Monday, July 26, 2010

new 'defmacro'

Here's a taste of what the new macros look like. It's just like syntax-rules, but without the redundant 'define-syntax'/'syntax-rules' layers. I've also lost a layer of parens by using something closer to the Irken pattern-matching syntax.

;; -*- Mode: Irken -*-

;; derived expressions

(defmacro and
  (and)                 -> #t
  (and test)            -> test
  (and test1 test2 ...) -> (if test1 (and test2 ...) #f)

;; *not* the same as a Scheme <or>, this returns a boolean.
(defmacro or
  (or) -> #f
  (or test) -> test
  (or test1 test2 ...) -> (if test1 #t (or test2 ...))

(defmacro let
  ;; normal <let> here, we just rename it to our core
  ;; binding construct, <let_splat>
  (let ((name val) ...) body1 body2 ...)
  -> (let_splat ((name val) ...) body1 body2 ...)

  ;; named let
  (let tag ((name val) ...) body1 body2 ...)
  -> (letrec ((tag (lambda (name ...) body1 body2 ...)))
       (tag val ...))

Wednesday, July 21, 2010

The Macro Wars

Back in the 80's I worked mostly in Common Lisp, and therefore learned the relatively simple 'defmacro'. Over the years of exposure to Scheme, I was often curious about the more sophisticated macro systems (I think first described in R4RS, but rarely implemented). Most Scheme systems still used a backquote-style macro facility.

I never tried the 'hygienic' macro systems, partly because I didn't really care about the issue they're trying to solve, but mostly because they're just dauntingly complex. One reason is that people still seem to have not settled on one standard system. Maybe R6RS has?

If you want a taste, look over the PLT/Racket documentation here and here.

This discussion covers the strain between the two camps pretty well.

I've not completely decided yet whether I'm going to add macros to Irken. I'm coding something up now to try out. If it's a win, it stays in. But I'm leaning toward something like 'syntax-rules', with a simplified syntax. Pattern-matching will be a natural fit with the rest of Irken. [Why does scheme require both 'define-syntax' and 'syntax-rules' as two separate forms, repeating the name of the macro yet again?]