Looks to me that this should be ok. GHC typechecks it fine. Simon | -----Original Message----- | From: Jonathon Bell [mailto:jbell@mathsoft.com] | Sent: 18 April 2001 19:20 | To: 'hugs-bugs@haskell.org' | Subject: possible bug | | | Hello there, | | I've been experimenting with the use of type dependencies in | type classes and have come across something i find | surprising. Could it in fact be a bug in the implementation? | I'm using Hugs Feb 2001, with switches +o and -98: | | > class Bug f a r | f a -> r where | > | > bug::f->a->r | > | > instance Bug (Int->r) Int r | >--instance ... | > instance (Bug f a r) => Bug f (c a) (c r) | > | > f:: Bug(Int->Int) a r => a->r | > f = bug(id::Int->Int) | | The above compiles fine and at the prompt .. | | Main> f (f [0::Int]) | | ...runs with an expected program error that member 'bug' has | not been defined. Fine. But | | Main> f (f (f [0::Int])) | | -- ...fails to compile with an unresolved overloading: | | *** ERROR - Unresolved overloading | *** Type : (Bug (Int->Int) Int a, Bug(Int->Int) a v => [b] | *** Expression : f (f (f [0])) | | which is a surprise. it appears as though the compiler is | failing to exploit the dependency '|f a->r' from which it | could infer that 'a' in the above message must in fact be | 'Int', etc... | | Many thanks for investigating this... | | ________________________________ | Jonathon Bell jbell@mathsoft.com | MathSoft, Inc. www.mathsoft.com | 101 Main St, Cambridge, MA 02142 | (617) 577-1017 x745 | | | _______________________________________________ | Hugs-Bugs mailing list | Hugs-Bugs@haskell.org | http://www.haskell.org/mailman/listinfo/hugs-bugs |