Ryan Ingram wrote:

On 12/3/07, Kalman Noel wrote:

...

You're confusing sum and product types. I'm not so sure; it looks like they already have that type (Exp) and wants to use AlgExp to hold the "folding" functions used.

Ah, I didn't catch that on the first read. I suppose Carlo should then tell us what Exp exactly looks like; and it would be nice, too, to explain to me what the function in question is supposed to achieve then. He doesn't seem to want to reduce the expression, after all. Kalman ---------------------------------------------------------------------- Free pop3 email with a spam filter. http://www.bluebottle.com/tag/5

I believe the exercise is about understanding folds. There are two references that are related to the exercise: A tutorial on the universality and expressiveness of fold, by Graham Hutton. Dealing with large bananas, by Ralf Lammel, etc. The last paper motivates well the need to gather all the function parameters to the fold (ie, the algebra) in a separate record structure. I don't think being told what to write will help you understand what is going on, which is simpler than it seems.

Brent Yorgey wrote:

One comment: it looks like (add exp1 exp2), (and exp1 exp2) and so on above are not correct. The second argument of foldExp is a value of type Exp, so you are pattern-matching on the constructors of Exp, and constructors are always uppercase. Perhaps Exp has constructors named Add, And, and so on? Then you would want to do something like

Yepp, it's my fault I wasn't too precise in my explanation. There's a declaration of the data type EXP, which is not difficult to gess with the things I've said: data Exp = LitI Int |LitB Bool |Add Exp Exp |And Exp Exp |If Exp Exp Exp I'm going to read carefully all of your answer now (thanks to all!!!), and then I'll tell you. ;) -- View this message in context: http://www.nabble.com/foild-function-for-expressions-tf4932877.html#a1415512... Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

On Mon, 2007-12-03 at 19:13 -0800, Stefan O'Rear wrote:

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.

This is an unusually poor post for you. Presuming you mean "initial" or "free" or "term" for "universal" then it does (presumably) have it with Exp. Assuming Exp is the expected thing (an AST) it is (combined with the constructors) the initial algebra. foldExp should quite definitely be the type it is and not be part of the record and the its implementation is so far correct (modulo syntactical errors that are potentially indicative of a deeper confusion).