Ashley Just leave out the existential in your defn of D! The (C a b) in the defn of f will do the job.
But your example nevertheless does expose a delicate interaction in the type checker. 5.02.2 happened not to expose the functional dependency, so it simply use the (C a b) that f provided. 5.03 does expose the dependency, and so makes the two b's the same, which is what gives the error message. It's a problem that some subtlety in the inference algorithm (I don't even know what it is) changes the behaviour. I don't know how to fix this, but perhaps someone else does. GHC makes all type abstractions and applications explicit, so the two b's really can't be the same: they don't have the same scope. Simon | -----Original Message----- | From: Ashley Yakeley [mailto:[EMAIL PROTECTED]] | Sent: 06 April 2002 02:00 | To: GHC List | Subject: Fundep/Existential Types in 5.03 | | | Consider this: | | module Test3 where | | class C a b | a -> b where | m :: a -> b | | data D a = forall b. (C a b) => MkD a | | f :: (C a b) => D a -> b | f (MkD a) = m a | | This compiles fine under GHC 5.02.2. But under 5.03, it gives | an error: | | Model/Test3.hs:9: | Inferred type is less polymorphic than expected | Quantified type variable `b' escapes | When checking an existential match that binds | and whose type is D a -> b1 | In the definition of `f': f (MkD a) = m a | | I consider that the 5.02.2 behaviour is preferable, and that | this is a | perfectly good program. 'b' does not escape because it is | fundep on 'a', | which is specified in the type-signature. There can be only one. | | What was changed in 5.03 and why? | | -- | Ashley Yakeley, Seattle WA | | _______________________________________________ | Glasgow-haskell-users mailing list | [EMAIL PROTECTED] | http://www.haskell.org/mailman/listinfo/glasgow-| haskell-users | _______________________________________________ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
