Automatically derived instances

[Marc A. Ziegert]

just try to compile it with

 ghc -fallow-overlapping-instances -Wall --make Main.hs

or inseart sth. like this at the first line:

 {-# OPTIONS -fglasgow-exts -fffi -fallow-undecidable-instances -fallow-overlapping-instances #-}

- marc



Am Montag, 29. August 2005 05:25 schrieb Juan Carlos Arevalo Baeza:
> David Menendez wrote:
> 
> >Juan Carlos Arevalo Baeza writes:
> >
> >  
> >
> >The way Haskell type classes
> >work, the overlap is determined without looking at the context, so "Show
> >a" will overlap with every possible instance for Show, including Show
> >Int, which is predefined.
> >  
> >
> 
>    Ah. :-P Bummer.
> 
> >I'm not sure what the official justification for that is, but reason is
> >to avoid situations like this:
> >
> >    class A t where a :: t
> >    class B t where b :: t
> >    class C t where c :: t
> >    
> >    instance A t => C t where c = a
> >    instance B t => C t where c = b
> >    
> >    instance A Char where a = 'a'
> >    instance B Char where b = 'b'
> >    
> >What should c :: Char evaluate to?
> >  
> >
> 
>    Right. Ambiguity. But as long as there's no ambiguity, there's no 
> reason to be this restrictive. Oh, well.
> 
> JCAB
> 
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