Interleaving streams in Racket

streams-interleave.rkt

#lang racket

;; produce a new stream with the elements of the given streams interleaved
(define (streams-interleave . streams)
  (letrec ([loop (lambda (streams) 
              (let ([next ((car streams))])
                (cons (car next)
                      (thunk (loop (append (cdr streams) (list (cdr next))))))))])
    (thunk (loop streams))))

;; using interleave on some streams
(define (ones) (cons 1 ones))
(define (twos) (cons 2 twos))
(define (threes) (cons 3 threes))
(define interleaved (streams-interleave ones twos threes))

The second letrec is unnecessary, a simple let would be enough.
And in fact, this doesn't work!
Try inserting the list of natural numbers and you'll get only 1s from it.

Thanks Pierre! Good catch, corrected ;-)