======interlace.scm======
<code shell>#!/usr/bin/env racket
#lang racket
</code>
zdvoji prvky v seznamu
<code scheme>
(define doublist
  (lambda (l)
    (foldr (lambda (e t) (cons e (cons e t)))
	   '()
	   l)))
</code>
(doublist '(1 2 3))
<code scheme>

(define (lreverse l) (foldl cons '() l))
</code>
(lreverse '(r e v e r se))


foldr f term list0 ... listn  
1 x 2 x 3
<code scheme>
(define interlace
  (lambda (l i)
    (if (or (null? l) (null? (car l)))
      l
      (cons (car l)
	    (foldr (lambda (e t) (cons i (cons e t)))
		   '()
		   (cdr l))))))

(interlace '() 'x)
(interlace '(1) 'x)
(interlace '(1 2 3) 'x)
(interlace '(1 2 3) '())
(interlace '(1 2 3) '(1 2 3))


</code>
(foldr +    0   '(1 2 3))
(foldr cons '() '(1 2 3))

 vim: syntax=racket