On Jun 30, 2009, at 2:44 AM, Conal Elliott wrote:
I just uploaded a new package [1] for generalized booleans, which provides type classes with generalizations of boolean values & operations, if-then-else, Eq and Ord. These values & types come up for me with every new deep DSEL, and I think they do for others as well. The design space has some tricky trade-offs, and I'm not positive I've found the optimum yet. Users & comments are very welcome. Please direct discussion to the haskell-cafe list (rather than haskell list).
class Bool f r | f -> r where bool :: r -> r -> f -> r false :: f true :: f
class Maybe f a r | f -> a, f -> r where maybe :: r -> (a -> r) -> f a -> r nothing :: f a just :: a -> f a
class Either f a b r | f -> a, f -> b, f -> r where either :: (a -> r) -> (b -> r) -> f a b -> r left :: a -> f a b right :: b -> f a b Currently we have a very limited and somewhat messy code base on github[1] which shows how to instantiate these types to get back the original Haskell functionality and how to produce JavaScript code that runs in a browser. The the JavaScript instance is very much the same as I used in my FRP to JS EDSL[4]. Next target will, off course, be Objective C. :-) Our code is not yet release worthy and probably never will be in this
Conal, Good work! Together with Tom Lokhorst I've been working on something very similar. We've been using a rather consistent way of eliminating data structures that scales well to other data types. Although we are also using functional dependencies I think we might want to change them to type families. Examples: form. But is would be very nice to see some kind of generalized prelude evolving.
[1]: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ Boolean
- Conal
-- Sebastiaan Visser [1] http://github.com/tomlokhorst/AwesomePrelude/tree/master [2] http://github.com/sebastiaanvisser/frp-js