Data declaration with constraint

Problem

Question

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.

Answer

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.

Solution

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

See also

• Henning Thielemann in Haskell-Cafe: Context for type parameters of type constructors
• Mark Nicholls in Haskell-Cafe: nice simple problem for someone struggling....