Data declaration with constraint

1 Problem

I have declared

data C a  =>  T 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

. I like to declare an instance for an type constructor class for the type constructor

T

but its methods require type constraints that depend on the particular type constructor

T

E.g.

instance Vector T where
   add (Cons x) (Cons y) = Cons (x+y)    -- requires Num constraint on type 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? You cannot. One should insert a discussion here, whether it is sensible to want this.

For now you have to stick to multi-parameter type classes,

where

T a

and

a

are separate arguments.