[Haskell-beginners] Re: Boilerplate Code

Christian Maeder Christian.Maeder at dfki.de
Tue Aug 3 13:45:08 EDT 2010

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.


> 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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