RE: [Haskell-cafe] coherence when overlapping?

12 Apr 2006

      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))
_________________________________________________________________
Download MSN Messenger emoticons and display pictures. 
http://ilovemessenger.msn.com/?mkt=en-sg