[Haskell-cafe] ANN: package Boolean: Generalized booleans
Sebastiaan Visser
sfvisser at cs.uu.nl
Tue Jun 30 04:34:11 EDT 2009
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
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