Looks to me that this should be ok. GHC typechecks it fine.
Simon
| -----Original Message-----
| From: Jonathon Bell [mailto:[EMAIL PROTECTED]]
| Sent: 18 April 2001 19:20
| To: '[EMAIL PROTECTED]'
| 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 [EMAIL PROTECTED]
| MathSoft, Inc. www.mathsoft.com
| 101 Main St, Cambridge, MA 02142
| (617) 577-1017 x745
|
|
| _______________________________________________
| Hugs-Bugs mailing list
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