On Mon, Dec 03, 2007 at 09:18:18AM -0800, Carlo Vivari wrote:
Hi! I'm a begginer in haskell and I have a problem with an exercise, I expect someone could help me:
In one hand I have a declaration of an algebra data, like this:
data AlgExp a = AlgExp { litI :: Int -> a, litB :: Bool -> a, add :: a -> a -> a, and :: a -> a -> a, ifte :: a -> a -> a -> a}
(being ifte an 'ifthenelse' expresion...)
What I want to do is to write a fold function for expressions, something like this:
foldExp :: AlgExp a -> Exp -> a foldExp alg (LitI i) = litI alg i foldExp alg (LitB i) = litB alg i foldExp alg (add exp1 exp2) = ¿¿¿??? foldExp alg (and exp1 exp2) = ¿¿¿??? foldExp alg (ifte exp1 exp2 exp3) = ¿¿¿???
..ETC
the fact is that I have no idea of what to do with the other expresions (add, and, and ifte)... I really don' t understand how to do this... It's clear that a fold function should colapse in one valour, but how can I espress it in the terms of the exercise?
For further information about the problem after this, it's suposed that I have to rewrite some functions for expresions but in terms of foldexp (the one I should write before)
The problem is that AlgExp defines an arbitrary algebra, but in order to fold you need a universal algebra. So it makes the most sense to add foldExp to the reccord. Stefan