NAME

t/op/lexicals.t - Lexical Ops

SYNOPSIS

    % prove t/op/lexicals.t

DESCRIPTION

Tests various lexical scratchpad operations, as described in PDD20.

  ;;; Indicate that the computation has failed, and that the program
  ;;; should try another path.  We rebind this variable as needed.
  (define fail
    (lambda () (error "Program failed")))

  ;;; Choose an arbitrary value and return it, with backtracking.
  ;;; You are not expected to understand this.
  (define (choose . all-choices)
    (let ((old-fail fail))
      (call-with-current-continuation
       (lambda (continuation)
         (define (try choices)
           (if (null? choices)
               (begin
                 (set! fail old-fail)
                 (fail))
               (begin
                 (set! fail
                      (lambda () (continuation (try (cdr choices)))))
                 (car choices))))
         (try all-choices)))))

  ;;; Find two numbers with a product of 15.
  (let ((x (choose 1 3 5))
        (y (choose 1 5 9)))
    (for-each display `("Trying " ,x " and " ,y #\newline))
    (unless (= (* x y) 15)
      (fail))
    (for-each display `("Found " ,x " * " ,y " = 15" #\newline)))