Right. The crash was definitely a bug, but it seems to have been fixed. The error message about ambiguity is just what you'd expect. So it seems that this is all fine. Simon | -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users- | bounces@haskell.org] On Behalf Of Geoffrey Alan Washburn | Sent: 04 June 2006 18:53 | To: glasgow-haskell-users@haskell.org | Cc: Stephanie C Weirich | Subject: Re: Bug? Or at least a better error message? | | | I just upgraded to ghc 6.5.20060603 and now I get the following error | | Prelude> :load strange.hs | [1 of 1] Compiling Testing ( strange.hs, interpreted ) | | strange.hs:9:13: | Ambiguous type variable `t' in the constraints: | `Foo t' arising from use of `bar' at strange.hs:9:13-19 | `Num t' arising from the literal `1' at strange.hs:9:18 | Probable fix: add a type signature that fixes these type | variable(s) | Failed, modules loaded: none. | | If I wrap "1" with ( :: Int) it seems to be accepted. | | | Geoffrey Alan Washburn wrote: | > While trying out the following example, in an attempt to learn something | > about the fiddly case where a type class instance tries to use an | > instance that is more specific than itself | > | > module Testing where | > | > class Foo a where { bar :: a -> Int } | > | > instance Foo Int where | > bar i = i | > | > instance Foo a => Foo [a] where | > bar [] = bar [1] | > bar ([x]) = 1 | > bar (x:xs) = (bar x) + (bar xs) | > | > It is kind of like polymorphic recursion, I suppose. I get the | > following exception | > | > [1 of 1] Compiling Testing ( strange.hs, interpreted ) | > *** Exception: typecheck/TcEnv.lhs:(365,0)-(392,32): Non-exhaustive | > patterns in function find_thing | > | > Is the example supposed to work? I'm trying to determine the source | > of a problem with type classes and GADTs and I figured this example | > using "normal" data types would be a good place to start in | > understanding what was going wrong. | > | > I was using ghci version 6.5.20060503. | | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users