From Newsgroup: comp.lang.scheme

Frode V. Fjeld wrote:

Raffaele Ricciardi <rfflrccrd@gmail.com> writes:

in CL, is there an idiomatic way to process more than one element of a sequence at a time?

Yes: (loop for (a b) on list by #'cddr collect (cons a b))

Or without LOOP, I'd do:

(do ((a (pop list) (pop list))
(b (pop list) (pop list))
(result nil))
((null list) (reverse result))
(push (cons a b) result))

Testing:

(setq lst '(p 2 q 3 r 4 s 5))

(do ((a (pop lst) (pop lst))
(b (pop lst) (pop lst))
(result nil))
((null lst) (reverse result))
(push (cons a b) result))

((P . 2) (Q . 3) (R . 4))

The last pair is missing.


Gauche Scheme

(define lst '(p 2 q 3 r 4 s 5))

(do@ ((:for a (pop! lst))
(:for b (pop! lst))
(result '()))
((begin (push! result (cons a b)) (null? lst))
(reverse result)))

((p . 2) (q . 3) (r . 4) (s . 5))


Given:

Use ":for" when the same expression is to be assigned
to the variable every time.

(define-syntax do@-aux
(syntax-rules ()
[(do* (inits ...) ((var update) ...) (test expr ...) stuff ...)
(let* (inits ...)
(if test
(begin expr ...)
(begin
(begin stuff ...)
(let loop ()
(begin (set! var update) ...)
(if test
(begin expr ...)
(begin stuff ...
(loop)))))))]))

(define-syntax do@
(syntax-rules (:for !!!)
[(do@ !!! (inits ...) (updates ...)
((:for var expr) more ...) until body ...)
(do@ !!! (inits ... (var expr)) (updates ... (var expr))
(more ...) until body ...)]
[(do@ !!! (inits ...) (updates ...)
((var init update) more ...) until body ...)
(do@ !!! (inits ... (var init)) (updates ... (var update))
(more ...) until body ...)]
[(do@ !!! (inits ...) (updates ...)
((var init) more ...) until body ...)
(do@ !!! (inits ... (var init)) (updates ... )
(more ...) until body ...)]
[(do@ !!! inits updates () until body ...)
(do@-aux inits updates until body ...)]
[(do@ inits-updates until stuff ...)
(do@ !!! () () inits-updates until stuff ...)]))
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