Type.hs

-- (c) The University of Glasgow 2006
-- (c) The GRASP/AQUA Project, Glasgow University, 1998
--
-- Type - public interface

{-# LANGUAGE CPP, FlexibleContexts #-}
{-# OPTIONS_GHC -fno-warn-orphans #-}

-- | Main functions for manipulating types and type-related things
module Type (
        -- Note some of this is just re-exports from TyCon..

        -- * Main data types representing Types
        -- $type_classification

        -- $representation_types
        TyThing(..), Type, ArgFlag(..), KindOrType, PredType, ThetaType,
        Var, TyVar, isTyVar, TyCoVar, TyBinder, TyVarBinder,

        -- ** Constructing and deconstructing types
        mkTyVarTy, mkTyVarTys, getTyVar, getTyVar_maybe, repGetTyVar_maybe,
        getCastedTyVar_maybe, tyVarKind,

        mkAppTy, mkAppTys, splitAppTy, splitAppTys, repSplitAppTys,
        splitAppTy_maybe, repSplitAppTy_maybe, tcRepSplitAppTy_maybe,

        mkFunTy, mkFunTys, splitFunTy, splitFunTy_maybe,
        splitFunTys, funResultTy, funArgTy,

        mkTyConApp, mkTyConTy,
        tyConAppTyCon_maybe, tyConAppTyConPicky_maybe,
        tyConAppArgs_maybe, tyConAppTyCon, tyConAppArgs,
        splitTyConApp_maybe, splitTyConApp, tyConAppArgN, nextRole,
        splitListTyConApp_maybe,
        repSplitTyConApp_maybe,

        mkForAllTy, mkForAllTys, mkInvForAllTys, mkSpecForAllTys,
        mkVisForAllTys, mkInvForAllTy,
        splitForAllTys, splitForAllTyVarBndrs,
        splitForAllTy_maybe, splitForAllTy,
        splitPiTy_maybe, splitPiTy, splitPiTys,
        mkPiTy, mkPiTys, mkTyConBindersPreferAnon,
        mkLamType, mkLamTypes,
        piResultTy, piResultTys,
        applyTysX, dropForAlls,

        mkNumLitTy, isNumLitTy,
        mkStrLitTy, isStrLitTy,

        mkCastTy, mkCoercionTy, splitCastTy_maybe,

        userTypeError_maybe, pprUserTypeErrorTy,

        coAxNthLHS,
        stripCoercionTy, splitCoercionType_maybe,

        splitPiTysInvisible, filterOutInvisibleTypes,
        filterOutInvisibleTyVars, partitionInvisibles,
        synTyConResKind,

        -- Analyzing types
        TyCoMapper(..), mapType, mapCoercion,

        -- (Newtypes)
        newTyConInstRhs,

        -- Pred types
        mkFamilyTyConApp,
        isDictLikeTy,
        mkPrimEqPred, mkReprPrimEqPred, mkPrimEqPredRole,
        equalityTyCon,
        mkHeteroPrimEqPred, mkHeteroReprPrimEqPred,
        mkClassPred,
        isClassPred, isEqPred, isNomEqPred,
        isIPPred, isIPPred_maybe, isIPTyCon, isIPClass,
        isCTupleClass,

        -- Deconstructing predicate types
        PredTree(..), EqRel(..), eqRelRole, classifyPredType,
        getClassPredTys, getClassPredTys_maybe,
        getEqPredTys, getEqPredTys_maybe, getEqPredRole,
        predTypeEqRel,

        -- ** Binders
        sameVis,
        mkTyVarBinder, mkTyVarBinders,
        mkAnonBinder,
        isAnonTyBinder, isNamedTyBinder,
        binderVar, binderVars, binderKind, binderArgFlag,
        tyBinderType,
        binderRelevantType_maybe, caseBinder,
        isVisibleArgFlag, isInvisibleArgFlag, isVisibleBinder, isInvisibleBinder,
        tyConBindersTyBinders,
        mkTyBinderTyConBinder,

        -- ** Common type constructors
        funTyCon,

        -- ** Predicates on types
        isTyVarTy, isFunTy, isDictTy, isPredTy, isCoercionTy,
        isCoercionTy_maybe, isCoercionType, isForAllTy,
        isPiTy, isTauTy, isFamFreeTy,

        -- (Lifting and boxity)
        isLiftedType_maybe, isUnliftedType, isUnboxedTupleType, isUnboxedSumType,
        isAlgType, isClosedAlgType,
        isPrimitiveType, isStrictType,
        isRuntimeRepTy, isRuntimeRepVar, isRuntimeRepKindedTy,
        dropRuntimeRepArgs,
        getRuntimeRep, getRuntimeRepFromKind,

        -- * Main data types representing Kinds
        Kind,

        -- ** Finding the kind of a type
        typeKind, isTypeLevPoly, resultIsLevPoly,

        -- ** Common Kind
        liftedTypeKind,

        -- * Type free variables
        tyCoFVsOfType, tyCoFVsBndr,
        tyCoVarsOfType, tyCoVarsOfTypes,
        tyCoVarsOfTypeDSet,
        coVarsOfType,
        coVarsOfTypes, closeOverKinds, closeOverKindsList,
        noFreeVarsOfType,
        splitVisVarsOfType, splitVisVarsOfTypes,
        expandTypeSynonyms,
        typeSize,

        -- * Well-scoped lists of variables
        dVarSetElemsWellScoped, toposortTyVars, tyCoVarsOfTypeWellScoped,
        tyCoVarsOfTypesWellScoped,

        -- * Type comparison
        eqType, eqTypeX, eqTypes, nonDetCmpType, nonDetCmpTypes, nonDetCmpTypeX,
        nonDetCmpTypesX, nonDetCmpTc,
        eqVarBndrs,

        -- * Forcing evaluation of types
        seqType, seqTypes,

        -- * Other views onto Types
        coreView, coreViewOneStarKind,

        tyConsOfType,

        -- * Main type substitution data types
        TvSubstEnv,     -- Representation widely visible
        TCvSubst(..),    -- Representation visible to a few friends

        -- ** Manipulating type substitutions
        emptyTvSubstEnv, emptyTCvSubst, mkEmptyTCvSubst,

        mkTCvSubst, zipTvSubst, mkTvSubstPrs,
        notElemTCvSubst,
        getTvSubstEnv, setTvSubstEnv,
        zapTCvSubst, getTCvInScope, getTCvSubstRangeFVs,
        extendTCvInScope, extendTCvInScopeList, extendTCvInScopeSet,
        extendTCvSubst, extendCvSubst,
        extendTvSubst, extendTvSubstBinder,
        extendTvSubstList, extendTvSubstAndInScope,
        extendTvSubstWithClone,
        isInScope, composeTCvSubstEnv, composeTCvSubst, zipTyEnv, zipCoEnv,
        isEmptyTCvSubst, unionTCvSubst,

        -- ** Performing substitution on types and kinds
        substTy, substTys, substTyWith, substTysWith, substTheta,
        substTyAddInScope,
        substTyUnchecked, substTysUnchecked, substThetaUnchecked,
        substTyWithUnchecked,
        substCoUnchecked, substCoWithUnchecked,
        substTyVarBndr, substTyVar, substTyVars,
        cloneTyVarBndr, cloneTyVarBndrs, lookupTyVar,

        -- * Pretty-printing
        pprType, pprParendType, pprTypeApp, pprTyThingCategory, pprShortTyThing,
        pprTvBndr, pprTvBndrs, pprForAll, pprUserForAll,
        pprSigmaType, ppSuggestExplicitKinds,
        pprTheta, pprThetaArrowTy, pprClassPred,
        pprKind, pprParendKind, pprSourceTyCon,
        TyPrec(..), maybeParen,
        pprTyVar, pprTyVars, pprTcAppTy, pprPrefixApp, pprArrowChain,

        -- * Tidying type related things up for printing
        tidyType,      tidyTypes,
        tidyOpenType,  tidyOpenTypes,
        tidyOpenKind,
        tidyTyCoVarBndr, tidyTyCoVarBndrs, tidyFreeTyCoVars,
        tidyOpenTyCoVar, tidyOpenTyCoVars,
        tidyTyVarOcc,
        tidyTopType,
        tidyKind,
        tidyTyVarBinder, tidyTyVarBinders
    ) where

#include "HsVersions.h"

-- We import the representation and primitive functions from TyCoRep.
-- Many things are reexported, but not the representation!

import Kind
import TyCoRep

-- friends:
import Var
import VarEnv
import VarSet
import NameEnv

import Class
import TyCon
import TysPrim
import {-# SOURCE #-} TysWiredIn ( listTyCon, typeNatKind
                                 , typeSymbolKind, liftedTypeKind )
import PrelNames
import CoAxiom
import {-# SOURCE #-} Coercion

-- others
import Util
import Outputable
import FastString
import Pair
import ListSetOps
import Digraph
import Unique ( nonDetCmpUnique )
import SrcLoc  ( SrcSpan )
import OccName ( OccName )
import Name    ( mkInternalName )

import Maybes           ( orElse )
import Data.Maybe       ( isJust, mapMaybe )
import Control.Monad    ( guard )
import Control.Arrow    ( first, second )

-- $type_classification
-- #type_classification#
--
-- Types are one of:
--
-- [Unboxed]            Iff its representation is other than a pointer
--                      Unboxed types are also unlifted.
--
-- [Lifted]             Iff it has bottom as an element.
--                      Closures always have lifted types: i.e. any
--                      let-bound identifier in Core must have a lifted
--                      type. Operationally, a lifted object is one that
--                      can be entered.
--                      Only lifted types may be unified with a type variable.
--
-- [Algebraic]          Iff it is a type with one or more constructors, whether
--                      declared with @data@ or @newtype@.
--                      An algebraic type is one that can be deconstructed
--                      with a case expression. This is /not/ the same as
--                      lifted types, because we also include unboxed
--                      tuples in this classification.
--
-- [Data]               Iff it is a type declared with @data@, or a boxed tuple.
--
-- [Primitive]          Iff it is a built-in type that can't be expressed in Haskell.
--
-- Currently, all primitive types are unlifted, but that's not necessarily
-- the case: for example, @Int@ could be primitive.
--
-- Some primitive types are unboxed, such as @Int#@, whereas some are boxed
-- but unlifted (such as @ByteArray#@).  The only primitive types that we
-- classify as algebraic are the unboxed tuples.
--
-- Some examples of type classifications that may make this a bit clearer are:
--
-- @
-- Type          primitive       boxed           lifted          algebraic
-- -----------------------------------------------------------------------------
-- Int#          Yes             No              No              No
-- ByteArray#    Yes             Yes             No              No
-- (\# a, b \#)  Yes             No              No              Yes
-- (\# a | b \#) Yes             No              No              Yes
-- (  a, b  )    No              Yes             Yes             Yes
-- [a]           No              Yes             Yes             Yes
-- @

-- $representation_types
-- A /source type/ is a type that is a separate type as far as the type checker is
-- concerned, but which has a more low-level representation as far as Core-to-Core
-- passes and the rest of the back end is concerned.
--
-- You don't normally have to worry about this, as the utility functions in
-- this module will automatically convert a source into a representation type
-- if they are spotted, to the best of it's abilities. If you don't want this
-- to happen, use the equivalent functions from the "TcType" module.

{-
************************************************************************
*                                                                      *
                Type representation
*                                                                      *
************************************************************************
-}

{-# INLINE coreView #-}
coreView :: Type -> Maybe Type
-- ^ This function Strips off the /top layer only/ of a type synonym
-- application (if any) its underlying representation type.
-- Returns Nothing if there is nothing to look through.
--
-- By being non-recursive and inlined, this case analysis gets efficiently
-- joined onto the case analysis that the caller is already doing
coreView (TyConApp tc tys) | Just (tenv, rhs, tys') <- expandSynTyCon_maybe tc tys
  = Just (mkAppTys (substTy (mkTvSubstPrs tenv) rhs) tys')
               -- The free vars of 'rhs' should all be bound by 'tenv', so it's
               -- ok to use 'substTy' here.
               -- See also Note [The substitution invariant] in TyCoRep.
               -- Its important to use mkAppTys, rather than (foldl AppTy),
               -- because the function part might well return a
               -- partially-applied type constructor; indeed, usually will!
coreView _ = Nothing

-- | Like 'coreView', but it also "expands" @Constraint@ to become
-- @TYPE LiftedRep@.
{-# INLINE coreViewOneStarKind #-}
coreViewOneStarKind :: Type -> Maybe Type
coreViewOneStarKind ty
  | Just ty' <- coreView ty   = Just ty'
  | TyConApp tc [] <- ty
  , isStarKindSynonymTyCon tc = Just liftedTypeKind
  | otherwise                 = Nothing

-----------------------------------------------
expandTypeSynonyms :: Type -> Type
-- ^ Expand out all type synonyms.  Actually, it'd suffice to expand out
-- just the ones that discard type variables (e.g.  type Funny a = Int)
-- But we don't know which those are currently, so we just expand all.
--
-- 'expandTypeSynonyms' only expands out type synonyms mentioned in the type,
-- not in the kinds of any TyCon or TyVar mentioned in the type.
--
-- Keep this synchronized with 'synonymTyConsOfType'
expandTypeSynonyms ty
  = go (mkEmptyTCvSubst in_scope) ty
  where
    in_scope = mkInScopeSet (tyCoVarsOfType ty)

    go subst (TyConApp tc tys)
      | Just (tenv, rhs, tys') <- expandSynTyCon_maybe tc expanded_tys
      = let subst' = mkTvSubst in_scope (mkVarEnv tenv)
            -- Make a fresh substitution; rhs has nothing to
            -- do with anything that has happened so far
            -- NB: if you make changes here, be sure to build an
            --     /idempotent/ substitution, even in the nested case
            --        type T a b = a -> b
            --        type S x y = T y x
            -- (Trac #11665)
        in  mkAppTys (go subst' rhs) tys'
      | otherwise
      = TyConApp tc expanded_tys
      where
        expanded_tys = (map (go subst) tys)

    go _     (LitTy l)     = LitTy l
    go subst (TyVarTy tv)  = substTyVar subst tv
    go subst (AppTy t1 t2) = mkAppTy (go subst t1) (go subst t2)
    go subst (FunTy arg res)
      = mkFunTy (go subst arg) (go subst res)
    go subst (ForAllTy (TvBndr tv vis) t)
      = let (subst', tv') = substTyVarBndrCallback go subst tv in
        ForAllTy (TvBndr tv' vis) (go subst' t)
    go subst (CastTy ty co)  = mkCastTy (go subst ty) (go_co subst co)
    go subst (CoercionTy co) = mkCoercionTy (go_co subst co)

    go_co subst (Refl r ty)
      = mkReflCo r (go subst ty)
       -- NB: coercions are always expanded upon creation
    go_co subst (TyConAppCo r tc args)
      = mkTyConAppCo r tc (map (go_co subst) args)
    go_co subst (AppCo co arg)
      = mkAppCo (go_co subst co) (go_co subst arg)
    go_co subst (ForAllCo tv kind_co co)
      = let (subst', tv', kind_co') = go_cobndr subst tv kind_co in
        mkForAllCo tv' kind_co' (go_co subst' co)
    go_co subst (CoVarCo cv)
      = substCoVar subst cv
    go_co subst (AxiomInstCo ax ind args)
      = mkAxiomInstCo ax ind (map (go_co subst) args)
    go_co subst (UnivCo p r t1 t2)
      = mkUnivCo (go_prov subst p) r (go subst t1) (go subst t2)
    go_co subst (SymCo co)
      = mkSymCo (go_co subst co)
    go_co subst (TransCo co1 co2)
      = mkTransCo (go_co subst co1) (go_co subst co2)
    go_co subst (NthCo n co)
      = mkNthCo n (go_co subst co)
    go_co subst (LRCo lr co)
      = mkLRCo lr (go_co subst co)
    go_co subst (InstCo co arg)
      = mkInstCo (go_co subst co) (go_co subst arg)
    go_co subst (CoherenceCo co1 co2)
      = mkCoherenceCo (go_co subst co1) (go_co subst co2)
    go_co subst (KindCo co)
      = mkKindCo (go_co subst co)
    go_co subst (SubCo co)
      = mkSubCo (go_co subst co)
    go_co subst (AxiomRuleCo ax cs) = AxiomRuleCo ax (map (go_co subst) cs)

    go_prov _     UnsafeCoerceProv    = UnsafeCoerceProv
    go_prov subst (PhantomProv co)    = PhantomProv (go_co subst co)
    go_prov subst (ProofIrrelProv co) = ProofIrrelProv (go_co subst co)
    go_prov _     p@(PluginProv _)    = p
    go_prov _     (HoleProv h)        = pprPanic "expandTypeSynonyms hit a hole" (ppr h)

      -- the "False" and "const" are to accommodate the type of
      -- substForAllCoBndrCallback, which is general enough to
      -- handle coercion optimization (which sometimes swaps the
      -- order of a coercion)
    go_cobndr subst = substForAllCoBndrCallback False (go_co subst) subst

{-
************************************************************************
*                                                                      *
   Analyzing types
*                                                                      *
************************************************************************

These functions do a map-like operation over types, performing some operation
on all variables and binding sites. Primarily used for zonking.

Note [Efficiency for mapCoercion ForAllCo case]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
As noted in Note [Forall coercions] in TyCoRep, a ForAllCo is a bit redundant.
It stores a TyVar and a Coercion, where the kind of the TyVar always matches
the left-hand kind of the coercion. This is convenient lots of the time, but
not when mapping a function over a coercion.

The problem is that tcm_tybinder will affect the TyVar's kind and
mapCoercion will affect the Coercion, and we hope that the results will be
the same. Even if they are the same (which should generally happen with
correct algorithms), then there is an efficiency issue. In particular,
this problem seems to make what should be a linear algorithm into a potentially
exponential one. But it's only going to be bad in the case where there's
lots of foralls in the kinds of other foralls. Like this:

  forall a : (forall b : (forall c : ...). ...). ...

This construction seems unlikely. So we'll do the inefficient, easy way
for now.

Note [Specialising mappers]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
These INLINABLE pragmas are indispensable. mapType/mapCoercion are used
to implement zonking, and it's vital that they get specialised to the TcM
monad. This specialisation happens automatically (that is, without a
SPECIALISE pragma) as long as the definitions are INLINABLE. For example,
this one change made a 20% allocation difference in perf/compiler/T5030.

-}

-- | This describes how a "map" operation over a type/coercion should behave
data TyCoMapper env m
  = TyCoMapper
      { tcm_smart :: Bool -- ^ Should the new type be created with smart
                         -- constructors?
      , tcm_tyvar :: env -> TyVar -> m Type
      , tcm_covar :: env -> CoVar -> m Coercion
      , tcm_hole  :: env -> CoercionHole -> Role
                  -> Type -> Type -> m Coercion
          -- ^ What to do with coercion holes. See Note [Coercion holes] in
          -- TyCoRep.

      , tcm_tybinder :: env -> TyVar -> ArgFlag -> m (env, TyVar)
          -- ^ The returned env is used in the extended scope
      }

{-# INLINABLE mapType #-}  -- See Note [Specialising mappers]
mapType :: Monad m => TyCoMapper env m -> env -> Type -> m Type
mapType mapper@(TyCoMapper { tcm_smart = smart, tcm_tyvar = tyvar
                           , tcm_tybinder = tybinder })
        env ty
  = go ty
  where
    go (TyVarTy tv) = tyvar env tv
    go (AppTy t1 t2) = mkappty <$> go t1 <*> go t2
    go t@(TyConApp _ []) = return t  -- avoid allocation in this exceedingly
                                     -- common case (mostly, for *)
    go (TyConApp tc tys) = mktyconapp tc <$> mapM go tys
    go (FunTy arg res)   = FunTy <$> go arg <*> go res
    go (ForAllTy (TvBndr tv vis) inner)
      = do { (env', tv') <- tybinder env tv vis
           ; inner' <- mapType mapper env' inner
           ; return $ ForAllTy (TvBndr tv' vis) inner' }
    go ty@(LitTy {})   = return ty
    go (CastTy ty co)  = mkcastty <$> go ty <*> mapCoercion mapper env co
    go (CoercionTy co) = CoercionTy <$> mapCoercion mapper env co

    (mktyconapp, mkappty, mkcastty)
      | smart     = (mkTyConApp, mkAppTy, mkCastTy)
      | otherwise = (TyConApp,   AppTy,   CastTy)

{-# INLINABLE mapCoercion #-}  -- See Note [Specialising mappers]
mapCoercion :: Monad m
            => TyCoMapper env m -> env -> Coercion -> m Coercion
mapCoercion mapper@(TyCoMapper { tcm_smart = smart, tcm_covar = covar
                               , tcm_hole = cohole, tcm_tybinder = tybinder })
            env co
  = go co
  where
    go (Refl r ty) = Refl r <$> mapType mapper env ty
    go (TyConAppCo r tc args)
      = mktyconappco r tc <$> mapM go args
    go (AppCo c1 c2) = mkappco <$> go c1 <*> go c2
    go (ForAllCo tv kind_co co)
      = do { kind_co' <- go kind_co
           ; (env', tv') <- tybinder env tv Inferred
           ; co' <- mapCoercion mapper env' co
           ; return $ mkforallco tv' kind_co' co' }
        -- See Note [Efficiency for mapCoercion ForAllCo case]
    go (CoVarCo cv) = covar env cv
    go (AxiomInstCo ax i args)
      = mkaxiominstco ax i <$> mapM go args
    go (UnivCo (HoleProv hole) r t1 t2)
      = cohole env hole r t1 t2
    go (UnivCo p r t1 t2)
      = mkunivco <$> go_prov p <*> pure r
                 <*> mapType mapper env t1 <*> mapType mapper env t2
    go (SymCo co) = mksymco <$> go co
    go (TransCo c1 c2) = mktransco <$> go c1 <*> go c2
    go (AxiomRuleCo r cos) = AxiomRuleCo r <$> mapM go cos
    go (NthCo i co)        = mknthco i <$> go co
    go (LRCo lr co)        = mklrco lr <$> go co
    go (InstCo co arg)     = mkinstco <$> go co <*> go arg
    go (CoherenceCo c1 c2) = mkcoherenceco <$> go c1 <*> go c2
    go (KindCo co)         = mkkindco <$> go co
    go (SubCo co)          = mksubco <$> go co

    go_prov UnsafeCoerceProv    = return UnsafeCoerceProv
    go_prov (PhantomProv co)    = PhantomProv <$> go co
    go_prov (ProofIrrelProv co) = ProofIrrelProv <$> go co
    go_prov p@(PluginProv _)    = return p
    go_prov (HoleProv _)        = panic "mapCoercion"

    ( mktyconappco, mkappco, mkaxiominstco, mkunivco
      , mksymco, mktransco, mknthco, mklrco, mkinstco, mkcoherenceco
      , mkkindco, mksubco, mkforallco)
      | smart
      = ( mkTyConAppCo, mkAppCo, mkAxiomInstCo, mkUnivCo
        , mkSymCo, mkTransCo, mkNthCo, mkLRCo, mkInstCo, mkCoherenceCo
        , mkKindCo, mkSubCo, mkForAllCo )
      | otherwise
      = ( TyConAppCo, AppCo, AxiomInstCo, UnivCo
        , SymCo, TransCo, NthCo, LRCo, InstCo, CoherenceCo
        , KindCo, SubCo, ForAllCo )

{-
************************************************************************
*                                                                      *
\subsection{Constructor-specific functions}
*                                                                      *
************************************************************************


---------------------------------------------------------------------
                                TyVarTy
                                ~~~~~~~
-}

-- | Attempts to obtain the type variable underlying a 'Type', and panics with the
-- given message if this is not a type variable type. See also 'getTyVar_maybe'
getTyVar :: String -> Type -> TyVar
getTyVar msg ty = case getTyVar_maybe ty of
                    Just tv -> tv
                    Nothing -> panic ("getTyVar: " ++ msg)

isTyVarTy :: Type -> Bool
isTyVarTy ty = isJust (getTyVar_maybe ty)

-- | Attempts to obtain the type variable underlying a 'Type'
getTyVar_maybe :: Type -> Maybe TyVar
getTyVar_maybe ty | Just ty' <- coreView ty = getTyVar_maybe ty'
                  | otherwise               = repGetTyVar_maybe ty

-- | If the type is a tyvar, possibly under a cast, returns it, along
-- with the coercion. Thus, the co is :: kind tv ~R kind type
getCastedTyVar_maybe :: Type -> Maybe (TyVar, Coercion)
getCastedTyVar_maybe ty | Just ty' <- coreView ty = getCastedTyVar_maybe ty'
getCastedTyVar_maybe (CastTy (TyVarTy tv) co)     = Just (tv, co)
getCastedTyVar_maybe (TyVarTy tv)
  = Just (tv, mkReflCo Nominal (tyVarKind tv))
getCastedTyVar_maybe _                            = Nothing

-- | Attempts to obtain the type variable underlying a 'Type', without
-- any expansion
repGetTyVar_maybe :: Type -> Maybe TyVar
repGetTyVar_maybe (TyVarTy tv) = Just tv
repGetTyVar_maybe _            = Nothing

{-
---------------------------------------------------------------------
                                AppTy
                                ~~~~~
We need to be pretty careful with AppTy to make sure we obey the
invariant that a TyConApp is always visibly so.  mkAppTy maintains the
invariant: use it.

Note [Decomposing fat arrow c=>t]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Can we unify (a b) with (Eq a => ty)?   If we do so, we end up with
a partial application like ((=>) Eq a) which doesn't make sense in
source Haskell.  In constrast, we *can* unify (a b) with (t1 -> t2).
Here's an example (Trac #9858) of how you might do it:
   i :: (Typeable a, Typeable b) => Proxy (a b) -> TypeRep
   i p = typeRep p

   j = i (Proxy :: Proxy (Eq Int => Int))
The type (Proxy (Eq Int => Int)) is only accepted with -XImpredicativeTypes,
but suppose we want that.  But then in the call to 'i', we end
up decomposing (Eq Int => Int), and we definitely don't want that.

This really only applies to the type checker; in Core, '=>' and '->'
are the same, as are 'Constraint' and '*'.  But for now I've put
the test in repSplitAppTy_maybe, which applies throughout, because
the other calls to splitAppTy are in Unify, which is also used by
the type checker (e.g. when matching type-function equations).

-}

-- | Applies a type to another, as in e.g. @k a@
mkAppTy :: Type -> Type -> Type
mkAppTy (TyConApp tc tys) ty2 = mkTyConApp tc (tys ++ [ty2])
mkAppTy ty1               ty2 = AppTy ty1 ty2
        -- Note that the TyConApp could be an
        -- under-saturated type synonym.  GHC allows that; e.g.
        --      type Foo k = k a -> k a
        --      type Id x = x
        --      foo :: Foo Id -> Foo Id
        --
        -- Here Id is partially applied in the type sig for Foo,
        -- but once the type synonyms are expanded all is well

mkAppTys :: Type -> [Type] -> Type
mkAppTys ty1                []   = ty1
mkAppTys (TyConApp tc tys1) tys2 = mkTyConApp tc (tys1 ++ tys2)
mkAppTys ty1                tys2 = foldl AppTy ty1 tys2

-------------
splitAppTy_maybe :: Type -> Maybe (Type, Type)
-- ^ Attempt to take a type application apart, whether it is a
-- function, type constructor, or plain type application. Note
-- that type family applications are NEVER unsaturated by this!
splitAppTy_maybe ty | Just ty' <- coreView ty
                    = splitAppTy_maybe ty'
splitAppTy_maybe ty = repSplitAppTy_maybe ty

-------------
repSplitAppTy_maybe :: Type -> Maybe (Type,Type)
-- ^ Does the AppTy split as in 'splitAppTy_maybe', but assumes that
-- any Core view stuff is already done
repSplitAppTy_maybe (FunTy ty1 ty2)   = Just (TyConApp funTyCon [ty1], ty2)
repSplitAppTy_maybe (AppTy ty1 ty2)   = Just (ty1, ty2)
repSplitAppTy_maybe (TyConApp tc tys)
  | mightBeUnsaturatedTyCon tc || tys `lengthExceeds` tyConArity tc
  , Just (tys', ty') <- snocView tys
  = Just (TyConApp tc tys', ty')    -- Never create unsaturated type family apps!
repSplitAppTy_maybe _other = Nothing

-- this one doesn't braek apart (c => t).
-- See Note [Decomposing fat arrow c=>t]
-- Defined here to avoid module loops between Unify and TcType.
tcRepSplitAppTy_maybe :: Type -> Maybe (Type,Type)
-- ^ Does the AppTy split as in 'tcSplitAppTy_maybe', but assumes that
-- any coreView stuff is already done. Refuses to look through (c => t)
tcRepSplitAppTy_maybe (FunTy ty1 ty2)
  | isConstraintKind (typeKind ty1)     = Nothing  -- See Note [Decomposing fat arrow c=>t]
  | otherwise                           = Just (TyConApp funTyCon [ty1], ty2)
tcRepSplitAppTy_maybe (AppTy ty1 ty2)   = Just (ty1, ty2)
tcRepSplitAppTy_maybe (TyConApp tc tys)
  | mightBeUnsaturatedTyCon tc || tys `lengthExceeds` tyConArity tc
  , Just (tys', ty') <- snocView tys
  = Just (TyConApp tc tys', ty')    -- Never create unsaturated type family apps!
tcRepSplitAppTy_maybe _other = Nothing
-------------
splitAppTy :: Type -> (Type, Type)
-- ^ Attempts to take a type application apart, as in 'splitAppTy_maybe',
-- and panics if this is not possible
splitAppTy ty = case splitAppTy_maybe ty of
                Just pr -> pr
                Nothing -> panic "splitAppTy"

-------------
splitAppTys :: Type -> (Type, [Type])
-- ^ Recursively splits a type as far as is possible, leaving a residual
-- type being applied to and the type arguments applied to it. Never fails,
-- even if that means returning an empty list of type applications.
splitAppTys ty = split ty ty []
  where
    split orig_ty ty args | Just ty' <- coreView ty = split orig_ty ty' args
    split _       (AppTy ty arg)        args = split ty ty (arg:args)
    split _       (TyConApp tc tc_args) args
      = let -- keep type families saturated
            n | mightBeUnsaturatedTyCon tc = 0
              | otherwise                  = tyConArity tc
            (tc_args1, tc_args2) = splitAt n tc_args
        in
        (TyConApp tc tc_args1, tc_args2 ++ args)
    split _   (FunTy ty1 ty2) args = ASSERT( null args )
                                     (TyConApp funTyCon [], [ty1,ty2])
    split orig_ty _           args = (orig_ty, args)

-- | Like 'splitAppTys', but doesn't look through type synonyms
repSplitAppTys :: Type -> (Type, [Type])
repSplitAppTys ty = split ty []
  where
    split (AppTy ty arg) args = split ty (arg:args)
    split (TyConApp tc tc_args) args
      = let n | mightBeUnsaturatedTyCon tc = 0
              | otherwise                  = tyConArity tc
            (tc_args1, tc_args2) = splitAt n tc_args
        in
        (TyConApp tc tc_args1, tc_args2 ++ args)
    split (FunTy ty1 ty2) args = ASSERT( null args )
                                 (TyConApp funTyCon [], [ty1, ty2])
    split ty args = (ty, args)

{-
                      LitTy
                      ~~~~~
-}

mkNumLitTy :: Integer -> Type
mkNumLitTy n = LitTy (NumTyLit n)

-- | Is this a numeric literal. We also look through type synonyms.
isNumLitTy :: Type -> Maybe Integer
isNumLitTy ty | Just ty1 <- coreView ty = isNumLitTy ty1
isNumLitTy (LitTy (NumTyLit n)) = Just n
isNumLitTy _                    = Nothing

mkStrLitTy :: FastString -> Type
mkStrLitTy s = LitTy (StrTyLit s)

-- | Is this a symbol literal. We also look through type synonyms.
isStrLitTy :: Type -> Maybe FastString
isStrLitTy ty | Just ty1 <- coreView ty = isStrLitTy ty1
isStrLitTy (LitTy (StrTyLit s)) = Just s
isStrLitTy _                    = Nothing


-- | Is this type a custom user error?
-- If so, give us the kind and the error message.
userTypeError_maybe :: Type -> Maybe Type
userTypeError_maybe t
  = do { (tc, _kind : msg : _) <- splitTyConApp_maybe t
          -- There may be more than 2 arguments, if the type error is
          -- used as a type constructor (e.g. at kind `Type -> Type`).

       ; guard (tyConName tc == errorMessageTypeErrorFamName)
       ; return msg }

-- | Render a type corresponding to a user type error into a SDoc.
pprUserTypeErrorTy :: Type -> SDoc
pprUserTypeErrorTy ty =
  case splitTyConApp_maybe ty of

    -- Text "Something"
    Just (tc,[txt])
      | tyConName tc == typeErrorTextDataConName
      , Just str <- isStrLitTy txt -> ftext str

    -- ShowType t
    Just (tc,[_k,t])
      | tyConName tc == typeErrorShowTypeDataConName -> ppr t

    -- t1 :<>: t2
    Just (tc,[t1,t2])
      | tyConName tc == typeErrorAppendDataConName ->
        pprUserTypeErrorTy t1 <> pprUserTypeErrorTy t2

    -- t1 :$$: t2
    Just (tc,[t1,t2])
      | tyConName tc == typeErrorVAppendDataConName ->
        pprUserTypeErrorTy t1 $$ pprUserTypeErrorTy t2

    -- An uneavaluated type function
    _ -> ppr ty




{-
---------------------------------------------------------------------
                                FunTy
                                ~~~~~
-}

isFunTy :: Type -> Bool
isFunTy ty = isJust (splitFunTy_maybe ty)

splitFunTy :: Type -> (Type, Type)
-- ^ Attempts to extract the argument and result types from a type, and
-- panics if that is not possible. See also 'splitFunTy_maybe'
splitFunTy ty | Just ty' <- coreView ty = splitFunTy ty'
splitFunTy (FunTy arg res) = (arg, res)
splitFunTy other           = pprPanic "splitFunTy" (ppr other)

splitFunTy_maybe :: Type -> Maybe (Type, Type)
-- ^ Attempts to extract the argument and result types from a type
splitFunTy_maybe ty | Just ty' <- coreView ty = splitFunTy_maybe ty'
splitFunTy_maybe (FunTy arg res) = Just (arg, res)
splitFunTy_maybe _               = Nothing

splitFunTys :: Type -> ([Type], Type)
splitFunTys ty = split [] ty ty
  where
    split args orig_ty ty | Just ty' <- coreView ty = split args orig_ty ty'
    split args _       (FunTy arg res) = split (arg:args) res res
    split args orig_ty _               = (reverse args, orig_ty)

funResultTy :: Type -> Type
-- ^ Extract the function result type and panic if that is not possible
funResultTy ty | Just ty' <- coreView ty = funResultTy ty'
funResultTy (FunTy _ res) = res
funResultTy ty            = pprPanic "funResultTy" (ppr ty)

funArgTy :: Type -> Type
-- ^ Extract the function argument type and panic if that is not possible
funArgTy ty | Just ty' <- coreView ty = funArgTy ty'
funArgTy (FunTy arg _res) = arg
funArgTy ty               = pprPanic "funArgTy" (ppr ty)

piResultTy :: Type -> Type ->  Type
piResultTy ty arg = case piResultTy_maybe ty arg of
                      Just res -> res
                      Nothing  -> pprPanic "piResultTy" (ppr ty $$ ppr arg)

piResultTy_maybe :: Type -> Type -> Maybe Type

-- ^ Just like 'piResultTys' but for a single argument
-- Try not to iterate 'piResultTy', because it's inefficient to substitute
-- one variable at a time; instead use 'piResultTys"
piResultTy_maybe ty arg
  | Just ty' <- coreView ty = piResultTy_maybe ty' arg

  | FunTy _ res <- ty
  = Just res

  | ForAllTy (TvBndr tv _) res <- ty
  = let empty_subst = mkEmptyTCvSubst $ mkInScopeSet $
                      tyCoVarsOfTypes [arg,res]
    in Just (substTy (extendTvSubst empty_subst tv arg) res)

  | otherwise
  = Nothing

-- | (piResultTys f_ty [ty1, .., tyn]) gives the type of (f ty1 .. tyn)
--   where f :: f_ty
-- 'piResultTys' is interesting because:
--      1. 'f_ty' may have more for-alls than there are args
--      2. Less obviously, it may have fewer for-alls
-- For case 2. think of:
--   piResultTys (forall a.a) [forall b.b, Int]
-- This really can happen, but only (I think) in situations involving
-- undefined.  For example:
--       undefined :: forall a. a
-- Term: undefined @(forall b. b->b) @Int
-- This term should have type (Int -> Int), but notice that
-- there are more type args than foralls in 'undefined's type.

-- If you edit this function, you may need to update the GHC formalism
-- See Note [GHC Formalism] in coreSyn/CoreLint.hs

-- This is a heavily used function (e.g. from typeKind),
-- so we pay attention to efficiency, especially in the special case
-- where there are no for-alls so we are just dropping arrows from
-- a function type/kind.
piResultTys :: Type -> [Type] -> Type
piResultTys ty [] = ty
piResultTys ty orig_args@(arg:args)
  | Just ty' <- coreView ty
  = piResultTys ty' orig_args

  | FunTy _ res <- ty
  = piResultTys res args

  | ForAllTy (TvBndr tv _) res <- ty
  = go (extendVarEnv emptyTvSubstEnv tv arg) res args

  | otherwise
  = pprPanic "piResultTys1" (ppr ty $$ ppr orig_args)
  where
    in_scope = mkInScopeSet (tyCoVarsOfTypes (ty:orig_args))

    go :: TvSubstEnv -> Type -> [Type] -> Type
    go tv_env ty [] = substTy (mkTvSubst in_scope tv_env) ty

    go tv_env ty all_args@(arg:args)
      | Just ty' <- coreView ty
      = go tv_env ty' all_args

      | FunTy _ res <- ty
      = go tv_env res args

      | ForAllTy (TvBndr tv _) res <- ty
      = go (extendVarEnv tv_env tv arg) res args

      | TyVarTy tv <- ty
      , Just ty' <- lookupVarEnv tv_env tv
        -- Deals with piResultTys (forall a. a) [forall b.b, Int]
      = piResultTys ty' all_args

      | otherwise
      = pprPanic "piResultTys2" (ppr ty $$ ppr orig_args $$ ppr all_args)

applyTysX :: [TyVar] -> Type -> [Type] -> Type
-- applyTyxX beta-reduces (/\tvs. body_ty) arg_tys
-- Assumes that (/\tvs. body_ty) is closed
applyTysX tvs body_ty arg_tys
  = ASSERT2( length arg_tys >= n_tvs, pp_stuff )
    ASSERT2( tyCoVarsOfType body_ty `subVarSet` mkVarSet tvs, pp_stuff )
    mkAppTys (substTyWith tvs (take n_tvs arg_tys) body_ty)
             (drop n_tvs arg_tys)
  where
    pp_stuff = vcat [ppr tvs, ppr body_ty, ppr arg_tys]
    n_tvs = length tvs

{-