import ErrUtils
import BasicTypes
import Bag
+
+import Maybes ( allMaybes )
import Util
import Outputable
import FastString
-- Predicates
go origin (PredTy p1) (PredTy p2) = uPred origin p1 p2
- -- Functions; just check the two parts
+ -- Coercion functions: (t1a ~ t1b) => t1c ~ (t2a ~ t2b) => t2c
+ go origin 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 origin (FunTy fun1 arg1) (FunTy fun2 arg2)
= do { coi_l <- uType origin fun1 fun2
; coi_r <- uType origin arg1 arg2
; return $ mkAppTyCoI coi_s coi_t }
go _ ty1 ty2
- | isSigmaTy ty1 || isSigmaTy ty2
+ | tcIsForAllTy ty1 || tcIsForAllTy ty2
+{-- | isSigmaTy ty1 || isSigmaTy ty2 --}
= unifySigmaTy origin ty1 ty2
-- Anything else fails
-- (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)
+-- (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 not (tv `elemVarSet` tyVarsOfType 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 -> 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 [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
+constraint simplifier. Assume that we have a wanted constraint:
+{
+ m1 ~ [F m2],
+ m2 ~ [F m3],
+ m3 ~ [F m4],
+ D m1,
+ D m2,
+ D m3
+}
+where D is some type class. If we eagerly unify m1 := [F m2], m2 := [F m3], m3 := [F m2],
+then, after zonking, our constraint simplifier will be faced with the following wanted
+constraint:
+{
+ D [F [F [F m4]]],
+ D [F [F m4]],
+ D [F m4]
+}
+which has to be flattened by the constraint solver. However, because the sharing is lost,
+an polynomially larger number of flatten skolems will be created and the constraint sets
+we are working with will be polynomially larger.
+
+Instead, if we defer the unifications m1 := [F m2], etc. we will only be generating three
+flatten skolems, which is the maximum possible sharing arising from the original constraint.
\begin{code}
data LookupTyVarResult -- The result of a lookupTcTyVar call
projects / ghc-hetmet.git / blobdiff