Delayed evaluation

Semantics: The delay construct is used together with the procedure force to implement lazy evaluation or call by need. (delay expression) returns an object called a promise which at some point in the future can be asked (by the force procedure) to evaluate expression, and deliver the resulting value. The effect of expression returning multiple values is unspecified.

Semantics: The expression (delay-force expression) is conceptually similar to (delay (force expression)), with the difference that forcing the result of delay-force will in effect result in a tail call to (force expression), while forcing the result of (delay (force expression)) might not. Thus iterative lazy algorithms that might result in a long series of chains of delay and force can be rewritten using delay-force to prevent consuming unbounded space during evaluation.

The force procedure forces the value of a promise created by delay, delay-force, or make-promise. If no value has been computed for the promise, then a value is computed and returned. The value of the promise must be cached (or "memoized") so that if it is forced a second time, the previously computed value is returned. Consequently, a delayed expression is evaluated using the parameter values and exception handler of the call to force which first requested its value. If promise is not a promise, it may be returned unchanged.

The promise? procedure returns #t if its argument is a promise, and #f otherwise. Note that promises are not necessarily disjoint from other Scheme types such as procedures.

The make-promise procedure returns a promise which, when forced, will return obj. It is similar to delay, but does not delay its argument: it is a procedure rather than syntax. If obj is already a promise, it is returned.