Consecutive sequences of elements matching a predicate in Racket (lazy-version)

in-groups.rkt

#lang racket
(define (in-groups keep? seq)
  (let loop ([rem seq]         ;; Remaining elements.
             [group empty])    ;; Accumulates the sequence until we spit it.
    (cond [(empty? rem)
           (if (empty? group)  ;; When hit the end condition, check if there
               empty-stream    ;; is something to spit.
               group)]
          [(keep? (first rem)) ;; Keep the element? Add to group.
           (loop (rest rem) (cons (first rem) group))]
          [(empty? group)      ;; Not keep and empty group? Just continue.
           (loop (rest rem) group)]
          [else                ;; Not keep and something to spit?
           (stream-cons group (loop (rest rem) empty))]))) ;; Lazy recurse!