Editing Scheme (programming language) (section)

===Delayed evaluation===
{{See also|Lazy evaluation}}
Scheme supports delayed evaluation through the <code>delay</code> form and the procedure <code>force</code>.

<syntaxhighlight lang="Scheme">(define a 10)
(define eval-aplus2 (delay (+ a 2)))
(set! a 20)
(force eval-aplus2)
===> 22
(define eval-aplus50 (delay (+ a 50)))
(let ((a 8))
  (force eval-aplus50))
===> 70
(set! a 100)
(force eval-aplus2)
===> 22
</syntaxhighlight>
The lexical context of the original definition of the promise is preserved, and its value is also preserved after the first use of <code>force</code>. The promise is only ever evaluated once.

These primitives, which produce or handle values known as [[Futures and promises|promises]], can be used to implement advanced [[lazy evaluation]] constructs such as [[stream (computing)|stream]]s.<ref name="srfi-41">{{Cite web |last=Philip L. Bewig |date=2008-01-24 |title=SRFI 41: Streams |url=http://srfi.schemers.org/srfi-41/srfi-41.html |access-date=2012-08-09 |publisher=The SRFI Editors, schemers.org}}</ref>

In the R6RS standard, these are no longer primitives, but instead, are provided as part of the R5RS compatibility library (rnrs r5rs (6)).

In R5RS, a suggested implementation of <code>delay</code> and <code>force</code> is given, implementing the promise as a procedure with no arguments (a [[thunk]]) and using [[memoization]] to ensure that it is only ever evaluated once, irrespective of the number of times <code>force</code> is called (R5RS sec. 6.4).<ref name="r5rs"/>

SRFI 41 enables the expression of both finite and infinite sequences with extraordinary economy. For example, this is a definition of the [[Fibonacci sequence]] using the functions defined in SRFI 41:<ref name="srfi-41"/>

<syntaxhighlight lang="Scheme">
;; Define the Fibonacci sequence:
(define fibs
  (stream-cons 0
    (stream-cons 1
      (stream-map +
        fibs
        (stream-cdr fibs)))))
;; Compute the hundredth number in the sequence:
(stream-ref fibs 99)
===>  218922995834555169026
</syntaxhighlight>