strange overlapping instance error

[haskell at list.mightyreason.com]

Dave Herman wrote:
> I'm a little mystified by an overlapping instance error I'm getting from 
> GHC (I'm using v6.6.1 in Windows). Here's a simple test case that 
> demonstrates the type error:
> 
> -----
> 
> {-# OPTIONS_GHC -fglasgow-exts #-}
> module Overlap where
> 
> class Needle a
> 
> instance Needle String
> 
> class Needle b => Haystack a b where
>   find :: a -> [b]
> 
> instance Needle a => Haystack a a where
>   find a = [a]
> 
> instance Haystack a b => Haystack [a] b where
>   find xs = concatMap find xs
> 
> data Tree = Leaf String
>           | Node [Tree]
> 
> instance Haystack Tree String where
>   find (Leaf s) = find s
>   find (Node ss) = concatMap find ss
> 
> -----
> 
> The error is:
> 
> overlap.hs:21:18:
>     Overlapping instances for Haystack String String
>       arising from use of `find' at overlap.hs:21:18-23
>     Matching instances:
>       instance (Needle a) => Haystack a a -- Defined at overlap.hs:11:0
>       instance (Haystack a b) => Haystack [a] b
>     -- Defined at overlap.hs:14:0
>     In the expression: find s
>     In the definition of `find': find (Leaf s) = find s
>     In the definition for method `find'
> 
> Now, I understand that String is [Char], but since the proposition 
> (Haystack Char String) is not true, I don't understand why the type 
> checker is claiming that the second instance declaration matches.
> 


The type checker does not look at the contexts such as your (Haystack a b =>) 
when choosing an instance.

After it chooses an instance it will look at the context for that instance.



-- 
Chris