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| author | simonpj@microsoft.com <unknown> | 2006-09-18 00:51:42 +0000 |
|---|---|---|
| committer | simonpj@microsoft.com <unknown> | 2006-09-18 00:51:42 +0000 |
| commit | c5d2d92ce6295d7236c9e8a66a47e741d6f5388b (patch) | |
| tree | de9c00dd54fd826000140b292dfa8c27d430d330 /compiler | |
| parent | 40f5a0759bd07308009c3ae8956dfa061c684ebd (diff) | |
| download | haskell-c5d2d92ce6295d7236c9e8a66a47e741d6f5388b.tar.gz | |
Comments
Diffstat (limited to 'compiler')
| -rw-r--r-- | compiler/typecheck/TcSimplify.lhs | 24 | ||||
| -rw-r--r-- | compiler/types/FunDeps.lhs | 1 |
2 files changed, 22 insertions, 3 deletions
diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs index 3c8160cbcb..8b3724a921 100644 --- a/compiler/typecheck/TcSimplify.lhs +++ b/compiler/typecheck/TcSimplify.lhs @@ -86,6 +86,24 @@ import DynFlags ( DynFlags(ctxtStkDepth), Notes on functional dependencies (a bug) -------------------------------------- +Consider this: + + class C a b | a -> b + class D a b | a -> b + + instance D a b => C a b -- Undecidable + -- (Not sure if it's crucial to this eg) + f :: C a b => a -> Bool + f _ = True + + g :: C a b => a -> Bool + g = f + +Here f typechecks, but g does not!! Reason: before doing improvement, +we reduce the (C a b1) constraint from the call of f to (D a b1). + +Here is a more complicated example: + | > class Foo a b | a->b | > | > class Bar a b | a->b @@ -257,9 +275,9 @@ any other type variables. - -------------------------------------- - Notes on ambiguity - -------------------------------------- +------------------------------------- + Note [Ambiguity] +------------------------------------- It's very hard to be certain when a type is ambiguous. Consider diff --git a/compiler/types/FunDeps.lhs b/compiler/types/FunDeps.lhs index 9347f5f665..7000075062 100644 --- a/compiler/types/FunDeps.lhs +++ b/compiler/types/FunDeps.lhs @@ -125,6 +125,7 @@ oclose preds fixed_tvs \begin{code} grow :: [PredType] -> TyVarSet -> TyVarSet +-- See Note [Ambiguity] in TcSimplify grow preds fixed_tvs | null preds = fixed_tvs | otherwise = loop fixed_tvs |