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
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: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe- | bounces@haskell.org] 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