[Haskell-beginners] Boilerplate Code

[matthew coolbeth]

Perhaps I am misunderstanding your question, but why not just skip defining
these predicates altogether?  You could just use pattern matching directly
when you want to check that something is a number, or that it is a symbol,
etc.

On Tue, Aug 3, 2010 at 07:51, Matt Andrew <mjsaand at gmail.com> wrote:

> 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?
>
> 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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>



-- 
mac
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