Seems I got ahead of myself with the bug search. I was thinking bug because when I ascribe a type, I expect the compiler to check and then respect it. With the "most general type" specification of the ":type" command in mind, this does make sense. Thanks for improving my internal notion of ":type".
My grumble may seem more legitimate from a library perspective. I implement a type-level function Append with three (preferably hidden) ancillary classes and a single instance in order to support the multiple modalities (in the Mercury sense) of the Append logic function. When a user defines another function that uses the append method, it's obfuscating for the user to see the internal classes in the inferred type. That's what I would like to workaround. If we consider class C the internal and consider class D and the function f the library's exposed interface, then I'd like to see C instead of D in the context of f and any function the user defines with f, especially when I have supplied a preferred type for f. > f :: D a => () -> a > f () = d > *> :t f > f :: (C a) => () -> a No dice? Thanks again, Nick On Sun, Dec 7, 2008 at 2:34 PM, Simon Peyton-Jones <[EMAIL PROTECTED]> wrote: > This is perfectly reasonable behavior I'm afraid. If you do ":info d" you'll > get d's original type signature. But ":type" takes an *arbitrary expression* > (in this case a single variable 'd', and figures out its most general type. > You could have said ":t (3*3)" for example. > > In this case, when inferring the most general type of the expression "d", GHC > tries to simplify the context (D a), and uses the instance declaration to > reduce it to (C a). And then it can't simplify it further. But you *might* > have had > instance C a > somewhere, in which case it'd have been able to simplify the (C a) away. So > GHC must try that route. If it fails, you want it to "back up" to a > notationally more convenient type, but GHC can't do that, I'm afraid > > Simon > > | -----Original Message----- > | From: [EMAIL PROTECTED] [mailto:haskell-cafe- > | [EMAIL PROTECTED] On Behalf Of Nicolas Frisby > | Sent: 06 December 2008 03:23 > | To: haskell Cafe > | Subject: [Haskell-cafe] know a workaround for greedy context reduction? > | > | With these three declarations > | > | {-# LANGUAGE FlexibleInstances #-} > | {-# LANGUAGE UndecidableInstances #-} > | > | class C a where c :: a > | class C a => D a where d :: a > | instance C a => D a where d = c > | > | ghci exhibits this behavior: > | > | *> :t d > | d :: (C a) => a > | > | Where I would prefer "d :: (D a) => a". In my actual examples, the > | context is much larger and I can't involve overlapping instances. Is > | there a known workaround? I didn't find a related bug on the GHC trac, > | and I don't know if other compilers behave in the same way. > | _______________________________________________ > | Haskell-Cafe mailing list > | Haskell-Cafe@haskell.org > | http://www.haskell.org/mailman/listinfo/haskell-cafe > > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
