Play audio slide show -- Keyboard shortcut: 'x'  Back to notes -- Keyboard shortcut: 'u'        next -- Keyboard shortcut: 'n'  Slide program -- Keyboard shortcut: 't'      The functions generate-leq and the helping function list-index .Lecture 4 - slide 34 : 34
Program 1
;; Generate a less than or equal predicate from the
;; enumeration-order. If p is the generated predicate,
;; (p x y) is true if and only if (selector x) comes before
;; (or at the same position) as (selector y) in the
;; enumeration-order. Thus, (selector x) is assumed to give a
;; value in enumeration-order. Comparison with elements in the
;; enumeration-list is done with eq?
(define (generate-leq enumeration-order selector)
  (lambda (x y)
     ; x and y supposed to be elements in enumeration order
     (let ((x-index (list-index (selector x) enumeration-order))
           (y-index (list-index (selector y) enumeration-order)))
       (<= x-index y-index))))

; A helping function of generate-leq.
; Return the position of e in lst. First is 1
; compare with eq?
; if e is not member of lst return (+ 1 (length lst))
(define (list-index e lst)
 (cond ((null? lst) 1)
       ((eq? (car lst) e) 1)
       (else (+ 1 (list-index e (cdr lst))))))