Bug? Or at least a better error message?

[Geoffrey Alan Washburn]

	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.