[Haskell-cafe] ANN: package Boolean: Generalized booleans

[Conal Elliott]

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 <sfvisser at 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
>
>
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