From haskell-cafe  Sat Dec 11 01:40:03 2004
From: Jorge Adriano Aires <jadrian () mat ! uc ! pt>
Date: Sat, 11 Dec 2004 01:40:03 +0000
To: haskell-cafe
Subject: [Haskell-cafe] Class Synonyms
Message-Id: <200412110140.03349.jadrian () mat ! uc ! pt>
X-MARC-Message: https://marc.info/?l=haskell-cafe&m=120491990139004

Hello! 

I got a multi-parameter type class:

> class Foo a b | a -> b where
>    foo_method1 :: ...
>    foo_method2 :: ...
>    ...

And some particular cases are important on their own, like the one where 'a' 
and 'b' are the same, I call elements with this property, Bar. So I defined:

class Foo a a => Bar a where

This is nice, now I can replace (F a a) for (Bar a) in the context of many 
functions. Less typing and it's more readable. Still, I have to define 
instances as:
 
> instance Foo A A where
>    foo_method1 = bla1
>    foo_method2 = bla2
> 
> instance Bar A where


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. Or even better, I could declare

> instance Bar A where
>    foo_method1 = bla1
>    foo_method2 = bla2


Is anything like this possible at all?
Thanks,

J.A.