-\begin{code}
-module Unify (
- -- Matching and unification
- tcMatchTys, tcMatchTyX, ruleMatchTyX, tcMatchPreds, MatchEnv(..),
-
- tcUnifyTys,
-
- gadtRefineTys, BindFlag(..),
+%
+% (c) The University of Glasgow 2006
+%
- coreRefineTys, dataConCanMatch, TypeRefinement,
+\begin{code}
+{-# OPTIONS -w #-}
+-- The above warning supression flag is a temporary kludge.
+-- While working on this module you are encouraged to remove it and fix
+-- any warnings in the module. See
+-- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
+-- for details
- -- Re-export
- MaybeErr(..)
+module Unify (
+ -- Matching of types:
+ -- the "tc" prefix indicates that matching always
+ -- respects newtypes (rather than looking through them)
+ tcMatchTy, tcMatchTys, tcMatchTyX,
+ ruleMatchTyX, tcMatchPreds, MatchEnv(..),
+
+ dataConCannotMatch
) where
#include "HsVersions.h"
-import Var ( Var, TyVar, tyVarKind )
+import Var
import VarEnv
import VarSet
-import Kind ( isSubKind )
-import Type ( typeKind, tyVarsOfType, tyVarsOfTypes, tyVarsOfTheta, mkTyVarTys,
- TvSubstEnv, emptyTvSubstEnv, TvSubst(..), substTy, tcEqTypeX,
- mkOpenTvSubst, tcView )
-import TypeRep ( Type(..), PredType(..), funTyCon )
-import DataCon ( DataCon, isVanillaDataCon, dataConResTys, dataConInstResTy )
-import Util ( snocView )
-import ErrUtils ( Message )
+import Type
+import TyCon
+import DataCon
+import TypeRep
import Outputable
+import Util
import Maybes
\end{code}
, me_env :: RnEnv2 -- Renaming envt for nested foralls
} -- In-scope set includes template tyvars
+tcMatchTy :: TyVarSet -- Template tyvars
+ -> Type -- Template
+ -> Type -- Target
+ -> Maybe TvSubst -- One-shot; in principle the template
+ -- variables could be free in the target
+
+tcMatchTy tmpls ty1 ty2
+ = case match menv emptyTvSubstEnv ty1 ty2 of
+ Just subst_env -> Just (TvSubst in_scope subst_env)
+ Nothing -> Nothing
+ where
+ menv = ME { me_tmpls = tmpls, me_env = mkRnEnv2 in_scope }
+ in_scope = mkInScopeSet (tmpls `unionVarSet` tyVarsOfType ty2)
+ -- We're assuming that all the interesting
+ -- tyvars in tys1 are in tmpls
+
tcMatchTys :: TyVarSet -- Template tyvars
- -> [Type] -- Template
- -> [Type] -- Target
- -> Maybe TvSubst -- One-shot; in principle the template
+ -> [Type] -- Template
+ -> [Type] -- Target
+ -> Maybe TvSubst -- One-shot; in principle the template
-- variables could be free in the target
tcMatchTys tmpls tys1 tys2
-- it respects NewTypes and PredType
match menv subst ty1 ty2 | Just ty1' <- tcView ty1 = match menv subst ty1' ty2
-match menv subst ty1 ty2 | Just ty2' <- tcView ty2 = match menv subst ty1 ty2'
+ | Just ty2' <- tcView ty2 = match menv subst ty1 ty2'
match menv subst (TyVarTy tv1) ty2
- | tv1 `elemVarSet` me_tmpls menv
+ | tv1' `elemVarSet` me_tmpls menv
= case lookupVarEnv subst tv1' of
- Nothing | any (inRnEnvR rn_env) (varSetElems (tyVarsOfType ty2))
- -> Nothing -- Occurs check
- | not (typeKind ty2 `isSubKind` tyVarKind tv1)
- -> Nothing -- Kind mis-match
- | otherwise
- -> Just (extendVarEnv subst tv1 ty2)
-
- Just ty1' | tcEqTypeX (nukeRnEnvL rn_env) ty1' ty2
+ Nothing -- No existing binding
+ | any (inRnEnvR rn_env) (varSetElems (tyVarsOfType ty2))
+ -> Nothing -- Occurs check
+ | not (typeKind ty2 `isSubKind` tyVarKind tv1)
+ -> Nothing -- Kind mis-match
+ | otherwise
+ -> Just (extendVarEnv subst tv1' ty2)
+
+ Just ty1' -- There is an existing binding; check whether ty2 matches it
+ | tcEqTypeX (nukeRnEnvL rn_env) ty1' ty2
-- ty1 has no locally-bound variables, hence nukeRnEnvL
-- Note tcEqType...we are doing source-type matching here
- -> Just subst
-
- other -> Nothing
+ -> Just subst
+ | otherwise -> Nothing -- ty2 doesn't match
+
| otherwise -- tv1 is not a template tyvar
= case ty2 of
; match menv subst' ty1b ty2b }
match menv subst (AppTy ty1a ty1b) ty2
| Just (ty2a, ty2b) <- repSplitAppTy_maybe ty2
+ -- 'repSplit' used because the tcView stuff is done above
= do { subst' <- match menv subst ty1a ty2a
; match menv subst' ty1b ty2b }
%************************************************************************
%* *
- Unification
+ GADTs
%* *
%************************************************************************
-\begin{code}
-tcUnifyTys :: (TyVar -> BindFlag)
- -> [Type] -> [Type]
- -> Maybe TvSubst -- A regular one-shot substitution
--- The two types may have common type variables, and indeed do so in the
--- second call to tcUnifyTys in FunDeps.checkClsFD
-tcUnifyTys bind_fn tys1 tys2
- = maybeErrToMaybe $ initUM bind_fn $
- do { subst_env <- unify_tys emptyTvSubstEnv tys1 tys2
-
- -- Find the fixed point of the resulting non-idempotent substitution
- ; let in_scope = mkInScopeSet (tvs1 `unionVarSet` tvs2)
- subst = TvSubst in_scope subst_env_fixpt
- subst_env_fixpt = mapVarEnv (substTy subst) subst_env
- ; return subst }
- where
- tvs1 = tyVarsOfTypes tys1
- tvs2 = tyVarsOfTypes tys2
+Note [Pruning dead case alternatives]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider data T a where
+ T1 :: T Int
+ T2 :: T a
-----------------------------
-dataConCanMatch :: DataCon -> [Type] -> Bool
--- Returns True iff the data con can match a scrutinee of type (T tys)
--- where T is the type constructor for the data con
-dataConCanMatch con tys
- | isVanillaDataCon con
- = True
- | otherwise
- = isSuccess $ initUM (\tv -> BindMe) $
- unify_tys emptyTvSubstEnv (dataConResTys con) tys
-
-----------------------------
-coreRefineTys :: DataCon -> [TyVar] -- Case pattern (con tv1 .. tvn ...)
- -> Type -- Type of scrutinee
- -> Maybe TypeRefinement
-
-type TypeRefinement = (TvSubstEnv, Bool)
- -- The Bool is True iff all the bindings in the
- -- env are for the pattern type variables
- -- In this case, there is no type refinement
- -- for already-in-scope type variables
-
--- Used by Core Lint and the simplifier.
-coreRefineTys con tvs scrut_ty
- = maybeErrToMaybe $ initUM (tryToBind tv_set) $
- do { -- Run the unifier, starting with an empty env
- ; subst_env <- unify emptyTvSubstEnv pat_res_ty scrut_ty
-
- -- Find the fixed point of the resulting non-idempotent substitution
- ; let subst = mkOpenTvSubst subst_env_fixpt
- subst_env_fixpt = mapVarEnv (substTy subst) subst_env
-
- ; return (subst_env_fixpt, all_bound_here subst_env) }
- where
- pat_res_ty = dataConInstResTy con (mkTyVarTys tvs)
-
- -- 'tvs' are the tyvars bound by the pattern
- tv_set = mkVarSet tvs
- all_bound_here env = all bound_here (varEnvKeys env)
- bound_here uniq = elemVarSetByKey uniq tv_set
-
--- This version is used by the type checker
-gadtRefineTys :: TvSubst
- -> DataCon -> [TyVar]
- -> [Type] -> [Type]
- -> MaybeErr Message (TvSubst, Bool)
--- The bool is True <=> the only *new* bindings are for pat_tvs
-
-gadtRefineTys (TvSubst in_scope env1) con pat_tvs pat_tys ctxt_tys
- = initUM (tryToBind tv_set) $
- do { -- Run the unifier, starting with an empty env
- ; env2 <- unify_tys env1 pat_tys ctxt_tys
-
- -- Find the fixed point of the resulting non-idempotent substitution
- ; let subst2 = TvSubst in_scope subst_env_fixpt
- subst_env_fixpt = mapVarEnv (substTy subst2) env2
-
- ; return (subst2, all_bound_here env2) }
- where
- -- 'tvs' are the tyvars bound by the pattern
- tv_set = mkVarSet pat_tvs
- all_bound_here env = all bound_here (varEnvKeys env)
- bound_here uniq = elemVarEnvByKey uniq env1 || elemVarSetByKey uniq tv_set
- -- The bool is True <=> the only *new* bindings are for pat_tvs
-
-----------------------------
-tryToBind :: TyVarSet -> TyVar -> BindFlag
-tryToBind tv_set tv | tv `elemVarSet` tv_set = BindMe
- | otherwise = AvoidMe
-\end{code}
+ newtype X = MkX Int
+ newtype Y = MkY Char
+ type family F a
+ type instance F Bool = Int
-%************************************************************************
-%* *
- The workhorse
-%* *
-%************************************************************************
+Now consider case x of { T1 -> e1; T2 -> e2 }
-\begin{code}
-unify :: TvSubstEnv -- An existing substitution to extend
- -> Type -> Type -- Types to be unified
- -> UM TvSubstEnv -- Just the extended substitution,
- -- Nothing if unification failed
--- We do not require the incoming substitution to be idempotent,
--- nor guarantee that the outgoing one is. That's fixed up by
--- the wrappers.
+The question before the house is this: if I know something about the type
+of x, can I prune away the T1 alternative?
--- Respects newtypes, PredTypes
+Suppose x::T Char. It's impossible to construct a (T Char) using T1,
+ Answer = YES (clearly)
-unify subst ty1 ty2 = -- pprTrace "unify" (ppr subst <+> pprParendType ty1 <+> pprParendType ty2) $
- unify_ subst ty1 ty2
+Suppose x::T (F a), where 'a' is in scope. Then 'a' might be instantiated
+to 'Bool', in which case x::T Int, so
+ ANSWER = NO (clearly)
--- in unify_, any NewTcApps/Preds should be taken at face value
-unify_ subst (TyVarTy tv1) ty2 = uVar False subst tv1 ty2
-unify_ subst ty1 (TyVarTy tv2) = uVar True subst tv2 ty1
+Suppose x::T X. Then *in Haskell* it's impossible to construct a (non-bottom)
+value of type (T X) using T1. But *in FC* it's quite possible. The newtype
+gives a coercion
+ CoX :: X ~ Int
+So (T CoX) :: T X ~ T Int; hence (T1 `cast` sym (T CoX)) is a non-bottom value
+of type (T X) constructed with T1. Hence
+ ANSWER = NO (surprisingly)
-unify_ subst ty1 ty2 | Just ty1' <- tcView ty1 = unify subst ty1' ty2
-unify_ subst ty1 ty2 | Just ty2' <- tcView ty2 = unify subst ty1 ty2'
+Furthermore, this can even happen; see Trac #1251. GHC's newtype-deriving
+mechanism uses a cast, just as above, to move from one dictionary to another,
+in effect giving the programmer access to CoX.
-unify_ subst (PredTy p1) (PredTy p2) = unify_pred subst p1 p2
+Finally, suppose x::T Y. Then *even in FC* we can't construct a
+non-bottom value of type (T Y) using T1. That's because we can get
+from Y to Char, but not to Int.
-unify_ subst t1@(TyConApp tyc1 tys1) t2@(TyConApp tyc2 tys2)
- | tyc1 == tyc2 = unify_tys subst tys1 tys2
-unify_ subst (FunTy ty1a ty1b) (FunTy ty2a ty2b)
- = do { subst' <- unify subst ty1a ty2a
- ; unify subst' ty1b ty2b }
+Here's a related question. data Eq a b where EQ :: Eq a a
+Consider
+ case x of { EQ -> ... }
- -- Applications need a bit of care!
- -- They can match FunTy and TyConApp, so use splitAppTy_maybe
- -- NB: we've already dealt with type variables and Notes,
- -- so if one type is an App the other one jolly well better be too
-unify_ subst (AppTy ty1a ty1b) ty2
- | Just (ty2a, ty2b) <- repSplitAppTy_maybe ty2
- = do { subst' <- unify subst ty1a ty2a
- ; unify subst' ty1b ty2b }
-
-unify_ subst ty1 (AppTy ty2a ty2b)
- | Just (ty1a, ty1b) <- repSplitAppTy_maybe ty1
- = do { subst' <- unify subst ty1a ty2a
- ; unify subst' ty1b ty2b }
-
-unify_ subst ty1 ty2 = failWith (misMatch ty1 ty2)
-
-------------------------------
-unify_pred subst (ClassP c1 tys1) (ClassP c2 tys2)
- | c1 == c2 = unify_tys subst tys1 tys2
-unify_pred subst (IParam n1 t1) (IParam n2 t2)
- | n1 == n2 = unify subst t1 t2
-unify_pred subst p1 p2 = failWith (misMatch (PredTy p1) (PredTy p2))
-
-------------------------------
-unify_tys = unifyList unify
-
-unifyList :: Outputable a
- => (TvSubstEnv -> a -> a -> UM TvSubstEnv)
- -> TvSubstEnv -> [a] -> [a] -> UM TvSubstEnv
-unifyList unifier subst orig_xs orig_ys
- = go subst orig_xs orig_ys
- where
- go subst [] [] = return subst
- go subst (x:xs) (y:ys) = do { subst' <- unifier subst x y
- ; go subst' xs ys }
- go subst _ _ = failWith (lengthMisMatch orig_xs orig_ys)
-
-------------------------------
-uVar :: Bool -- Swapped
- -> TvSubstEnv -- An existing substitution to extend
- -> TyVar -- Type variable to be unified
- -> Type -- with this type
- -> UM TvSubstEnv
-
-uVar swap subst tv1 ty
- = -- Check to see whether tv1 is refined by the substitution
- case (lookupVarEnv subst tv1) of
- -- Yes, call back into unify'
- Just ty' | swap -> unify subst ty ty'
- | otherwise -> unify subst ty' ty
- -- No, continue
- Nothing -> uUnrefined subst tv1 ty ty
-
-
-uUnrefined :: TvSubstEnv -- An existing substitution to extend
- -> TyVar -- Type variable to be unified
- -> Type -- with this type
- -> Type -- (de-noted version)
- -> UM TvSubstEnv
-
--- We know that tv1 isn't refined
-
-uUnrefined subst tv1 ty2 ty2'
- | Just ty2'' <- tcView ty2'
- = uUnrefined subst tv1 ty2 ty2'' -- Unwrap synonyms
- -- This is essential, in case we have
- -- type Foo a = a
- -- and then unify a :=: Foo a
-
-uUnrefined subst tv1 ty2 (TyVarTy tv2)
- | tv1 == tv2 -- Same type variable
- = return subst
-
- -- Check to see whether tv2 is refined
- | Just ty' <- lookupVarEnv subst tv2
- = uUnrefined subst tv1 ty' ty'
-
- -- So both are unrefined; next, see if the kinds force the direction
- | k1 == k2 -- Can update either; so check the bind-flags
- = do { b1 <- tvBindFlag tv1
- ; b2 <- tvBindFlag tv2
- ; case (b1,b2) of
- (BindMe, _) -> bind tv1 ty2
-
- (AvoidMe, BindMe) -> bind tv2 ty1
- (AvoidMe, _) -> bind tv1 ty2
-
- (WildCard, WildCard) -> return subst
- (WildCard, Skolem) -> return subst
- (WildCard, _) -> bind tv2 ty1
-
- (Skolem, WildCard) -> return subst
- (Skolem, Skolem) -> failWith (misMatch ty1 ty2)
- (Skolem, _) -> bind tv2 ty1
- }
-
- | k1 `isSubKind` k2 = bindTv subst tv2 ty1 -- Must update tv2
- | k2 `isSubKind` k1 = bindTv subst tv1 ty2 -- Must update tv1
-
- | otherwise = failWith (kindMisMatch tv1 ty2)
- where
- ty1 = TyVarTy tv1
- k1 = tyVarKind tv1
- k2 = tyVarKind tv2
- bind tv ty = return (extendVarEnv subst tv ty)
-
-uUnrefined subst tv1 ty2 ty2' -- ty2 is not a type variable
- | tv1 `elemVarSet` substTvSet subst (tyVarsOfType ty2')
- = failWith (occursCheck tv1 ty2) -- Occurs check
- | not (k2 `isSubKind` k1)
- = failWith (kindMisMatch tv1 ty2) -- Kind check
- | otherwise
- = bindTv subst tv1 ty2 -- Bind tyvar to the synonym if poss
- where
- k1 = tyVarKind tv1
- k2 = typeKind ty2'
-
-substTvSet :: TvSubstEnv -> TyVarSet -> TyVarSet
--- Apply the non-idempotent substitution to a set of type variables,
--- remembering that the substitution isn't necessarily idempotent
-substTvSet subst tvs
- = foldVarSet (unionVarSet . get) emptyVarSet tvs
- where
- get tv = case lookupVarEnv subst tv of
- Nothing -> unitVarSet tv
- Just ty -> substTvSet subst (tyVarsOfType ty)
-
-bindTv subst tv ty -- ty is not a type variable
- = do { b <- tvBindFlag tv
- ; case b of
- Skolem -> failWith (misMatch (TyVarTy tv) ty)
- WildCard -> return subst
- other -> return (extendVarEnv subst tv ty)
- }
-\end{code}
+Suppose x::Eq Int Char. Is the alternative dead? Clearly yes.
-%************************************************************************
-%* *
- Unification monad
-%* *
-%************************************************************************
+What about x::Eq Int a, in a context where we have evidence that a~Char.
+Then again the alternative is dead.
-\begin{code}
-data BindFlag
- = BindMe -- A regular type variable
- | AvoidMe -- Like BindMe but, given the choice, avoid binding it
-
- | Skolem -- This type variable is a skolem constant
- -- Don't bind it; it only matches itself
-
- | WildCard -- This type variable matches anything,
- -- and does not affect the substitution
-
-newtype UM a = UM { unUM :: (TyVar -> BindFlag)
- -> MaybeErr Message a }
-
-instance Monad UM where
- return a = UM (\tvs -> Succeeded a)
- fail s = UM (\tvs -> Failed (text s))
- m >>= k = UM (\tvs -> case unUM m tvs of
- Failed err -> Failed err
- Succeeded v -> unUM (k v) tvs)
-
-initUM :: (TyVar -> BindFlag) -> UM a -> MaybeErr Message a
-initUM badtvs um = unUM um badtvs
-
-tvBindFlag :: TyVar -> UM BindFlag
-tvBindFlag tv = UM (\tv_fn -> Succeeded (tv_fn tv))
-
-failWith :: Message -> UM a
-failWith msg = UM (\tv_fn -> Failed msg)
-
-maybeErrToMaybe :: MaybeErr fail succ -> Maybe succ
-maybeErrToMaybe (Succeeded a) = Just a
-maybeErrToMaybe (Failed m) = Nothing
-
-------------------------------
-repSplitAppTy_maybe :: Type -> Maybe (Type,Type)
--- Like Type.splitAppTy_maybe, but any coreView 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) = case snocView tys of
- Just (tys', ty') -> Just (TyConApp tc tys', ty')
- Nothing -> Nothing
-repSplitAppTy_maybe other = Nothing
-\end{code}
+ Summary
-%************************************************************************
-%* *
- Error reporting
- We go to a lot more trouble to tidy the types
- in TcUnify. Maybe we'll end up having to do that
- here too, but I'll leave it for now.
-%* *
-%************************************************************************
+We are really doing a test for unsatisfiability of the type
+constraints implied by the match. And that is clearly, in general, a
+hard thing to do.
+
+However, since we are simply dropping dead code, a conservative test
+suffices. There is a continuum of tests, ranging from easy to hard, that
+drop more and more dead code.
+
+For now we implement a very simple test: type variables match
+anything, type functions (incl newtypes) match anything, and only
+distinct data types fail to match. We can elaborate later.
\begin{code}
-misMatch t1 t2
- = ptext SLIT("Can't match types") <+> quotes (ppr t1) <+>
- ptext SLIT("and") <+> quotes (ppr t2)
-
-lengthMisMatch tys1 tys2
- = sep [ptext SLIT("Can't match unequal length lists"),
- nest 2 (ppr tys1), nest 2 (ppr tys2) ]
-
-kindMisMatch tv1 t2
- = vcat [ptext SLIT("Can't match kinds") <+> quotes (ppr (tyVarKind tv1)) <+>
- ptext SLIT("and") <+> quotes (ppr (typeKind t2)),
- ptext SLIT("when matching") <+> quotes (ppr tv1) <+>
- ptext SLIT("with") <+> quotes (ppr t2)]
-
-occursCheck tv ty
- = hang (ptext SLIT("Can't construct the infinite type"))
- 2 (ppr tv <+> equals <+> ppr ty)
+dataConCannotMatch :: [Type] -> DataCon -> Bool
+-- Returns True iff the data con *definitely cannot* match a
+-- scrutinee of type (T tys)
+-- where T is the type constructor for the data con
+--
+dataConCannotMatch tys con
+ | null eq_spec = False -- Common
+ | all isTyVarTy tys = False -- Also common
+ | otherwise
+ = cant_match_s (map (substTyVar subst . fst) eq_spec)
+ (map snd eq_spec)
+ where
+ dc_tvs = dataConUnivTyVars con
+ eq_spec = dataConEqSpec con
+ subst = zipTopTvSubst dc_tvs tys
+
+ cant_match_s :: [Type] -> [Type] -> Bool
+ cant_match_s tys1 tys2 = ASSERT( equalLength tys1 tys2 )
+ or (zipWith cant_match tys1 tys2)
+
+ cant_match :: Type -> Type -> Bool
+ cant_match t1 t2
+ | Just t1' <- coreView t1 = cant_match t1' t2
+ | Just t2' <- coreView t2 = cant_match t1 t2'
+
+ cant_match (FunTy a1 r1) (FunTy a2 r2)
+ = cant_match a1 a2 || cant_match r1 r2
+
+ cant_match (TyConApp tc1 tys1) (TyConApp tc2 tys2)
+ | isDataTyCon tc1 && isDataTyCon tc2
+ = tc1 /= tc2 || cant_match_s tys1 tys2
+
+ cant_match (FunTy {}) (TyConApp tc _) = isDataTyCon tc
+ cant_match (TyConApp tc _) (FunTy {}) = isDataTyCon tc
+ -- tc can't be FunTyCon by invariant
+
+ cant_match (AppTy f1 a1) ty2
+ | Just (f2, a2) <- repSplitAppTy_maybe ty2
+ = cant_match f1 f2 || cant_match a1 a2
+ cant_match ty1 (AppTy f2 a2)
+ | Just (f1, a1) <- repSplitAppTy_maybe ty1
+ = cant_match f1 f2 || cant_match a1 a2
+
+ cant_match ty1 ty2 = False -- Safe!
+
+-- Things we could add;
+-- foralls
+-- look through newtypes
+-- take account of tyvar bindings (EQ example above)
\end{code}
\ No newline at end of file
projects / ghc-hetmet.git / blobdiff