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" <[EMAIL PROTECTED]>
To: "william kim" <[EMAIL PROTECTED]>,<haskell-cafe@haskell.org>
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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