guile partition-by

partition-by.scm

(define* (partition-by f l #:optional (p eqv?))
  (if (null? l)
      '()
      (let A ((L '()) (M (list (car l))) (R (cdr l)) (f1 (f (car l))))
        (if (null? R)
            (reverse (cons (reverse M) L))
            (let* ((R1 (car R)) (f2 (f R1)) (R2 (cdr R)))
              (if (p f1 f2)
                  (A L (cons R1 M) R2 f2)
                  (A (cons (reverse M) L) (list R1) R2 f2)))))))

(partition-by
 (let ((counter 0) (truth #f))
   (lambda (x)
     (if (< counter 5)
         (begin
           (set! counter (1+ counter))
           truth)
         (begin
           (set! counter 1)
           (set! truth (not truth))
           truth))))
 '(1 2 3 4 5 6 7 8 9 0 11 12 13 14 15 16 17 18 19 20) eq?)


(partition-by
 identity
 '(1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20)
 (let ((counter 1))
   (lambda (a b)
     (if (< counter 5)
         (begin
           (set! counter (1+ counter))
           #t)
         (begin
           (set! counter 1)
           #f)))))