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

It compiles fine :-) If you want, you can take a look at this [1] version of the code (or the tarball [2]). Load Main.hs in GHCi and run `test`, it will evaluate to `True`. Then, in the source code, switch the type of `test` from `P.Bool` to `Js JsBool` (in the comments) and run `test` again, now it will evaluate to a JavaScript expression. The code is still very immature (basically only that one example works), but it demonstrates our intend. - Tom [1]: http://github.com/tomlokhorst/AwesomePrelude/tree/master [2]: http://github.com/tomlokhorst/AwesomePrelude/tarball/master On Tue, Jun 30, 2009 at 8:21 PM, Conal Elliott wrote:

Does this code compile?  I'd expect that

  instance Bool (Js JsBool) (Js r) where

violates the fundep, since it applies for *all* values of r, not just to one.

   - Conal

On Tue, Jun 30, 2009 at 8:53 AM, Sebastiaan Visser wrote:

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On Jun 30, 2009, at 5:24 PM, Conal Elliott wrote:

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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?

Yes, that is really what I mean. This can be used to enforce that the return value of elimination can be restricted by the boolean type. This is especially useful when using GADTs to encode your domain language.

For example, take this simple JavaScript language:  data Js a where    Prim :: String -> Js a                -- Primitive embedding.    App :: Js (a -> b) -> Js a -> Js b    -- Function application.

 data JsBool

Now the functional dependencies can be used to enforce that eliminating booleans in the Js domain always returns a value in the Js domain:  instance Bool (Js JsBool) (Js r) where    bool f t c = Prim "(function ifthenelse (f, t, c) c ? t : f)" `App` f `App` t `App` c    true  = Prim "true"    false = Prim "false" Getting rid of this fundep and using type families will probably be a lot more intuitive.

Any suggestions on how to enforce elimination to be able to go from `Js JsBool -> Js r' using other techniques?

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Regards,   - Conal

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-- Sebastiaan Visser

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