Automatically derived instances

[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