[plt-scheme] a simple macro redux

On Tue, 21 Nov 2006, michael rice wrote:

> Let's forget about "syntax" for a minute. Here is a
> regular non-hygienic macro that works fine in Common
> Lisp but bombs in Scheme:
>
> How would one write the macro mac in Scheme?


Hi Michael,

You'll want to read the note at the very bottom of the defmacro.ss 
documentation.

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
Important: define-macro is still restricted by MzScheme's phase separation 
rules. This means that a macro cannot access run-time bindings because it 
is executed in the syntax expansion phase. Translating code that involves 
define-macro or defmacro from an implementation without this restriction 
usually implies separating macro related functionality into a 
begin-for-syntax or a module (that will be imported with 
require-for-syntax) and properly distinguishing syntactic information from 
run-time information.
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

So that issue about phase-separation can't be ignored.  Since your MAC 
macro depends on FUN, FUN still needs to be defined to be visible at the 
right syntax expansion phase.


This can be remedied:

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
(require (lib "defmacro.ss"))

(begin-for-syntax
   (define fun
    (lambda (n)
      (cond ((zero? n) '())
            (else (cons n (fun (- n 1))))))))

(define-macro (mac . n) `(+ ,@(fun n)))
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;


Once you have this, you'll get a different error message, but you should 
be able to instantly see how to fix it.


Good luck to you!