[Haskell-cafe] A Very Simple Type Class Question

[Tom Ellis]

On Tue, Nov 11, 2014 at 03:59:50PM -0500, Larry Lee wrote:
> I have a very simple problem.
> I have a class and want to define a function in that class that
> returns a different instance of the same class.
> 
> I tried accomplishing this as follows:
> 
>     class A a where
>       f :: A b => a -> b
> 
> 
> This fails however when I try to instantiate it. For example:
> 
>     instance A String where
>       f x = x

The `f` that you defined has type `String -> String`.  However, given the
definition of the class `A` it should have been of type `A b => String ->
b`.

What exactly are you trying to do here?  This is probably not the way to go
about it.

Tom