Hi, all!
Using `for/fold' with more than one accumulator is inconvenient, when
there is a need for auxiliary operations with accumulators before return.
For example:
(define (partition pred lst)
(define-values (a1 a2)
(for/fold ([acc1 null]
[acc2 null])
([v (in-list lst)])
(if (pred v)
(values (cons v acc1) acc2)
(values acc1 (cons v acc2)))))
(values (reverse a1)
(reverse a2)))
In example, it's impossible to reverse accumulators without using
intermediate definitions and applying `values' again.
IMHO, very often in `loop with accumulator' cases, there is a final
operation with accumulator before return.
Is it possible to add ability for defining such final operation as
optional or keyword argument?
For example:
(define (partition pred lst)
(for/fold ([acc1 null reverse]
[acc2 null reverse])
([v (in-list lst)])
(if (pred v)
(values (cons v acc1) acc2)
(values acc1 (cons v acc2))))))
... or even better:
(define (partition pred lst)
(for/fold ([acc1 null #:do-finally reverse]
[acc2 null #:do-finally reverse])
([v (in-list lst)])
(if (pred v)
(values (cons v acc1) acc2)
(values acc1 (cons v acc2))))))
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