Ambiguous types

[Janis Voigtlaender]

Ashley Yakeley wrote:
> 
> At 2002-01-21 00:25, Janis Voigtlaender wrote:
> 
> >Hope that helps and doesn't just make the story more obscure ;-)
> 
> I'm not convinced. Here's a simplified case...
> 
> This compiles fine:
> 
>   emptyList :: [a]
>   emptyList = []
> 
>   isempty :: Bool
>   isempty = null emptyList
> 
> But this gives an error:
> 
>   emptyList :: (Ord a) => [a]
>   emptyList = []
> 
>   isempty :: Bool
>   isempty = null emptyList
> 
> ...

Ok, it seems that the type error in Daniel's program occured for a
simpler reason than I suspected. But my objection to the program
persists: to have a polymorphic (==) that compares lists with the same
element types is different from having a (==) that compares polymorphic
lists.

--
Janis Voigtlaender
http://wwwtcs.inf.tu-dresden.de/~voigt/
mailto:voigt@tcs.inf.tu-dresden.de