associations.scm

(use srfi-1)

(define (advance ls n)
  (if (< n 1)
      ls
      (advance (cdr ls) (- n 1))))

(define (subset ls start count)
  (define (build-cell it i)
    (if (= i count)
        '()
        (cons (car it) (build-cell (cdr it) (+ i 1)))))
  (build-cell (advance ls start) 0))

(define (cartesian-product list-a list-b)
  (append-map 
    (lambda (a) 
      (map (lambda (b) 
             (list a b)) list-b)) list-a))

(define (associations items)
  (define (build-sub k n all)
    (if (= k n) 
        all
        (let ((left-set (associations (subset items 0 k)))
              (right-set (associations (subset items k (- n k)))))
          (build-sub (+ k 1) n (append (cartesian-product left-set right-set) all)))))
  (let ((n (length items)))
    (if (<= n 1)
        items
        (build-sub 1 n '()))))

(define (input-loop)
  (begin
    (display "enter the list of symbols to associate: ")

    (map (lambda (x) 
           (display x)
           (newline))
          (associations (read)))
    (input-loop)))

(input-loop)