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:

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{-# 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

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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