On 3 August 2010 19:50, Kyle Murphy <orclev@gmail.com> 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 <orclev@gmail.com> 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 <orclev@gmail.com> 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 <rpglover64@gmail.com> 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
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