[Haskell-cafe] Re: Ideas of a polymorphic Tree in Haskell

[Edgar Z. Alvarenga]

On Thu, 13/May/2010 at 18:57 +0100, Maciej Piechotka wrote:
 
> Hmm. What GDAT/existential do you use (for lazy people who do not want
> to read paper)? 

The GADT that I refered was from my faileds attempts.

> How is it programmed in Lisp?

The paper don't give much details, but by what I undertood, if a random
generated program don't compile it's discarted.

> [...]
> 
> Both compiles but I'm not sure if they are what you want.

What I want is to can create trees like:

U (a -> b) (U (c -> a) V) 

Or:

B (a -> b -> c) (U (c -> a) V) (U (c -> b) V)

And convert this to a function c -> b. I think I have achieved this now,
but really don't understand exactly how it works. Look at this:

data Tree a b c where
    B :: (a -> d -> b) -> Tree e a c -> Tree g d c -> Tree a b c
    U :: (a -> b) -> Tree d a c -> Tree a b c
    V :: (c -> d) -> Tree c d c

treeToFunc :: Tree a b c -> c -> b
treeToFunc (B f l r) = f <$> (treeToFunc l) <*> (treeToFunc r)
treeToFunc (U f u) = f.(treeToFunc u)
treeToFunc (V f) = f

The only problem is that I need to create (V id) in the last leaf
everytime:

*Main> let r = B (\x y -> (fromInteger  x) + y) (U floor (V id)) (U (**2) (V id))
*Main> (treeToFunc r) 3
12.0

Do you see any way to simplify this solution?
Thanks,
Edgar

> Regards



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