy,
      isSigTyVar, isOverlappableTyVar,  isTyConableTyVar,
      metaTvRef,
      isFlexi, isIndirect, isRuntimeUnkSkol,

      --------------------------------
      -- Builders
      mkPhiTy, mkSigmaTy,

      --------------------------------
      -- Splitters
      -- These are important because they do not look through newtypes
      tcView,
      tcSplitForAllTys, tcSplitPhiTy, tcSplitPredFunTy_maybe,
      tcSplitFunTy_maybe, tcSplitFunTys, tcFunArgTy, tcFunResultTy, tcSplitFunTysN,
      tcSplitTyConApp, tcSplitTyConApp_maybe, tcTyConAppTyCon, tcTyConAppArgs,
      tcSplitAppTy_maybe, tcSplitAppTy, tcSplitAppTys, repSplitAppTy_maybe,
      tcInstHeadTyNotSynonym, tcInstHeadTyAppAllTyVars,
      tcGetTyVar_maybe, tcGetTyVar,
      tcSplitSigmaTy, tcDeepSplitSigmaTy_maybe,

      ---------------------------------
      -- Predicates.
      -- Again, newtypes are opaque
      tcEqType, tcEqTypes, tcEqPred, tcCmpType, tcCmpTypes, tcCmpPred, tcEqTypeX,
      eqKind,
      isSigmaTy, isOverloadedTy,
      isDoubleTy, isFloatTy, isIntTy, isWordTy, isStringTy,
      isIntegerTy, isBoolTy, isUnitTy, isCharTy,
      isTauTy, isTauTyCon, tcIsTyVarTy, tcIsForAllTy,
      isSynFamilyTyConApp,

      ---------------------------------
      -- Misc type manipulators
      deNoteType,
      orphNamesOfType, orphNamesOfDFunHead,
      getDFunTyKey,

      ---------------------------------
      -- Predicate types
      getClassPredTys_maybe, getClassPredTys,
      isClassPred, isTyVarClassPred, isEqPred,
      mkClassPred, mkIPPred, tcSplitPredTy_maybe,
      mkDictTy, evVarPred,
      isPredTy, isDictTy, isDictLikeTy,
      tcSplitDFunTy, tcSplitDFunHead, predTyUnique,
      isIPPred,
      mkMinimalBySCs, transSuperClasses, immSuperClasses,

      -- * Tidying type related things up for printing
      tidyType,      tidyTypes,
      tidyOpenType,  tidyOpenTypes,
      tidyTyVarBndr, tidyFreeTyVars,
      tidyOpenTyVar, tidyOpenTyVars,
      tidyTopType,   tidyPred,
      tidyKind,

      ---------------------------------
      -- Foreign import and export
      isFFIArgumentTy,     -- :: DynFlags -> Safety -> Type -> Bool
      isFFIImportResultTy, -- :: DynFlags -> Type -> Bool
      isFFIExportResultTy, -- :: Type -> Bool
      isFFIExternalTy,     -- :: Type -> Bool
      isFFIDynArgumentTy,  -- :: Type -> Bool
      isFFIDynResultTy,    -- :: Type -> Bool
      isFFIPrimArgumentTy, -- :: DynFlags -> Type -> Bool
      isFFIPrimResultTy,   -- :: DynFlags -> Type -> Bool
      isFFILabelTy,        -- :: Type -> Bool
      isFFIDotnetTy,       -- :: DynFlags -> Type -> Bool
      isFFIDotnetObjTy,    -- :: Type -> Bool
      isFFITy,	       -- :: Type -> Bool
      isFunPtrTy,          -- :: Type -> Bool
      tcSplitIOType_maybe, -- :: Type -> Maybe Type

      --------------------------------
      -- Rexported from Coercion
      typeKind,

      --------------------------------
      -- Rexported from Type
      Kind, 	-- Stuff to do with kinds is insensitive to pre/post Tc
      unliftedTypeKind, liftedTypeKind, argTypeKind,
      openTypeKind, mkArrowKind, mkArrowKinds,
      isLiftedTypeKind, isUnliftedTypeKind, isSubOpenTypeKind,
      isSubArgTypeKind, isSubKind, splitKindFunTys, defaultKind,
      kindVarRef, mkKindVar,

      Type, PredType(..), ThetaType,
      mkForAllTy, mkForAllTys,
      mkFunTy, mkFunTys, zipFunTys,
      mkTyConApp, mkAppTy, mkAppTys, applyTy, applyTys,
      mkTyVarTy, mkTyVarTys, mkTyConTy, mkPredTy, mkPredTys,

      -- Type substitutions
      TvSubst(..), 	-- Representation visible to a few friends
      TvSubstEnv, emptyTvSubst, substEqSpec,
      mkOpenTvSubst, zipOpenTvSubst, zipTopTvSubst,
      mkTopTvSubst, notElemTvSubst, unionTvSubst,
      getTvSubstEnv, setTvSubstEnv, getTvInScope, extendTvInScope, lookupTyVar,
      extendTvSubst, extendTvSubstList, isInScope, mkTvSubst, zipTyEnv,
      substTy, substTys, substTyWith, substTheta, substTyVar, substTyVars, substTyVarBndr,

      isUnLiftedType,	-- Source types are always lifted
      isUnboxedTupleType,	-- Ditto
      isPrimitiveType,

      tyVarsOfType, tyVarsOfTypes, tyVarsOfPred, tyVarsOfTheta,
      tcTyVarsOfType, tcTyVarsOfTypes, tcTyVarsOfPred, exactTyVarsOfType,
      exactTyVarsOfTypes,

      pprKind, pprParendKind,
      pprType, pprParendType, pprTypeApp, pprTyThingCategory,
      pprPred, pprTheta, pprThetaArrow, pprClassPred

      ) where

    #include "HsVersions.h"

    -- friends:
    import TypeRep
    import Class
    import Var
    import ForeignCall
    import VarSet
    import Type
    import Coercion
    import TyCon

    -- others:
    import DynFlags
    import Name
    import NameSet
    import VarEnv
    import PrelNames
    import TysWiredIn
    import BasicTypes
    import Util
    import Maybes
    import ListSetOps
    import Outputable
    import FastString

    import Data.List( mapAccumL )
    import Data.IORef

\end{code} %************************************************************************ %* * \subsection{Types} %* * %************************************************************************ The type checker divides the generic Type world into the following more structured beasts: sigma ::= forall tyvars. phi -- A sigma type is a qualified type -- -- Note that even if 'tyvars' is empty, theta -- may not be: e.g. (?x::Int) => Int -- Note that 'sigma' is in prenex form: -- all the foralls are at the front. -- A 'phi' type has no foralls to the right of -- an arrow phi :: theta => rho rho ::= sigma -> rho | tau -- A 'tau' type has no quantification anywhere -- Note that the args of a type constructor must be taus tau ::= tyvar | tycon tau_1 .. tau_n | tau_1 tau_2 | tau_1 -> tau_2 -- In all cases, a (saturated) type synonym application is legal, -- provided it expands to the required form. \begin{code}

    type TcTyVar = TyVar  	-- Used only during type inference
    type TcCoVar = CoVar  	-- Used only during type inference; mutable
    type TcType = Type 	-- A TcType can have mutable type variables
    	-- Invariant on ForAllTy in TcTypes:
    	-- 	forall a. T
    	-- a cannot occur inside a MutTyVar in T; that is,
    	-- T is "flattened" before quantifying over a

    -- These types do not have boxy type variables in them
    type TcPredType     = PredType
    type TcThetaType    = ThetaType
    type TcSigmaType    = TcType
    type TcRhoType      = TcType
    type TcTauType      = TcType
    type TcKind         = Kind
    type TcTyVarSet     = TyVarSet

\end{code} %************************************************************************ %* * \subsection{TyVarDetails} %* * %************************************************************************ TyVarDetails gives extra info about type variables, used during type checking. It's attached to mutable type variables only. It's knot-tied back to Var.lhs. There is no reason in principle why Var.lhs shouldn't actually have the definition, but it "belongs" here. Note [Signature skolems] ~~~~~~~~~~~~~~~~~~~~~~~~ Consider this x :: [a] y :: b (x,y,z) = ([y,z], z, head x) Here, x and y have type sigs, which go into the environment. We used to instantiate their types with skolem constants, and push those types into the RHS, so we'd typecheck the RHS with type ( [a*], b*, c ) where a*, b* are skolem constants, and c is an ordinary meta type varible. The trouble is that the occurrences of z in the RHS force a* and b* to be the *same*, so we can't make them into skolem constants that don't unify with each other. Alas. One solution would be insist that in the above defn the programmer uses the same type variable in both type signatures. But that takes explanation. The alternative (currently implemented) is to have a special kind of skolem constant, SigTv, which can unify with other SigTvs. These are *not* treated as righd for the purposes of GADTs. And they are used *only* for pattern bindings and mutually recursive function bindings. See the function TcBinds.tcInstSig, and its use_skols parameter. \begin{code}

    -- A TyVarDetails is inside a TyVar
    data TcTyVarDetails
      = SkolemTv      -- A skolem
           Bool       -- True <=> this skolem type variable can be overlapped
                      --          when looking up instances
                      -- See Note [Binding when looking up instances] in InstEnv

      | RuntimeUnk    -- Stands for an as-yet-unknown type in the GHCi
                      -- interactive context

      | FlatSkol TcType
               -- The "skolem" obtained by flattening during
        	   -- constraint simplification

               -- In comments we will use the notation alpha[flat = ty]
               -- to represent a flattening skolem variable alpha
               -- identified with type ty.

      | MetaTv MetaInfo (IORef MetaDetails)

    vanillaSkolemTv, superSkolemTv :: TcTyVarDetails
    -- See Note [Binding when looking up instances] in InstEnv
    vanillaSkolemTv = SkolemTv False  -- Might be instantiated
    superSkolemTv   = SkolemTv True   -- Treat this as a completely distinct type

    data MetaDetails
      = Flexi  -- Flexi type variables unify to become Indirects
      | Indirect TcType

    instance Outputable MetaDetails where
      ppr Flexi         = ptext (sLit "Flexi")
      ppr (Indirect ty) = ptext (sLit "Indirect") <+> ppr ty

    data MetaInfo
       = TauTv	   -- This MetaTv is an ordinary unification variable
         		   -- A TauTv is always filled in with a tau-type, which
    		   -- never contains any ForAlls

       | SigTv Name	   -- A variant of TauTv, except that it should not be
    		   -- unified with a type, only with a type variable
    		   -- SigTvs are only distinguished to improve error messages
    		   --      see Note [Signature skolems]
    		   --      The MetaDetails, if filled in, will
    		   --      always be another SigTv or a SkolemTv
    		   -- The Name is the name of the function from whose
    		   -- type signature we got this skolem

       | TcsTv	   -- A MetaTv allocated by the constraint solver
         		   -- Its particular property is that it is always "touchable"
    		   -- Nevertheless, the constraint solver has to try to guess
    		   -- what type to instantiate it to

    -------------------------------------
    -- UserTypeCtxt describes the origin of the polymorphic type
    -- in the places where we need to an expression has that type

    data UserTypeCtxt
      = FunSigCtxt Name	-- Function type signature
    			-- Also used for types in SPECIALISE pragmas
      | ExprSigCtxt		-- Expression type signature
      | ConArgCtxt Name	-- Data constructor argument
      | TySynCtxt Name	-- RHS of a type synonym decl
      | GenPatCtxt		-- Pattern in generic decl
    			-- 	f{| a+b |} (Inl x) = ...
      | LamPatSigCtxt		-- Type sig in lambda pattern
    			-- 	f (x::t) = ...
      | BindPatSigCtxt	-- Type sig in pattern binding pattern
    			--	(x::t, y) = e
      | ResSigCtxt		-- Result type sig
    			-- 	f x :: t = ....
      | ForSigCtxt Name	-- Foreign inport or export signature
      | DefaultDeclCtxt	-- Types in a default declaration
      | SpecInstCtxt	-- SPECIALISE instance pragma
      | ThBrackCtxt		-- Template Haskell type brackets [t| ... |]

      | GenSigCtxt          -- Higher-rank or impredicative situations
                            -- e.g. (f e) where f has a higher-rank type
                            -- We might want to elaborate this

    -- Notes re TySynCtxt
    -- We allow type synonyms that aren't types; e.g.  type List = []
    --
    -- If the RHS mentions tyvars that aren't in scope, we'll
    -- quantify over them:
    --	e.g. 	type T = a->a
    -- will become	type T = forall a. a->a
    --
    -- With gla-exts that's right, but for H98 we should complain.

    ---------------------------------
    -- Kind variables:

    mkKindName :: Unique -> Name
    mkKindName unique = mkSystemName unique kind_var_occ

    kindVarRef :: KindVar -> IORef MetaDetails
    kindVarRef tc =
      ASSERT ( isTcTyVar tc )
      case tcTyVarDetails tc of
        MetaTv TauTv ref -> ref
        _                -> pprPanic "kindVarRef" (ppr tc)

    mkKindVar :: Unique -> IORef MetaDetails -> KindVar
    mkKindVar u r
      = mkTcTyVar (mkKindName u)
                  tySuperKind  -- not sure this is right,
                                -- do we need kind vars for
                                -- coercions?
                  (MetaTv TauTv r)

    kind_var_occ :: OccName	-- Just one for all KindVars
    			-- They may be jiggled by tidying
    kind_var_occ = mkOccName tvName "k"

\end{code} %************************************************************************ %* * Pretty-printing %* * %************************************************************************ \begin{code}

    pprTcTyVarDetails :: TcTyVarDetails -> SDoc
    -- For debugging
    pprTcTyVarDetails (SkolemTv {})        = ptext (sLit "sk")
    pprTcTyVarDetails (RuntimeUnk {})      = ptext (sLit "rt")
    pprTcTyVarDetails (FlatSkol {})        = ptext (sLit "fsk")
    pprTcTyVarDetails (MetaTv TauTv _)     = ptext (sLit "tau")
    pprTcTyVarDetails (MetaTv TcsTv _)     = ptext (sLit "tcs")
    pprTcTyVarDetails (MetaTv (SigTv _) _) = ptext (sLit "sig")

    pprUserTypeCtxt :: UserTypeCtxt -> SDoc
    pprUserTypeCtxt (FunSigCtxt n)  = ptext (sLit "the type signature for") <+> quotes (ppr n)
    pprUserTypeCtxt ExprSigCtxt     = ptext (sLit "an expression type signature")
    pprUserTypeCtxt (ConArgCtxt c)  = ptext (sLit "the type of the constructor") <+> quotes (ppr c)
    pprUserTypeCtxt (TySynCtxt c)   = ptext (sLit "the RHS of the type synonym") <+> quotes (ppr c)
    pprUserTypeCtxt GenPatCtxt      = ptext (sLit "the type pattern of a generic definition")
    pprUserTypeCtxt ThBrackCtxt     = ptext (sLit "a Template Haskell quotation [t|...|]")
    pprUserTypeCtxt LamPatSigCtxt   = ptext (sLit "a pattern type signature")
    pprUserTypeCtxt BindPatSigCtxt  = ptext (sLit "a pattern type signature")
    pprUserTypeCtxt ResSigCtxt      = ptext (sLit "a result type signature")
    pprUserTypeCtxt (ForSigCtxt n)  = ptext (sLit "the foreign declaration for") <+> quotes (ppr n)
    pprUserTypeCtxt DefaultDeclCtxt = ptext (sLit "a type in a `default' declaration")
    pprUserTypeCtxt SpecInstCtxt    = ptext (sLit "a SPECIALISE instance pragma")
    pprUserTypeCtxt GenSigCtxt      = ptext (sLit "a type expected by the context")

\end{code} %************************************************************************ %* * \subsection{TidyType} %* * %************************************************************************ \begin{code}

    -- | This tidies up a type for printing in an error message, or in
    -- an interface file.
    --
    -- It doesn't change the uniques at all, just the print names.
    tidyTyVarBndr :: TidyEnv -> TyVar -> (TidyEnv, TyVar)
    tidyTyVarBndr env@(tidy_env, subst) tyvar
      = case tidyOccName tidy_env occ1 of
          (tidy', occ') -> ((tidy', subst'), tyvar'')
    	where
              subst' = extendVarEnv subst tyvar tyvar''
              tyvar' = setTyVarName tyvar name'

              name' = tidyNameOcc name occ'

                    -- Don't forget to tidy the kind for coercions!
    	  tyvar'' | isCoVar tyvar = setTyVarKind tyvar' kind'
    		  | otherwise	  = tyvar'
    	  kind'  = tidyType env (tyVarKind tyvar)
      where
        name = tyVarName tyvar
        occ  = getOccName name
        -- System Names are for unification variables;
        -- when we tidy them we give them a trailing "0" (or 1 etc)
        -- so that they don't take precedence for the un-modified name
        occ1 | isSystemName name = mkTyVarOcc (occNameString occ ++ "0")
             | otherwise         = occ

    ---------------
    tidyFreeTyVars :: TidyEnv -> TyVarSet -> TidyEnv
    -- ^ Add the free 'TyVar's to the env in tidy form,
    -- so that we can tidy the type they are free in
    tidyFreeTyVars env tyvars = fst (tidyOpenTyVars env (varSetElems tyvars))

    ---------------
    tidyOpenTyVars :: TidyEnv -> [TyVar] -> (TidyEnv, [TyVar])
    tidyOpenTyVars env tyvars = mapAccumL tidyOpenTyVar env tyvars

    ---------------
    tidyOpenTyVar :: TidyEnv -> TyVar -> (TidyEnv, TyVar)
    -- ^ Treat a new 'TyVar' as a binder, and give it a fresh tidy name
    -- using the environment if one has not already been allocated. See
    -- also 'tidyTyVarBndr'
    tidyOpenTyVar env@(_, subst) tyvar
      = case lookupVarEnv subst tyvar of
    	Just tyvar' -> (env, tyvar')		-- Already substituted
    	Nothing	    -> tidyTyVarBndr env tyvar	-- Treat it as a binder

    ---------------
    tidyType :: TidyEnv -> Type -> Type
    tidyType env@(_, subst) ty
      = go ty
      where
        go (TyVarTy tv)	    = case lookupVarEnv subst tv of
    				Nothing  -> expand tv
    				Just tv' -> expand tv'
        go (TyConApp tycon tys) = let args = map go tys
    			      in args `seqList` TyConApp tycon args
        go (PredTy sty)	    = PredTy (tidyPred env sty)
        go (AppTy fun arg)	    = (AppTy $! (go fun)) $! (go arg)
        go (FunTy fun arg)	    = (FunTy $! (go fun)) $! (go arg)
        go (ForAllTy tv ty)	    = ForAllTy tvp $! (tidyType envp ty)
    			      where
    			        (envp, tvp) = tidyTyVarBndr env tv

        -- Expand FlatSkols, the skolems introduced by flattening process
        -- We don't want to show them in type error messages
        expand tv | isTcTyVar tv
                  , FlatSkol ty <- tcTyVarDetails tv
                  = go ty
                  | otherwise
                  = TyVarTy tv

    ---------------
    tidyTypes :: TidyEnv -> [Type] -> [Type]
    tidyTypes env tys = map (tidyType env) tys

    ---------------
    tidyPred :: TidyEnv -> PredType -> PredType
    tidyPred env (IParam n ty)     = IParam n (tidyType env ty)
    tidyPred env (ClassP clas tys) = ClassP clas (tidyTypes env tys)
    tidyPred env (EqPred ty1 ty2)  = EqPred (tidyType env ty1) (tidyType env ty2)

    ---------------
    -- | Grabs the free type variables, tidies them
    -- and then uses 'tidyType' to work over the type itself
    tidyOpenType :: TidyEnv -> Type -> (TidyEnv, Type)
    tidyOpenType env ty
      = (env', tidyType env' ty)
      where
        env' = tidyFreeTyVars env (tyVarsOfType ty)

    ---------------
    tidyOpenTypes :: TidyEnv -> [Type] -> (TidyEnv, [Type])
    tidyOpenTypes env tys = mapAccumL tidyOpenType env tys

    ---------------
    -- | Calls 'tidyType' on a top-level type (i.e. with an empty tidying environment)
    tidyTopType :: Type -> Type
    tidyTopType ty = tidyType emptyTidyEnv ty

    ---------------
    tidyKind :: TidyEnv -> Kind -> (TidyEnv, Kind)
    tidyKind env k = tidyOpenType env k

\end{code} %************************************************************************ %* * Predicates %* * %************************************************************************ \begin{code}

    isImmutableTyVar :: TyVar -> Bool

    isImmutableTyVar tv
      | isTcTyVar tv = isSkolemTyVar tv
      | otherwise    = True

    isTyConableTyVar, isSkolemTyVar, isOverlappableTyVar,
      isMetaTyVar :: TcTyVar -> Bool

    isTyConableTyVar tv
    	-- True of a meta-type variable that can be filled in
    	-- with a type constructor application; in particular,
    	-- not a SigTv
      = ASSERT( isTcTyVar tv)
        case tcTyVarDetails tv of
    	MetaTv (SigTv _) _ -> False
    	_                  -> True

    isSkolemTyVar tv
      = ASSERT2( isTcTyVar tv, ppr tv )
        case tcTyVarDetails tv of
            SkolemTv {}   -> True
            FlatSkol {}   -> True
            RuntimeUnk {} -> True
            MetaTv {}     -> False

    isOverlappableTyVar tv
      = ASSERT( isTcTyVar tv )
        case tcTyVarDetails tv of
            SkolemTv overlappable -> overlappable
            _                     -> False

    isMetaTyVar tv
      = ASSERT2( isTcTyVar tv, ppr tv )
        case tcTyVarDetails tv of
    	MetaTv _ _ -> True
    	_          -> False

    isMetaTyVarTy :: TcType -> Bool
    isMetaTyVarTy (TyVarTy tv) = isMetaTyVar tv
    isMetaTyVarTy _            = False

    isSigTyVar :: Var -> Bool
    isSigTyVar tv
      = ASSERT( isTcTyVar tv )
        case tcTyVarDetails tv of
    	MetaTv (SigTv _) _ -> True
    	_                  -> False

    metaTvRef :: TyVar -> IORef MetaDetails
    metaTvRef tv
      = ASSERT2( isTcTyVar tv, ppr tv )
        case tcTyVarDetails tv of
    	MetaTv _ ref -> ref
    	_          -> pprPanic "metaTvRef" (ppr tv)

    isFlexi, isIndirect :: MetaDetails -> Bool
    isFlexi Flexi = True
    isFlexi _     = False

    isIndirect (Indirect _) = True
    isIndirect _            = False

    isRuntimeUnkSkol :: TyVar -> Bool
    -- Called only in TcErrors; see Note [Runtime skolems] there
    isRuntimeUnkSkol x
      | isTcTyVar x, RuntimeUnk <- tcTyVarDetails x = True
      | otherwise                                   = False

\end{code} %************************************************************************ %* * \subsection{Tau, sigma and rho} %* * %************************************************************************ \begin{code}

    mkSigmaTy :: [TyVar] -> [PredType] -> Type -> Type
    mkSigmaTy tyvars theta tau = mkForAllTys tyvars (mkPhiTy theta tau)

    mkPhiTy :: [PredType] -> Type -> Type
    mkPhiTy theta ty = foldr (\p r -> mkFunTy (mkPredTy p) r) ty theta

\end{code} @isTauTy@ tests for nested for-alls. It should not be called on a boxy type. \begin{code}

    isTauTy :: Type -> Bool
    isTauTy ty | Just ty' <- tcView ty = isTauTy ty'
    isTauTy (TyVarTy _)	  = True
    isTauTy (TyConApp tc tys) = all isTauTy tys && isTauTyCon tc
    isTauTy (AppTy a b)	  = isTauTy a && isTauTy b
    isTauTy (FunTy a b)	  = isTauTy a && isTauTy b
    isTauTy (PredTy _)	  = True		-- Don't look through source types
    isTauTy _    		  = False

    isTauTyCon :: TyCon -> Bool
    -- Returns False for type synonyms whose expansion is a polytype
    isTauTyCon tc
      | isClosedSynTyCon tc = isTauTy (snd (synTyConDefn tc))
      | otherwise           = True

    ---------------
    getDFunTyKey :: Type -> OccName -- Get some string from a type, to be used to
    				-- construct a dictionary function name
    getDFunTyKey ty | Just ty' <- tcView ty = getDFunTyKey ty'
    getDFunTyKey (TyVarTy tv)    = getOccName tv
    getDFunTyKey (TyConApp tc _) = getOccName tc
    getDFunTyKey (AppTy fun _)   = getDFunTyKey fun
    getDFunTyKey (FunTy _ _)     = getOccName funTyCon
    getDFunTyKey (ForAllTy _ t)  = getDFunTyKey t
    getDFunTyKey ty		     = pprPanic "getDFunTyKey" (pprType ty)
    -- PredTy shouldn't happen

\end{code} %************************************************************************ %* * \subsection{Expanding and splitting} %* * %************************************************************************ These tcSplit functions are like their non-Tc analogues, but a) they do not look through newtypes b) they do not look through PredTys However, they are non-monadic and do not follow through mutable type variables. It's up to you to make sure this doesn't matter. \begin{code}

    tcSplitForAllTys :: Type -> ([TyVar], Type)
    tcSplitForAllTys ty = split ty ty []
       where
         split orig_ty ty tvs | Just ty' <- tcView ty = split orig_ty ty' tvs
         split _ (ForAllTy tv ty) tvs
           | not (isCoVar tv) = split ty ty (tv:tvs)
         split orig_ty _ tvs = (reverse tvs, orig_ty)

    tcIsForAllTy :: Type -> Bool
    tcIsForAllTy ty | Just ty' <- tcView ty = tcIsForAllTy ty'
    tcIsForAllTy (ForAllTy tv _) = not (isCoVar tv)
    tcIsForAllTy _               = False

    tcSplitPredFunTy_maybe :: Type -> Maybe (PredType, Type)
    -- Split off the first predicate argument from a type
    tcSplitPredFunTy_maybe ty | Just ty' <- tcView ty = tcSplitPredFunTy_maybe ty'
    tcSplitPredFunTy_maybe (ForAllTy tv ty)
      | isCoVar tv = Just (coVarPred tv, ty)
    tcSplitPredFunTy_maybe (FunTy arg res)
      | Just p <- tcSplitPredTy_maybe arg = Just (p, res)
    tcSplitPredFunTy_maybe _
      = Nothing

    tcSplitPhiTy :: Type -> (ThetaType, Type)
    tcSplitPhiTy ty
      = split ty []
      where
        split ty ts
          = case tcSplitPredFunTy_maybe ty of
    	  Just (pred, ty) -> split ty (pred:ts)
    	  Nothing         -> (reverse ts, ty)

    tcSplitSigmaTy :: Type -> ([TyVar], ThetaType, Type)
    tcSplitSigmaTy ty = case tcSplitForAllTys ty of
    			(tvs, rho) -> case tcSplitPhiTy rho of
    					(theta, tau) -> (tvs, theta, tau)

    -----------------------
    tcDeepSplitSigmaTy_maybe
      :: TcSigmaType -> Maybe ([TcType], [TyVar], ThetaType, TcSigmaType)
    -- Looks for a *non-trivial* quantified type, under zero or more function arrows
    -- By "non-trivial" we mean either tyvars or constraints are non-empty

    tcDeepSplitSigmaTy_maybe ty
      | Just (arg_ty, res_ty)           <- tcSplitFunTy_maybe ty
      , Just (arg_tys, tvs, theta, rho) <- tcDeepSplitSigmaTy_maybe res_ty
      = Just (arg_ty:arg_tys, tvs, theta, rho)

      | (tvs, theta, rho) <- tcSplitSigmaTy ty
      , not (null tvs && null theta)
      = Just ([], tvs, theta, rho)

      | otherwise = Nothing

    -----------------------
    tcTyConAppTyCon :: Type -> TyCon
    tcTyConAppTyCon ty = case tcSplitTyConApp_maybe ty of
    			Just (tc, _) -> tc
    			Nothing	     -> pprPanic "tcTyConAppTyCon" (pprType ty)

    tcTyConAppArgs :: Type -> [Type]
    tcTyConAppArgs ty = case tcSplitTyConApp_maybe ty of
    			Just (_, args) -> args
    			Nothing	       -> pprPanic "tcTyConAppArgs" (pprType ty)

    tcSplitTyConApp :: Type -> (TyCon, [Type])
    tcSplitTyConApp ty = case tcSplitTyConApp_maybe ty of
    			Just stuff -> stuff
    			Nothing	   -> pprPanic "tcSplitTyConApp" (pprType ty)

    tcSplitTyConApp_maybe :: Type -> Maybe (TyCon, [Type])
    tcSplitTyConApp_maybe ty | Just ty' <- tcView ty = tcSplitTyConApp_maybe ty'
    tcSplitTyConApp_maybe (TyConApp tc tys) = Just (tc, tys)
    tcSplitTyConApp_maybe (FunTy arg res)   = Just (funTyCon, [arg,res])
    	-- Newtypes are opaque, so they may be split
    	-- However, predicates are not treated
    	-- as tycon applications by the type checker
    tcSplitTyConApp_maybe _                 = Nothing

    -----------------------
    tcSplitFunTys :: Type -> ([Type], Type)
    tcSplitFunTys ty = case tcSplitFunTy_maybe ty of
    			Nothing	       -> ([], ty)
    			Just (arg,res) -> (arg:args, res')
    				       where
    					  (args,res') = tcSplitFunTys res

    tcSplitFunTy_maybe :: Type -> Maybe (Type, Type)
    tcSplitFunTy_maybe ty | Just ty' <- tcView ty           = tcSplitFunTy_maybe ty'
    tcSplitFunTy_maybe (FunTy arg res) | not (isPredTy arg) = Just (arg, res)
    tcSplitFunTy_maybe _                                    = Nothing
    	-- Note the (not (isPredTy arg)) guard
    	-- Consider	(?x::Int) => Bool
    	-- We don't want to treat this as a function type!
    	-- A concrete example is test tc230:
    	--	f :: () -> (?p :: ()) => () -> ()
    	--
    	--	g = f () ()

    tcSplitFunTysN
    	:: TcRhoType
    	-> Arity		-- N: Number of desired args
    	-> ([TcSigmaType], 	-- Arg types (N or fewer)
    	    TcSigmaType)	-- The rest of the type

    tcSplitFunTysN ty n_args
      | n_args == 0
      = ([], ty)
      | Just (arg,res) <- tcSplitFunTy_maybe ty
      = case tcSplitFunTysN res (n_args - 1) of
    	(args, res) -> (arg:args, res)
      | otherwise
      = ([], ty)

    tcSplitFunTy :: Type -> (Type, Type)
    tcSplitFunTy  ty = expectJust "tcSplitFunTy" (tcSplitFunTy_maybe ty)

    tcFunArgTy :: Type -> Type
    tcFunArgTy    ty = fst (tcSplitFunTy ty)

    tcFunResultTy :: Type -> Type
    tcFunResultTy ty = snd (tcSplitFunTy ty)

    -----------------------
    tcSplitAppTy_maybe :: Type -> Maybe (Type, Type)
    tcSplitAppTy_maybe ty | Just ty' <- tcView ty = tcSplitAppTy_maybe ty'
    tcSplitAppTy_maybe ty = repSplitAppTy_maybe ty

    tcSplitAppTy :: Type -> (Type, Type)
    tcSplitAppTy ty = case tcSplitAppTy_maybe ty of
    		    Just stuff -> stuff
    		    Nothing    -> pprPanic "tcSplitAppTy" (pprType ty)

    tcSplitAppTys :: Type -> (Type, [Type])
    tcSplitAppTys ty
      = go ty []
      where
        go ty args = case tcSplitAppTy_maybe ty of
    		   Just (ty', arg) -> go ty' (arg:args)
    		   Nothing	   -> (ty,args)

    -----------------------
    tcGetTyVar_maybe :: Type -> Maybe TyVar
    tcGetTyVar_maybe ty | Just ty' <- tcView ty = tcGetTyVar_maybe ty'
    tcGetTyVar_maybe (TyVarTy tv)   = Just tv
    tcGetTyVar_maybe _              = Nothing

    tcGetTyVar :: String -> Type -> TyVar
    tcGetTyVar msg ty = expectJust msg (tcGetTyVar_maybe ty)

    tcIsTyVarTy :: Type -> Bool
    tcIsTyVarTy ty = maybeToBool (tcGetTyVar_maybe ty)

    -----------------------
    tcSplitDFunTy :: Type -> ([TyVar], Class, [Type])
    -- Split the type of a dictionary function
    -- We don't use tcSplitSigmaTy,  because a DFun may (with NDP)
    -- have non-Pred arguments, such as
    --     df :: forall m. (forall b. Eq b => Eq (m b)) -> C m
    tcSplitDFunTy ty
      = case tcSplitForAllTys ty                 of { (tvs, rho)  ->
        case tcSplitDFunHead (drop_pred_tys rho) of { (clas, tys) ->
        (tvs, clas, tys) }}
      where
        -- Discard the context of the dfun.  This can be a mix of
        -- coercion and class constraints; or (in the general NDP case)
        -- some other function argument
        drop_pred_tys ty | Just ty' <- tcView ty = drop_pred_tys ty'
        drop_pred_tys (ForAllTy tv ty) = ASSERT( isCoVar tv ) drop_pred_tys ty
        drop_pred_tys (FunTy _ ty)     = drop_pred_tys ty
        drop_pred_tys ty               = ty

    tcSplitDFunHead :: Type -> (Class, [Type])
    tcSplitDFunHead tau
      = case tcSplitPredTy_maybe tau of
    	Just (ClassP clas tys) -> (clas, tys)
    	_ -> pprPanic "tcSplitDFunHead" (ppr tau)

    tcInstHeadTyNotSynonym :: Type -> Bool
    -- Used in Haskell-98 mode, for the argument types of an instance head
    -- These must not be type synonyms, but everywhere else type synonyms
    -- are transparent, so we need a special function here
    tcInstHeadTyNotSynonym ty
      = case ty of
            TyConApp tc _ -> not (isSynTyCon tc)
            _ -> True

    tcInstHeadTyAppAllTyVars :: Type -> Bool
    -- Used in Haskell-98 mode, for the argument types of an instance head
    -- These must be a constructor applied to type variable arguments
    tcInstHeadTyAppAllTyVars ty
      = case ty of
    	TyConApp _ tys  -> ok tys
    	FunTy arg res   -> ok [arg, res]
    	_               -> False
      where
    	-- Check that all the types are type variables,
    	-- and that each is distinct
        ok tys = equalLength tvs tys && hasNoDups tvs
    	   where
    	     tvs = mapCatMaybes get_tv tys

        get_tv (TyVarTy tv)  = Just tv	-- through synonyms
        get_tv _             = Nothing

\end{code} %************************************************************************ %* * \subsection{Predicate types} %* * %************************************************************************ \begin{code}

    evVarPred :: EvVar -> PredType
    evVarPred var
      = case tcSplitPredTy_maybe (varType var) of
          Just pred -> pred
          Nothing   -> pprPanic "evVarPred" (ppr var <+> ppr (varType var))

    tcSplitPredTy_maybe :: Type -> Maybe PredType
       -- Returns Just for predicates only
    tcSplitPredTy_maybe ty | Just ty' <- tcView ty = tcSplitPredTy_maybe ty'
    tcSplitPredTy_maybe (PredTy p)    = Just p
    tcSplitPredTy_maybe _             = Nothing

    predTyUnique :: PredType -> Unique
    predTyUnique (IParam n _)    = getUnique (ipNameName n)
    predTyUnique (ClassP clas _) = getUnique clas
    predTyUnique (EqPred a b)    = pprPanic "predTyUnique" (ppr (EqPred a b))

\end{code} --------------------- Dictionary types --------------------------------- \begin{code}

    mkClassPred :: Class -> [Type] -> PredType
    mkClassPred clas tys = ClassP clas tys

    isClassPred :: PredType -> Bool
    isClassPred (ClassP _ _) = True
    isClassPred _            = False

    isTyVarClassPred :: PredType -> Bool
    isTyVarClassPred (ClassP _ tys) = all tcIsTyVarTy tys
    isTyVarClassPred _              = False

    getClassPredTys_maybe :: PredType -> Maybe (Class, [Type])
    getClassPredTys_maybe (ClassP clas tys) = Just (clas, tys)
    getClassPredTys_maybe _                 = Nothing

    getClassPredTys :: PredType -> (Class, [Type])
    getClassPredTys (ClassP clas tys) = (clas, tys)
    getClassPredTys _ = panic "getClassPredTys"

    mkDictTy :: Class -> [Type] -> Type
    mkDictTy clas tys = mkPredTy (ClassP clas tys)

    isDictLikeTy :: Type -> Bool
    -- Note [Dictionary-like types]
    isDictLikeTy ty | Just ty' <- tcView ty = isDictTy ty'
    isDictLikeTy (PredTy p) = isClassPred p
    isDictLikeTy (TyConApp tc tys)
      | isTupleTyCon tc     = all isDictLikeTy tys
    isDictLikeTy _          = False

\end{code} Superclasses \begin{code}

    mkMinimalBySCs :: [PredType] -> [PredType]
    -- Remove predicates that can be deduced from others by superclasses
    mkMinimalBySCs ptys = [ ploc |  ploc <- ptys
                                 ,  ploc `not_in_preds` rec_scs ]
     where
       rec_scs = concatMap trans_super_classes ptys
       not_in_preds p ps = null (filter (tcEqPred p) ps)
       trans_super_classes (ClassP cls tys) = transSuperClasses cls tys
       trans_super_classes _other_pty       = []

    transSuperClasses :: Class -> [Type] -> [PredType]
    transSuperClasses cls tys
      = foldl (\pts p -> trans_sc p ++ pts) [] $
        immSuperClasses cls tys
      where trans_sc :: PredType -> [PredType]
            trans_sc this_pty@(ClassP cls tys)
              = foldl (\pts p -> trans_sc p ++ pts) [this_pty] $
                immSuperClasses cls tys
            trans_sc pty = [pty]

    immSuperClasses :: Class -> [Type] -> [PredType]
    immSuperClasses cls tys
      = substTheta (zipTopTvSubst tyvars tys) sc_theta
      where (tyvars,sc_theta,_,_) = classBigSig cls

\end{code} Note [Dictionary-like types] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Being "dictionary-like" means either a dictionary type or a tuple thereof. In GHC 6.10 we build implication constraints which construct such tuples, and if we land up with a binding t :: (C [a], Eq [a]) t = blah then we want to treat t as cheap under "-fdicts-cheap" for example. (Implication constraints are normally inlined, but sadly not if the occurrence is itself inside an INLINE function! Until we revise the handling of implication constraints, that is.) This turned out to be important in getting good arities in DPH code. Example: class C a class D a where { foo :: a -> a } instance C a => D (Maybe a) where { foo x = x } bar :: (C a, C b) => a -> b -> (Maybe a, Maybe b) {-# INLINE bar #-} bar x y = (foo (Just x), foo (Just y)) Then 'bar' should jolly well have arity 4 (two dicts, two args), but we ended up with something like bar = __inline_me__ (\d1,d2. let t :: (D (Maybe a), D (Maybe b)) = ... in \x,y. <blah>) This is all a bit ad-hoc; eg it relies on knowing that implication constraints build tuples. --------------------- Implicit parameters --------------------------------- \begin{code}

    mkIPPred :: IPName Name -> Type -> PredType
    mkIPPred ip ty = IParam ip ty

    isIPPred :: PredType -> Bool
    isIPPred (IParam _ _) = True
    isIPPred _            = False

\end{code} --------------------- Equality predicates --------------------------------- \begin{code}

    substEqSpec :: TvSubst -> [(TyVar,Type)] -> [(TcType,TcType)]
    substEqSpec subst eq_spec = [ (substTyVar subst tv, substTy subst ty)
    			    | (tv,ty) <- eq_spec]

\end{code} %************************************************************************ %* * \subsection{Predicates} %* * %************************************************************************ isSigmaTy returns true of any qualified type. It doesn't *necessarily* have any foralls. E.g. f :: (?x::Int) => Int -> Int \begin{code}

    isSigmaTy :: Type -> Bool
    isSigmaTy ty | Just ty' <- tcView ty = isSigmaTy ty'
    isSigmaTy (ForAllTy _ _) = True
    isSigmaTy (FunTy a _)    = isPredTy a
    isSigmaTy _              = False

    isOverloadedTy :: Type -> Bool
    -- Yes for a type of a function that might require evidence-passing
    -- Used only by bindLocalMethods
    -- NB: be sure to check for type with an equality predicate; hence isCoVar
    isOverloadedTy ty | Just ty' <- tcView ty = isOverloadedTy ty'
    isOverloadedTy (ForAllTy tv ty) = isCoVar tv || isOverloadedTy ty
    isOverloadedTy (FunTy a _)      = isPredTy a
    isOverloadedTy _                = False

    isPredTy :: Type -> Bool	-- Belongs in TcType because it does
    				-- not look through newtypes, or predtypes (of course)
    isPredTy ty | Just ty' <- tcView ty = isPredTy ty'
    isPredTy (PredTy _) = True
    isPredTy _          = False

\end{code} \begin{code}

    isFloatTy, isDoubleTy, isIntegerTy, isIntTy, isWordTy, isBoolTy,
        isUnitTy, isCharTy :: Type -> Bool
    isFloatTy      = is_tc floatTyConKey
    isDoubleTy     = is_tc doubleTyConKey
    isIntegerTy    = is_tc integerTyConKey
    isIntTy        = is_tc intTyConKey
    isWordTy       = is_tc wordTyConKey
    isBoolTy       = is_tc boolTyConKey
    isUnitTy       = is_tc unitTyConKey
    isCharTy       = is_tc charTyConKey

    isStringTy :: Type -> Bool
    isStringTy ty
      = case tcSplitTyConApp_maybe ty of
          Just (tc, [arg_ty]) -> tc == listTyCon && isCharTy arg_ty
          _                   -> False

    is_tc :: Unique -> Type -> Bool
    -- Newtypes are opaque to this
    is_tc uniq ty = case tcSplitTyConApp_maybe ty of
    			Just (tc, _) -> uniq == getUnique tc
    			Nothing	     -> False

\end{code} \begin{code}

    -- NB: Currently used in places where we have already expanded type synonyms;
    --     hence no 'coreView'.  This could, however, be changed without breaking
    --     any code.
    isSynFamilyTyConApp :: TcTauType -> Bool
    isSynFamilyTyConApp (TyConApp tc tys) = isSynFamilyTyCon tc &&
                                          length tys == tyConArity tc
    isSynFamilyTyConApp _other            = False

\end{code} %************************************************************************ %* * \subsection{Misc} %* * %************************************************************************ \begin{code}

    deNoteType :: Type -> Type
    -- Remove all *outermost* type synonyms and other notes
    deNoteType ty | Just ty' <- tcView ty = deNoteType ty'
    deNoteType ty = ty

\end{code} \begin{code}

    tcTyVarsOfType :: Type -> TcTyVarSet
    -- Just the *TcTyVars* free in the type
    -- (Types.tyVarsOfTypes finds all free TyVars)
    tcTyVarsOfType (TyVarTy tv)	    = if isTcTyVar tv then unitVarSet tv
    						      else emptyVarSet
    tcTyVarsOfType (TyConApp _ tys)     = tcTyVarsOfTypes tys
    tcTyVarsOfType (PredTy sty)	    = tcTyVarsOfPred sty
    tcTyVarsOfType (FunTy arg res)	    = tcTyVarsOfType arg `unionVarSet` tcTyVarsOfType res
    tcTyVarsOfType (AppTy fun arg)	    = tcTyVarsOfType fun `unionVarSet` tcTyVarsOfType arg
    tcTyVarsOfType (ForAllTy tyvar ty)  = (tcTyVarsOfType ty `delVarSet` tyvar)
                                          `unionVarSet` tcTyVarsOfTyVar tyvar
    	-- We do sometimes quantify over skolem TcTyVars

    tcTyVarsOfTyVar :: TcTyVar -> TyVarSet
    tcTyVarsOfTyVar tv | isCoVar tv = tcTyVarsOfType (tyVarKind tv)
                       | otherwise  = emptyVarSet

    tcTyVarsOfTypes :: [Type] -> TyVarSet
    tcTyVarsOfTypes tys = foldr (unionVarSet.tcTyVarsOfType) emptyVarSet tys

    tcTyVarsOfPred :: PredType -> TyVarSet
    tcTyVarsOfPred (IParam _ ty)  	= tcTyVarsOfType ty
    tcTyVarsOfPred (ClassP _ tys) 	= tcTyVarsOfTypes tys
    tcTyVarsOfPred (EqPred ty1 ty2) = tcTyVarsOfType ty1 `unionVarSet` tcTyVarsOfType ty2

\end{code} Note [Silly type synonym] ~~~~~~~~~~~~~~~~~~~~~~~~~ Consider type T a = Int What are the free tyvars of (T x)? Empty, of course! Here's the example that Ralf Laemmel showed me: foo :: (forall a. C u a -> C u a) -> u mappend :: Monoid u => u -> u -> u bar :: Monoid u => u bar = foo (\t -> t `mappend` t) We have to generalise at the arg to f, and we don't want to capture the constraint (Monad (C u a)) because it appears to mention a. Pretty silly, but it was useful to him. exactTyVarsOfType is used by the type checker to figure out exactly which type variables are mentioned in a type. It's also used in the smart-app checking code --- see TcExpr.tcIdApp On the other hand, consider a *top-level* definition f = (\x -> x) :: T a -> T a If we don't abstract over 'a' it'll get fixed to GHC.Prim.Any, and then if we have an application like (f "x") we get a confusing error message involving Any. So the conclusion is this: when generalising - at top level use tyVarsOfType - in nested bindings use exactTyVarsOfType See Trac #1813 for example. \begin{code}

    exactTyVarsOfType :: TcType -> TyVarSet
    -- Find the free type variables (of any kind)
    -- but *expand* type synonyms.  See Note [Silly type synonym] above.
    exactTyVarsOfType ty
      = go ty
      where
        go ty | Just ty' <- tcView ty = go ty'	-- This is the key line
        go (TyVarTy tv)         = unitVarSet tv
        go (TyConApp _ tys)     = exactTyVarsOfTypes tys
        go (PredTy ty)	    = go_pred ty
        go (FunTy arg res)	    = go arg `unionVarSet` go res
        go (AppTy fun arg)	    = go fun `unionVarSet` go arg
        go (ForAllTy tyvar ty)  = delVarSet (go ty) tyvar
                                  `unionVarSet` go_tv tyvar

        go_pred (IParam _ ty)    = go ty
        go_pred (ClassP _ tys)   = exactTyVarsOfTypes tys
        go_pred (EqPred ty1 ty2) = go ty1 `unionVarSet` go ty2

        go_tv tyvar | isCoVar tyvar = go (tyVarKind tyvar)
                    | otherwise     = emptyVarSet

    exactTyVarsOfTypes :: [TcType] -> TyVarSet
    exactTyVarsOfTypes tys = foldr (unionVarSet . exactTyVarsOfType) emptyVarSet tys

\end{code} Find the free tycons and classes of a type. This is used in the front end of the compiler. \begin{code}

    orphNamesOfType :: Type -> NameSet
    orphNamesOfType ty | Just ty' <- tcView ty = orphNamesOfType ty'
    		-- Look through type synonyms (Trac #4912)
    orphNamesOfType (TyVarTy _)		   = emptyNameSet
    orphNamesOfType (TyConApp tycon tys)       = unitNameSet (getName tycon)
                                                 `unionNameSets` orphNamesOfTypes tys
    orphNamesOfType (PredTy (IParam _ ty))    = orphNamesOfType ty
    orphNamesOfType (PredTy (ClassP cl tys))  = unitNameSet (getName cl)
                                                `unionNameSets` orphNamesOfTypes tys
    orphNamesOfType (PredTy (EqPred ty1 ty2)) = orphNamesOfType ty1
                                                `unionNameSets` orphNamesOfType ty2
    orphNamesOfType (FunTy arg res)	    = orphNamesOfType arg `unionNameSets` orphNamesOfType res
    orphNamesOfType (AppTy fun arg)	    = orphNamesOfType fun `unionNameSets` orphNamesOfType arg
    orphNamesOfType (ForAllTy _ ty)	    = orphNamesOfType ty

    orphNamesOfTypes :: [Type] -> NameSet
    orphNamesOfTypes tys = foldr (unionNameSets . orphNamesOfType) emptyNameSet tys

    orphNamesOfDFunHead :: Type -> NameSet
    -- Find the free type constructors and classes
    -- of the head of the dfun instance type
    -- The 'dfun_head_type' is because of
    --	instance Foo a => Baz T where ...
    -- The decl is an orphan if Baz and T are both not locally defined,
    --	even if Foo *is* locally defined
    orphNamesOfDFunHead dfun_ty
      = case tcSplitSigmaTy dfun_ty of
    	(_, _, head_ty) -> orphNamesOfType head_ty

\end{code} %************************************************************************ %* * \subsection[TysWiredIn-ext-type]{External types} %* * %************************************************************************ The compiler's foreign function interface supports the passing of a restricted set of types as arguments and results (the restricting factor being the ) \begin{code}

    tcSplitIOType_maybe :: Type -> Maybe (TyCon, Type, CoercionI)
    -- (isIOType t) returns Just (IO,t',co)
    --				if co : t ~ IO t'
    --		returns Nothing otherwise
    tcSplitIOType_maybe ty
      = case tcSplitTyConApp_maybe ty of
    	-- This split absolutely has to be a tcSplit, because we must
    	-- see the IO type; and it's a newtype which is transparent to splitTyConApp.

    	Just (io_tycon, [io_res_ty])
    	   |  io_tycon `hasKey` ioTyConKey
    	   -> Just (io_tycon, io_res_ty, IdCo ty)

    	Just (tc, tys)
    	   | not (isRecursiveTyCon tc)
    	   , Just (ty, co1) <- instNewTyCon_maybe tc tys
    		  -- Newtypes that require a coercion are ok
    	   -> case tcSplitIOType_maybe ty of
    		Nothing		    -> Nothing
    		Just (tc, ty', co2) -> Just (tc, ty', co1 `mkTransCoI` co2)

    	_ -> Nothing

    isFFITy :: Type -> Bool
    -- True for any TyCon that can possibly be an arg or result of an FFI call
    isFFITy ty = checkRepTyCon legalFFITyCon ty

    isFFIArgumentTy :: DynFlags -> Safety -> Type -> Bool
    -- Checks for valid argument type for a 'foreign import'
    isFFIArgumentTy dflags safety ty
       = checkRepTyCon (legalOutgoingTyCon dflags safety) ty

    isFFIExternalTy :: Type -> Bool
    -- Types that are allowed as arguments of a 'foreign export'
    isFFIExternalTy ty = checkRepTyCon legalFEArgTyCon ty

    isFFIImportResultTy :: DynFlags -> Type -> Bool
    isFFIImportResultTy dflags ty
      = checkRepTyCon (legalFIResultTyCon dflags) ty

    isFFIExportResultTy :: Type -> Bool
    isFFIExportResultTy ty = checkRepTyCon legalFEResultTyCon ty

    isFFIDynArgumentTy :: Type -> Bool
    -- The argument type of a foreign import dynamic must be Ptr, FunPtr, Addr,
    -- or a newtype of either.
    isFFIDynArgumentTy = checkRepTyConKey [ptrTyConKey, funPtrTyConKey]

    isFFIDynResultTy :: Type -> Bool
    -- The result type of a foreign export dynamic must be Ptr, FunPtr, Addr,
    -- or a newtype of either.
    isFFIDynResultTy = checkRepTyConKey [ptrTyConKey, funPtrTyConKey]

    isFFILabelTy :: Type -> Bool
    -- The type of a foreign label must be Ptr, FunPtr, Addr,
    -- or a newtype of either.
    isFFILabelTy = checkRepTyConKey [ptrTyConKey, funPtrTyConKey]

    isFFIPrimArgumentTy :: DynFlags -> Type -> Bool
    -- Checks for valid argument type for a 'foreign import prim'
    -- Currently they must all be simple unlifted types.
    isFFIPrimArgumentTy dflags ty
       = checkRepTyCon (legalFIPrimArgTyCon dflags) ty

    isFFIPrimResultTy :: DynFlags -> Type -> Bool
    -- Checks for valid result type for a 'foreign import prim'
    -- Currently it must be an unlifted type, including unboxed tuples.
    isFFIPrimResultTy dflags ty
       = checkRepTyCon (legalFIPrimResultTyCon dflags) ty

    isFFIDotnetTy :: DynFlags -> Type -> Bool
    isFFIDotnetTy dflags ty
      = checkRepTyCon (\ tc -> (legalFIResultTyCon dflags tc ||
    			   isFFIDotnetObjTy ty || isStringTy ty)) ty
    	-- NB: isStringTy used to look through newtypes, but
    	--     it no longer does so.  May need to adjust isFFIDotNetTy
    	--     if we do want to look through newtypes.

    isFFIDotnetObjTy :: Type -> Bool
    isFFIDotnetObjTy ty
      = checkRepTyCon check_tc t_ty
      where
       (_, t_ty) = tcSplitForAllTys ty
       check_tc tc = getName tc == objectTyConName

    isFunPtrTy :: Type -> Bool
    isFunPtrTy = checkRepTyConKey [funPtrTyConKey]

    checkRepTyCon :: (TyCon -> Bool) -> Type -> Bool
    -- Look through newtypes, but *not* foralls
    -- Should work even for recursive newtypes
    -- eg Manuel had:	newtype T = MkT (Ptr T)
    checkRepTyCon check_tc ty
      = go [] ty
      where
        go rec_nts ty
          | Just (tc,tys) <- splitTyConApp_maybe ty
          = case carefullySplitNewType_maybe rec_nts tc tys of
          	   Just (rec_nts', ty') -> go rec_nts' ty'
    	   Nothing	   	-> check_tc tc
          | otherwise
          = False

    checkRepTyConKey :: [Unique] -> Type -> Bool
    -- Like checkRepTyCon, but just looks at the TyCon key
    checkRepTyConKey keys
      = checkRepTyCon (\tc -> tyConUnique tc `elem` keys)

\end{code} ---------------------------------------------- These chaps do the work; they are not exported ---------------------------------------------- \begin{code}

    legalFEArgTyCon :: TyCon -> Bool
    legalFEArgTyCon tc
      -- It's illegal to make foreign exports that take unboxed
      -- arguments.  The RTS API currently can't invoke such things.  --SDM 7/2000
      = boxedMarshalableTyCon tc

    legalFIResultTyCon :: DynFlags -> TyCon -> Bool
    legalFIResultTyCon dflags tc
      | tc == unitTyCon         = True
      | otherwise	            = marshalableTyCon dflags tc

    legalFEResultTyCon :: TyCon -> Bool
    legalFEResultTyCon tc
      | tc == unitTyCon         = True
      | otherwise               = boxedMarshalableTyCon tc

    legalOutgoingTyCon :: DynFlags -> Safety -> TyCon -> Bool
    -- Checks validity of types going from Haskell -> external world
    legalOutgoingTyCon dflags _ tc
      = marshalableTyCon dflags tc

    legalFFITyCon :: TyCon -> Bool
    -- True for any TyCon that can possibly be an arg or result of an FFI call
    legalFFITyCon tc
      = isUnLiftedTyCon tc || boxedMarshalableTyCon tc || tc == unitTyCon

    marshalableTyCon :: DynFlags -> TyCon -> Bool
    marshalableTyCon dflags tc
      =  (xopt Opt_UnliftedFFITypes dflags
          && isUnLiftedTyCon tc
          && not (isUnboxedTupleTyCon tc)
          && case tyConPrimRep tc of	-- Note [Marshalling VoidRep]
    	   VoidRep -> False
    	   _       -> True)
      || boxedMarshalableTyCon tc

    boxedMarshalableTyCon :: TyCon -> Bool
    boxedMarshalableTyCon tc
       = getUnique tc `elem` [ intTyConKey, int8TyConKey, int16TyConKey
    			 , int32TyConKey, int64TyConKey
    			 , wordTyConKey, word8TyConKey, word16TyConKey
    			 , word32TyConKey, word64TyConKey
    			 , floatTyConKey, doubleTyConKey
    			 , ptrTyConKey, funPtrTyConKey
    			 , charTyConKey
    			 , stablePtrTyConKey
    			 , boolTyConKey
    			 ]

    legalFIPrimArgTyCon :: DynFlags -> TyCon -> Bool
    -- Check args of 'foreign import prim', only allow simple unlifted types.
    -- Strictly speaking it is unnecessary to ban unboxed tuples here since
    -- currently they're of the wrong kind to use in function args anyway.
    legalFIPrimArgTyCon dflags tc
      = xopt Opt_UnliftedFFITypes dflags
        && isUnLiftedTyCon tc
        && not (isUnboxedTupleTyCon tc)

    legalFIPrimResultTyCon :: DynFlags -> TyCon -> Bool
    -- Check result type of 'foreign import prim'. Allow simple unlifted
    -- types and also unboxed tuple result types '... -> (# , , #)'
    legalFIPrimResultTyCon dflags tc
      = xopt Opt_UnliftedFFITypes dflags
        && isUnLiftedTyCon tc
        && (isUnboxedTupleTyCon tc
            || case tyConPrimRep tc of	-- Note [Marshalling VoidRep]
    	   VoidRep -> False
    	   _       -> True)

\end{code} Note [Marshalling VoidRep] ~~~~~~~~~~~~~~~~~~~~~~~~~~ We don't treat State# (whose PrimRep is VoidRep) as marshalable. In turn that means you can't write foreign import foo :: Int -> State# RealWorld Reason: the back end falls over with panic "primRepHint:VoidRep"; and there is no compelling reason to permit it