Data declaration with constraint

Problem

Question

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

Answer

In Haskell 98, 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. GHC as of version 7.2 disallows them by default (turn back on with -XDatatypeContexts).

Solution

You could use ghc's Generalised Algebraic Data Structures (GADTs) to add an implicit context to the data constructors.

There has been some discussion about whether it is sensible to want this.

A Haskell 98 workaround is to use multi-parameter type classes, where T a and a are separate arguments.

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....