chickadee » scheme » force

Forces the value of promise (see "delayed evaluation"). If no value has been computed for the promise, then a value is computed and returned. The value of the promise is cached (or "memoized") so that if it is forced a second time, the previously computed value is returned.

(force (delay (+ 1 2)))           ===>  3
(let ((p (delay (+ 1 2))))
  (list (force p) (force p)))  
                                       ===>  (3 3)

(define a-stream
  (letrec ((next
            (lambda (n)
              (cons n (delay (next (+ n 1)))))))
    (next 0)))
(define head car)
(define tail
  (lambda (stream) (force (cdr stream))))

(head (tail (tail a-stream)))  
                                       ===>  2

Force and delay are mainly intended for programs written in functional style. The following examples should not be considered to illustrate good programming style, but they illustrate the property that only one value is computed for a promise, no matter how many times it is forced.

(define count 0)
(define p
  (delay (begin (set! count (+ count 1))
                (if (> count x)
                    count
                    (force p)))))
(define x 5)
p                             ===>  a promise
(force p)                     ===>  6
p                             ===>  a promise, still
(begin (set! x 10)
       (force p))             ===>  6

Here is a possible implementation of delay and force. Promises are implemented here as procedures of no arguments, and force simply calls its argument:

(define force
  (lambda (object)
    (object)))

We define the expression

(delay <expression>)

to have the same meaning as the procedure call

(make-promise (lambda () <expression>))

as follows

(define-syntax delay
  (syntax-rules ()
    ((delay expression)
     (make-promise (lambda () expression))))),

where make-promise is defined as follows:

(define make-promise
  (lambda (proc)
    (let ((result-ready? #f)
          (result #f))
      (lambda ()
        (if result-ready?
            result
            (let ((x (proc)))
              (if result-ready?
                  result
                  (begin (set! result-ready? #t)
                         (set! result x)
                         result))))))))

Rationale: A promise may refer to its own value, as in the last example above. Forcing such a promise may cause the promise to be forced a second time before the value of the first force has been computed. This complicates the definition of make-promise.

Various extensions to this semantics of delay and force are supported in some implementations:

• Calling force on an object that is not a promise may simply return the object.
• It may be the case that there is no means by which a promise can be operationally distinguished from its forced value. That is, expressions like the following may evaluate to either #t or to #f, depending on the implementation:

   (eqv? (delay 1) 1)                  ===>  unspecified
   (pair? (delay (cons 1 2)))          ===>  unspecified

• Some implementations may implement "implicit forcing," where the value of a promise is forced by primitive procedures like cdr and +:

   (+ (delay (* 3 7)) 13)          ===>  34