[racket-dev] Identifier macros

I learned about identifier macros mostly from scrambling through the  
racket sources. There they are often seen in places where macros are used  
instead of functions to avoid the cost of a function call. Normally.  
macros have the drawback that they can't be passed around or applied like  
functions.

(define-syntax-rule (call/print fun args ...)
   (begin
     (printf "call ~s\n" (cons 'fun '(args ...)))
     (fun args ...)))

(call/print printf "~a ~a\n" 1 2) ; works

(apply call/print printf "~a ~a\n" '(1 2)) ; fails with "use does not  
match pattern"

That's where identifier macros come into play:

(define-syntax call/print
   (syntax-id-rules ()
     [(_ fun args ...)
      (begin
        (printf "call ~s\n" (cons 'fun '(args ...)))
        (fun args ...))]
     [_ (lambda (fun . args)
          (printf "call ~s\n" (cons 'fun args))
          (apply fun args))]))

(call/print printf "~a ~a\n" 1 2) ; works with first rule
(apply call/print printf "~a ~a\n" '(1 2)) ; works with the second rule

Like this both cases work and the user must not know if, what he calls is  
a function or a macros. The advantage for the user is that he gets a  
speedup in the common case and the developer can optimize functions  
without breaking code.

Maybe that would be a good addition for the "Optimization" section.

Tobias


On Tue, 04 Dec 2012 23:16:17 +0100, Carl Eastlund <cce at ccs.neu.edu> wrote:

> I'm confused about so-called "identifier macros".  I had thought that
> ordinary transformers bound to functions could only transform
> application-position references to themselves, and that a "set!
> transformer" was required to transform bare references and set!
> references.  Turns out I was wrong, set! transformers are in fact only
> required to transform set! references.  So why are they listed under the
> documentation for "identifier macros" and why does that documentation  
> imply
> that normal transformers can't handle bare references?
>
> Carl Eastlund


-- 
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Tobias Hammer
DLR / Institute of Robotics and Mechatronics
Muenchner Str. 20, D-82234 Wessling
Tel.: 08153/28-1487
Mail: tobias.hammer at dlr.de