Data declaration with constraint

Revision as of 17:35, 21 December 2007 by Lemming (Talk | contribs)

1 Problem

I have declared

data T a  =  C a => Cons a

and I hoped that now the type checker knows, that every value of type

T a

satisfies the type constraint on

a

Only functions can have type constraints.

The type constraint of a

data

only refers to the constructors.

The designers of Haskell 98 do now think, that it was a bad decision to allow constraints on constructors.

But how can one bake type constraints into a type ? ...