Re: filter

[Lassi Kortela]

> I suggest, to include a filter routine with two values, the sublist
> which passes the test and the sublist which fails

In Scheme that procedure is called `partition`, also from SRFI 1:
https://srfi.schemers.org/srfi-1/srfi-1.html#partition

`filter` returns one list containing the values that pass the test:
https://srfi.schemers.org/srfi-1/srfi-1.html#filter

You can define `partition` in a few lines:

(define (partition pass? xs)
  (let loop ((xs xs) (pass '()) (fail '()))
    (if (null? xs) (values (reverse pass) (reverse fail))
        (if (pass? (car xs))
            (loop (cdr xs) (cons (car xs) pass) fail)
            (loop (cdr xs) pass (cons (car xs) fail))))))

(partition even? '(0 1 2 3 4 5 6 7 8 9)) => (0 2 4 6 8); (1 3 5 7 9)