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