On Sat, Feb 20, 2010 at 3:30 AM, Andrew Coppin
Have I just invented arrows?
No... you have a data type which is *an* Arrow (probably/almost). The pure implementation bugs me because of its use of undefined. Might still be okay though. I would be more comfortable if it could not output until it has *some* input; i.e. data Property' a b = forall s. Property' s (a -> s -> (b,s)) Anyway, for yours: try to implement (.) :: Property b c -> Property a b -> Property a c, and first :: Property a b -> Property (a,c) (b,c). Then you will have an arrow. This is a Causal Commutative Arrow, even, similar to the kinds of things that are done in Yampa. It might be more recognizable as the non-recursive version of: newtype Property a b = Property b (a -> Property a b) Luke