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Re: perhaps I've missed something ...



At 5:37 PM -0500 1/20/00, Lars Thomas Hansen wrote:
 >... but I fail to see the appeal of this mechanism. As far as I can
 >tell, a scheme implementation which conforms to this SRFI allows me
 >to write
 >
 >(set! (car x) 5)
 >
 >rather than
 >
 >(set-car! x 5)

It's useful for syntactic (rather than procedural) abstraction.  It
allows you to write macros like this:

  ;; inc! increments whatever is stored in the "location" that
  ;; is its argument.

  (define-syntax inc!
    (syntax-rules ()
      ((inc! ?loc)
       (set! ?loc (+ ?loc 1)))))

Without SRFI-17, you can at most pass a variable as a location:

  (inc! x)  => (set! x (+ x 1))

but with SRFI-17 you can do better:

  (inc! (car x)) => (set! (car x) (+ (car x) 1))
  (inc! (vector-ref v i)) => (set! (vector-ref v i) (+ (vector-ref v i) 1))


Let me clarify. I have nothing against syntactic abstraction. My concern is primarily with the unnecessary overloading of the set! primitive. Replace set! with set-location! (or set-l! if you prefer) and you have a language extension which
a) I would not personally use, but
b) I would not object to (much).

john clements