I've seen this expressed with GADTs (which I guess many things can), though I'm not sure if it's the best way, and I'm no type system wizard. If I recall correctly, this use is normally called phantom types. {-# LANGUAGE EmptyDataDecls, GADTs #-} data Even data Odd data Foo a where One :: Foo Odd Two :: Foo Even Three :: Foo Odd Four :: Foo Even Then you can write functions that accept only evens: f :: Foo Even -> ... or only odds: f :: Foo Odd -> ... or either: f :: Foo a -> ... Hope that helps, -Ross On Jul 27, 2009, at 12:01 AM, Brian Troutwine wrote:
Hello all.
I would like to define a data type that is the super-set of several types and then each of the proper subset types. For example:
data Foo = One | Two | Three | Four data Odd = One | Three data Even = Two | Four
This, of course, does not work. It seems that such a thing should possible to express entirely in the type system, but I cannot think of how. Would someone be so kind as to explain how this sort of thing can be accomplished?
Thanks, Brian _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe