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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