[Haskell-cafe] type classes

[Jules Bean]

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!

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!

Jules