[Haskell-cafe] type classes

Jules Bean jules at jellybean.co.uk
Fri Dec 14 09:48:19 EST 2007

Peter Padawitz wrote:
> I'd like to define several instances of the same type class with the 
> same type variable instance. Only method instances differ. How can I do 
> this without writing copies of the type class?

newtypes and modules have both been suggested.

I have another suggestion:


Don't use typeclasses.

The only useful thing about typeclasses is that they are a kind of 
type-indexed family of dictionaries. If you don't want to use the type 
indexin, then don't use classes. Just use your own kind of dictionary.

E.g., instead of:

class Foo a where { bar :: a -> Int; baz :: a -> String }

instance Foo Double ...
instance Foo Double ... -- bother, I wanted a different Double instance!

you should just have:

data Foo a = Foo { bar :: a -> Int, baz :: a -> String }

foo1 :: Foo Double
foo1 = Foo { ... }

foo2 :: Foo Double
foo2 = Foo { ... }

-- now I can have as many 'instances' for the same type as I want!


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