import TcErrors ( typeExtraInfoMsg, unifyCtxt )
import TcMType
-import TcEnv
import TcIface
import TcRnMonad
import TcType
import ErrUtils
import BasicTypes
import Bag
+
+import Maybes ( allMaybes )
import Util
import Outputable
import FastString
-- Returns a wrapper of shape ty_actual ~ ty_expected
tcSubType origin skol_info ty_actual ty_expected
| isSigmaTy ty_actual
- = do { let extra_tvs = tyVarsOfType ty_actual
- ; (sk_wrap, inst_wrap)
- <- tcGen skol_info extra_tvs ty_expected $ \ _ sk_rho -> do
+ = do { (sk_wrap, inst_wrap)
+ <- tcGen skol_info ty_expected $ \ _ sk_rho -> do
{ (in_wrap, in_rho) <- deeplyInstantiate origin ty_actual
; coi <- unifyType in_rho sk_rho
; return (coiToHsWrapper coi <.> in_wrap) }
%************************************************************************
\begin{code}
-tcGen :: SkolemInfo -> TcTyVarSet -> TcType
+tcGen :: SkolemInfo -> TcType
-> ([TcTyVar] -> TcRhoType -> TcM result)
-> TcM (HsWrapper, result)
-- The expression has type: spec_ty -> expected_ty
-tcGen skol_info extra_tvs
- expected_ty thing_inside -- We expect expected_ty to be a forall-type
- -- If not, the call is a no-op
+tcGen skol_info expected_ty thing_inside
+ -- We expect expected_ty to be a forall-type
+ -- If not, the call is a no-op
= do { traceTc "tcGen" empty
; (wrap, tvs', given, rho') <- deeplySkolemise skol_info expected_ty
text "expected_ty" <+> ppr expected_ty,
text "inst ty" <+> ppr tvs' <+> ppr rho' ]
- -- In 'free_tvs' we must check that the "forall_tvs" havn't been constrained
+ -- Generally we must check that the "forall_tvs" havn't been constrained
-- The interesting bit here is that we must include the free variables
-- of the expected_ty. Here's an example:
-- runST (newVar True)
-- for (newVar True), with s fresh. Then we unify with the runST's arg type
-- forall s'. ST s' a. That unifies s' with s, and a with MutVar s Bool.
-- So now s' isn't unconstrained because it's linked to a.
- -- Conclusion: pass the free vars of the expected_ty to checkConsraints
- ; let free_tvs = tyVarsOfType expected_ty `unionVarSet` extra_tvs
+ --
+ -- However [Oct 10] now that the untouchables are a range of
+ -- TcTyVars, all tihs is handled automatically with no need for
+ -- extra faffing around
- ; (ev_binds, result) <- checkConstraints skol_info free_tvs tvs' given $
+ ; (ev_binds, result) <- checkConstraints skol_info tvs' given $
thing_inside tvs' rho'
; return (wrap <.> mkWpLet ev_binds, result) }
-- often empty, in which case mkWpLet is a no-op
checkConstraints :: SkolemInfo
- -> TcTyVarSet -- Free variables (other than the type envt)
- -- for the skolem escape check
-> [TcTyVar] -- Skolems
-> [EvVar] -- Given
-> TcM result
-> TcM (TcEvBinds, result)
-checkConstraints skol_info free_tvs skol_tvs given thing_inside
+checkConstraints skol_info skol_tvs given thing_inside
| null skol_tvs && null given
= do { res <- thing_inside; return (emptyTcEvBinds, res) }
-- Just for efficiency. We check every function argument with
-- tcPolyExpr, which uses tcGen and hence checkConstraints.
| otherwise
- = do { (ev_binds, wanted, result) <- newImplication skol_info free_tvs
- skol_tvs given thing_inside
- ; emitConstraints wanted
- ; return (ev_binds, result) }
+ = newImplication skol_info skol_tvs given thing_inside
-newImplication :: SkolemInfo -> TcTyVarSet -> [TcTyVar]
+newImplication :: SkolemInfo -> [TcTyVar]
-> [EvVar] -> TcM result
- -> TcM (TcEvBinds, WantedConstraints, result)
-newImplication skol_info free_tvs skol_tvs given thing_inside
+ -> TcM (TcEvBinds, result)
+newImplication skol_info skol_tvs given thing_inside
= ASSERT2( all isTcTyVar skol_tvs, ppr skol_tvs )
ASSERT2( all isSkolemTyVar skol_tvs, ppr skol_tvs )
- do { gbl_tvs <- tcGetGlobalTyVars
- ; free_tvs <- zonkTcTyVarsAndFV free_tvs
- ; let untch = gbl_tvs `unionVarSet` free_tvs
-
- ; (result, wanted) <- getConstraints $
- setUntouchables untch $
- thing_inside
+ do { ((result, untch), wanted) <- captureConstraints $
+ captureUntouchables $
+ thing_inside
; if isEmptyBag wanted && not (hasEqualities given)
-- Optimisation : if there are no wanteds, and the givens
-- we don't want to lose the "inaccessible alternative"
-- error check
then
- return (emptyTcEvBinds, emptyWanteds, result)
+ return (emptyTcEvBinds, result)
else do
{ ev_binds_var <- newTcEvBinds
; lcl_env <- getLclTypeEnv
, ic_binds = ev_binds_var
, ic_loc = loc }
- ; return (TcEvBinds ev_binds_var, unitBag (WcImplic implic), result) } }
+ ; emitConstraint (WcImplic implic)
+ ; return (TcEvBinds ev_binds_var, result) } }
\end{code}
%************************************************************************
= do { traceTc "u_tys " $ vcat
[ sep [ ppr orig_ty1, text "~", ppr orig_ty2]
, ppr origin]
- ; coi <- go origin orig_ty1 orig_ty2
+ ; coi <- go orig_ty1 orig_ty2
; case coi of
ACo co -> traceTc "u_tys yields coercion:" (ppr co)
IdCo _ -> traceTc "u_tys yields no coercion" empty
bale_out :: [EqOrigin] -> TcM a
bale_out origin = failWithMisMatch origin
- go :: [EqOrigin] -> TcType -> TcType -> TcM CoercionI
+ go :: TcType -> TcType -> TcM CoercionI
-- The arguments to 'go' are always semantically identical
-- to orig_ty{1,2} except for looking through type synonyms
-- Note that we pass in *original* (before synonym expansion),
-- so that type variables tend to get filled in with
-- the most informative version of the type
- go origin (TyVarTy tyvar1) ty2 = uVar origin NotSwapped tyvar1 ty2
- go origin ty1 (TyVarTy tyvar2) = uVar origin IsSwapped tyvar2 ty1
+ go (TyVarTy tyvar1) ty2 = uVar origin NotSwapped tyvar1 ty2
+ go ty1 (TyVarTy tyvar2) = uVar origin IsSwapped tyvar2 ty1
-- Expand synonyms:
-- see Note [Unification and synonyms]
-- Do this after the variable case so that we tend to unify
- -- variables with un-expended type synonym
- go origin ty1 ty2
- | Just ty1' <- tcView ty1 = uType origin ty1' ty2
- | Just ty2' <- tcView ty2 = uType origin ty1 ty2'
+ -- variables with un-expanded type synonym
+ --
+ -- Also NB that we recurse to 'go' so that we don't push a
+ -- new item on the origin stack. As a result if we have
+ -- type Foo = Int
+ -- and we try to unify Foo ~ Bool
+ -- we'll end up saying "can't match Foo with Bool"
+ -- rather than "can't match "Int with Bool". See Trac #4535.
+ go ty1 ty2
+ | Just ty1' <- tcView ty1 = go ty1' ty2
+ | Just ty2' <- tcView ty2 = go ty1 ty2'
+
-- Predicates
- go origin (PredTy p1) (PredTy p2) = uPred origin p1 p2
-
- -- Functions; just check the two parts
- go origin (FunTy fun1 arg1) (FunTy fun2 arg2)
+ go (PredTy p1) (PredTy p2) = uPred origin p1 p2
+
+ -- Coercion functions: (t1a ~ t1b) => t1c ~ (t2a ~ t2b) => t2c
+ go ty1 ty2
+ | Just (t1a,t1b,t1c) <- splitCoPredTy_maybe ty1,
+ Just (t2a,t2b,t2c) <- splitCoPredTy_maybe ty2
+ = do { co1 <- uType origin t1a t2a
+ ; co2 <- uType origin t1b t2b
+ ; co3 <- uType origin t1c t2c
+ ; return $ mkCoPredCoI co1 co2 co3 }
+
+ -- Functions (or predicate functions) just check the two parts
+ go (FunTy fun1 arg1) (FunTy fun2 arg2)
= do { coi_l <- uType origin fun1 fun2
; coi_r <- uType origin arg1 arg2
; return $ mkFunTyCoI coi_l coi_r }
-- Always defer if a type synonym family (type function)
-- is involved. (Data families behave rigidly.)
- go origin ty1@(TyConApp tc1 _) ty2
+ go ty1@(TyConApp tc1 _) ty2
| isSynFamilyTyCon tc1 = uType_defer origin ty1 ty2
- go origin ty1 ty2@(TyConApp tc2 _)
+ go ty1 ty2@(TyConApp tc2 _)
| isSynFamilyTyCon tc2 = uType_defer origin ty1 ty2
- go origin (TyConApp tc1 tys1) (TyConApp tc2 tys2)
+ go (TyConApp tc1 tys1) (TyConApp tc2 tys2)
| tc1 == tc2 -- See Note [TyCon app]
= do { cois <- uList origin uType tys1 tys2
; return $ mkTyConAppCoI tc1 cois }
-- See Note [Care with type applications]
- go origin (AppTy s1 t1) ty2
+ go (AppTy s1 t1) ty2
| Just (s2,t2) <- tcSplitAppTy_maybe ty2
= do { coi_s <- uType_np origin s1 s2 -- See Note [Unifying AppTy]
; coi_t <- uType origin t1 t2
; return $ mkAppTyCoI coi_s coi_t }
- go origin ty1 (AppTy s2 t2)
+ go ty1 (AppTy s2 t2)
| Just (s1,t1) <- tcSplitAppTy_maybe ty1
= do { coi_s <- uType_np origin s1 s2
; coi_t <- uType origin t1 t2
; return $ mkAppTyCoI coi_s coi_t }
- go _ ty1 ty2
- | isSigmaTy ty1 || isSigmaTy ty2
+ go ty1 ty2
+ | tcIsForAllTy ty1 || tcIsForAllTy ty2
= unifySigmaTy origin ty1 ty2
-- Anything else fails
- go origin _ _ = bale_out origin
+ go _ _ = bale_out origin
unifySigmaTy :: [EqOrigin] -> TcType -> TcType -> TcM CoercionI
unifySigmaTy origin ty1 ty2
in_scope = mkInScopeSet (mkVarSet skol_tvs)
phi1 = substTy (mkTvSubst in_scope (zipTyEnv tvs1 tys)) body1
phi2 = substTy (mkTvSubst in_scope (zipTyEnv tvs2 tys)) body2
- untch = tyVarsOfType ty1 `unionVarSet` tyVarsOfType ty2
-
- ; (coi, lie) <- getConstraints $
- setUntouchables untch $
- uType origin phi1 phi2
+-- untch = tyVarsOfType ty1 `unionVarSet` tyVarsOfType ty2
+ ; ((coi, _untch), lie) <- captureConstraints $
+ captureUntouchables $
+ uType origin phi1 phi2
-- Check for escape; e.g. (forall a. a->b) ~ (forall a. a->a)
; let bad_lie = filterBag is_bad lie
is_bad w = any (`elemVarSet` tyVarsOfWanted w) skol_tvs
-- (checkTauTvUpdate tv ty)
-- We are about to update the TauTv tv with ty.
-- Check (a) that tv doesn't occur in ty (occurs check)
--- (b) that ty is a monotype
--- (c) that kind(ty) is a sub-kind of kind(tv)
--- (d) that ty does not contain any type families, see Note [SHARING]
+-- (b) that kind(ty) is a sub-kind of kind(tv)
+-- (c) that ty does not contain any type families, see Note [Type family sharing]
--
-- We have two possible outcomes:
-- (1) Return the type to update the type variable with,
checkTauTvUpdate tv ty
= do { ty' <- zonkTcType ty
- ; if ok ty' && (typeKind ty' `isSubKind` tyVarKind tv)
- then return (Just ty')
+ ; if typeKind ty' `isSubKind` tyVarKind tv then
+ case ok ty' of
+ Nothing -> return Nothing
+ Just ty'' -> return (Just ty'')
else return Nothing }
- where ok :: TcType -> Bool
- -- Check that tv is not among the free variables of
- -- the type and that the type is type-family-free.
- ok (TyVarTy tv') = not (tv == tv')
- ok (TyConApp tc tys) = all ok tys && not (isSynFamilyTyCon tc)
- ok (PredTy sty) = ok_pred sty
- ok (FunTy arg res) = ok arg && ok res
- ok (AppTy fun arg) = ok fun && ok arg
- ok (ForAllTy _tv1 ty1) = ok ty1
-
- ok_pred (IParam _ ty) = ok ty
- ok_pred (ClassP _ tys) = all ok tys
- ok_pred (EqPred ty1 ty2) = ok ty1 && ok ty2
+
+ where ok :: TcType -> Maybe TcType
+ ok (TyVarTy tv') | not (tv == tv') = Just (TyVarTy tv')
+ ok this_ty@(TyConApp tc tys)
+ | not (isSynFamilyTyCon tc), Just tys' <- allMaybes (map ok tys)
+ = Just (TyConApp tc tys')
+ | isSynTyCon tc, Just ty_expanded <- tcView this_ty
+ = ok ty_expanded -- See Note [Type synonyms and the occur check]
+ ok (PredTy sty) | Just sty' <- ok_pred sty = Just (PredTy sty')
+ ok (FunTy arg res) | Just arg' <- ok arg, Just res' <- ok res
+ = Just (FunTy arg' res')
+ ok (AppTy fun arg) | Just fun' <- ok fun, Just arg' <- ok arg
+ = Just (AppTy fun' arg')
+ ok (ForAllTy tv1 ty1) | Just ty1' <- ok ty1 = Just (ForAllTy tv1 ty1')
+ -- Fall-through
+ ok _ty = Nothing
+
+ ok_pred (IParam nm ty) | Just ty' <- ok ty = Just (IParam nm ty')
+ ok_pred (ClassP cl tys)
+ | Just tys' <- allMaybes (map ok tys)
+ = Just (ClassP cl tys')
+ ok_pred (EqPred ty1 ty2)
+ | Just ty1' <- ok ty1, Just ty2' <- ok ty2
+ = Just (EqPred ty1' ty2')
+ -- Fall-through
+ ok_pred _pty = Nothing
\end{code}
-Note [SHARING]
+Note [Type synonyms and the occur check]
+~~~~~~~~~~~~~~~~~~~~
+Generally speaking we need to update a variable with type synonyms not expanded, which
+improves later error messages, except for when looking inside a type synonym may help resolve
+a spurious occurs check error. Consider:
+ type A a = ()
+
+ f :: (A a -> a -> ()) -> ()
+ f = \ _ -> ()
+
+ x :: ()
+ x = f (\ x p -> p x)
+
+We will eventually get a constraint of the form t ~ A t. The ok function above will
+properly expand the type (A t) to just (), which is ok to be unified with t. If we had
+unified with the original type A t, we would lead the type checker into an infinite loop.
+
+Hence, if the occurs check fails for a type synonym application, then (and *only* then),
+the ok function expands the synonym to detect opportunities for occurs check success using
+the underlying definition of the type synonym.
+
+The same applies later on in the constraint interaction code; see TcInteract,
+function @occ_check_ok@.
+
+
+Note [Type family sharing]
~~~~~~~~~~~~~~
We must avoid eagerly unifying type variables to types that contain function symbols,
because this may lead to loss of sharing, and in turn, in very poor performance of the