Hi Sebastiaan,
I like your extensions of generalized booleans to other common Haskell
types!
I also prefer using type families to fundeps. In this case I didn't because
of some awkwardness with vector operations, but I'm going to try again.
I'm confused about your particular fundep choice. For instance,
class Bool f r | f -> r where
bool :: r -> r -> f -> r
false :: f
true :: f
Do you *really* mean that the boolean type f determines the value type r?
Regards, - Conal
On Tue, Jun 30, 2009 at 1:34 AM, Sebastiaan Visser
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).
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:
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 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