[Haskell-beginners] Re: Boilerplate Code

[Alex Rozenshteyn]

That is a dirty, dirty hack.

On Tue, Aug 3, 2010 at 8:45 PM, Christian Maeder
<Christian.Maeder at 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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          Alex R
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