Thank you Simon. But I am still confused by the exact definition of coherence in the case of overlapping. Does the standard coherence theorem apply? If yes, how? If no, is there a theorem? Is there any write-up on this? Thanks. --william

From: "Simon Peyton-Jones" To: "william kim" , Subject: RE: [Haskell-cafe] coherence when overlapping? Date: Wed, 12 Apr 2006 17:43:33 +0100

| In the GHC documentation which describes the extension of overlapping | instances, an example similar to the following is given. | | >class C a where | > f:a -> a | >instance C Int where | > f=e1 | >instance C a where | > f=e2 | > | >let g x = f x | >in g 1 | | In this case GHC takes an ¡°incoherent¡± decision by taking the second | instance as an instantiation of function f even it is executed with an input | of type Int.

No it doesn't (ghc 6.4.1). I've just tried it. It uses the C Int instance, for exactly the reason you describe.

There is a flag -fallow-incoherent-instances that _does_ allow incoherence

Simon

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

module Main where

class C a where f::a -> a instance C Int where f x = x+1 instance C a where f x = x

main = print (let g x = f x in g (1::Int))

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