[Haskell-beginners] Re: Classes in Polymorphic Constructors

On 23/08/2010 18:32, Ertugrul Soeylemez wrote:

John Smith wrote:

...

Why can polymorphic data constructors not be instantiated as a class? (not sure if I used the right terminology there)

For example,

a = [1,True]::[Show] b = "foo":a c = map show b

This would obviate the need for wrapper types

Your type signature doesn't make sense. You are confusing type classes with types. Show is a type class.

My question was why doesn't the type system allow classes to be used in this way. It's the equivalent of a base class or interface in OO.