On Tue, Jun 30, 2009 at 1:34 AM, Sebastiaan Visser <sfvisser@cs.uu.nl> wrote:

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