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List:       haskell-cafe
Subject:    Re: [Haskell-cafe] Class Synonyms
From:       Stefan Holdermans <sholderm () students ! cs ! uu ! nl>
Date:       2004-12-11 8:55:51
Message-ID: 7594AAA0-4B52-11D9-91A0-000D934175F6 () students ! cs ! uu ! nl
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Jorge,

> I'd like to achieve something better than this. I was looking for 
> something
> like "class synonyms". So if I declared an instance Foo A A, I 
> automagicaly
> had Bar A, because they'd be the same.

Using -fallow-undecidable-instances (in GHC) you could do:

\begin{code}
class Foo a b | a -> b where
   foo_method1 :: ...
   foo_method2 :: ...
   ...

class (Foo a a) => Bar a where

instance (Foo a a) => Bar a where   -- replaces all your other Bar 
instances

instance Foo Integer Integer where
   foo_method1 = ...
   foo_method2 = ...
\end{code}

HTH,

Stefan


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