Bug? Or at least a better error message?

[Simon Peyton-Jones]

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 at haskell.org
[mailto:glasgow-haskell-users-
| bounces at haskell.org] On Behalf Of Geoffrey Alan Washburn
| Sent: 04 June 2006 18:53
| To: glasgow-haskell-users at 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.
| 
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