Re: [Haskell-beginners] Re: Boilerplate Code

Why do you need them to be Typeable? toConstr has the following type: toConstr :: (Data a) => a -> Constr Best, On 3 August 2010 19:50, Kyle Murphy wrote:

I was close, this actually does what was asked:

import Data.Data

typeChecker :: (Typeable a, Typeable b, Data a, Data b) => a -> b -> Bool typeChecker a b = toConstr a == toConstr b

-R. Kyle Murphy -- Curiosity was framed, Ignorance killed the cat.

On Tue, Aug 3, 2010 at 14:42, Kyle Murphy wrote:

...

Actually looking at the original question I'm not sure my code does what was intended. I was looking at does some type (a b) == (a c), which wasn't exactly the question. Oh well, back to the drawing board.

-R. Kyle Murphy -- Curiosity was framed, Ignorance killed the cat.

On Tue, Aug 3, 2010 at 14:38, Kyle Murphy wrote:

...

Less of a dirty dirty hack (requires that SchemeVal be an instance of Typeable):

import Data.Typeable import Data.Maybe

typeChecker :: (Typeable a, Typeable b) => a -> b -> Bool typeChecker a b = f a == f b where f :: (Typeable a) => a -> Maybe TypeRep f = listToMaybe . typeRepArgs . typeOf

-R. Kyle Murphy -- Curiosity was framed, Ignorance killed the cat.

On Tue, Aug 3, 2010 at 13:51, Alex Rozenshteyn wrote:

...

That is a dirty, dirty hack.

On Tue, Aug 3, 2010 at 8:45 PM, Christian Maeder < Christian.Maeder@dfki.de> wrote:

...

Matt Andrew schrieb:

...

Hi all,

I am in the process of writing a Scheme interpreter/compiler in Haskell as my first serious project after learning the basics of Haskell. The goal is to really get a feel for Haskell. I am trying to accomplish this as much as I can on my own, but am referring to Jonathan Tang's 'Write Yourself a Scheme in 48 hours' whenever I get really stuck.

I have a question regarding a pattern that I have found within my code for which I cannot seem to find an abstraction.

I am implementing some of the primitive Scheme type-checker functions with the following code:

numberP :: SchemeVal -> SchemeVal numberP (Number _) = Bool True numberP _ = Bool False

boolP :: SchemeVal -> SchemeVal boolP (Bool _) = Bool True boolP _ = Bool False

symbolP :: SchemeVal -> SchemeVal symbolP (Atom _) = Bool True symbolP _ = Bool False

This is a pattern that I could easily provide an abstraction for with a Lisp macro, but I'm having trouble discovering if/how it's possible to do so elegantly in Haskell. The closest (but obviously incorrect) code to what I'm trying to accomplish would be:

typeChecker :: SchemeVal -> SchemeVal -> SchemeVal typeChecker (cons _) (cons2 _) = Bool $ cons == cons2

I understand this code drastically misunderstands how pattern matching works, but (hopefully) it expresses what I'm trying to accomplish. Anyone have any suggestions?

typeChecker s1 s2 = let f = takeWhile isAlphaNum . show in Bool $ f s1 == f s2

hoping that my "f" just extracts the constructor as string.

C.

...

I do realise that such an abstraction is barely worth it for the amount of code it will save, but this exercise is about learning the ins and outs of Haskell.

Appreciate you taking the time to read this,

Matt Andrew

---

Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

-- () ascii ribbon campaign - against html e-mail /\ www.asciiribbon.org - against proprietary attachments

Alex R

_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

-- Ozgur Akgun