Hi all,
Common Lisp has a very useful idiom of SETF that can set values to arbitrary
places. For example, one can set (1 2)-th item of an array A as:
(SETF (AREF A 1 2) 20.0d0)
Scheme preferred way is to use `*-set!` procedures. However, sometimes it is
inconvenient. For example, if I want to swap two elements in the array/vector,
I would like to use `swap`:
(define-syntax swap
(syntax-rules ()
((swap ?place1 ?place2 ?tmp)
(begin
(set! ?tmp ?place1)
(set! ?place1 ?place2)
(set! ?place2 ?place1)))))
But it will only work for vectors if `set!` can act as CL's SETF on generalised
places, such as
(swap (vector-ref v 5) (vector-ref v 3) tmp)
SRFI-17 contains the definition of such a `set!` and I can use it from there
(AFAIR Racket supports it).
I was wondering if there is more Racket-way of doing this? And is there maybe a
reason why `set!` wasn't given the same power as SETF?
For example, I was thinking of defining syntax to access my implementation of
multidimensional arrays
as
(define-syntax aref
(syntax-rules (set!)
[(set! (aref ?a ?i ...) ?v) (array-set! ?a ?i ... ?v)]
[(aref ?a ?i ...) (array-ref ?a ?i ...)]))
but obviously it won't work now as `set!` expects the `id` not an expression
(`syntax-id-rules` won't work either as `aref` is not an `id`).
Regards, Alexey
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