[Haskell-beginners] Boilerplate Code

[Alex Rozenshteyn]

I have never used Data.Typeable, but maybe it could be made relevant here?

On Tue, Aug 3, 2010 at 4:18 PM, Brent Yorgey <byorgey at seas.upenn.edu> wrote:

> On Tue, Aug 03, 2010 at 09:51:45PM +1000, Matt Andrew 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:
>
> It isn't really possible to abstract this any further in Haskell.
> Constructors are rather magical functions, but they are still
> functions, and like other functions cannot be compared for equality
> directly.  Pattern-matching them is the only sort of equality
> comparison you get.
>
> With that said, your intuition to use Lisp macros is a good one.
> Haskell has a similar metaprogramming facility called Template
> Haskell, which could easily be used to automatically generate these
> sorts of functions.  Of course, it's a little more complicated than
> Lisp macros since Haskell syntax is so much more complex than Lisp's
> -- but given that, on the whole it's not so bad.  I wouldn't use TH to
> generate just the three functions you showed -- but I would certainly
> consider it for ten.
>
> -Brent
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          Alex R
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