Data declaration with constraint
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
I like to declare an instance for an type constructor class for the type constructor
but its methods require type constraints that depend on the particular type constructor
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,
T a and
a are separate arguments.