Promises are a convenient way to defer a calculation until its result is actually needed, and to run such a calculation only once. Also see SRFI-45 - Primitives for Expressing Iterative Lazy Algorithms.
Return a promise object which holds the given expr expression,
ready to be evaluated by a later
Return the value obtained from evaluating the expr in the given promise p. If p has previously been forced then its expr is not evaluated again, instead the value obtained at that time is simply returned.
force, an expr can call
force again on
its own promise, resulting in a recursive evaluation of that
expr. The first evaluation to return gives the value for the
promise. Higher evaluations run to completion in the normal way, but
their results are ignored,
force always returns the first