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Tue, 03 Jun 2003 10:02:07 +1000
Gnus/5.090019 (Oort Gnus v0.19) Emacs/21.2 (gnu/linux)
Getting delay described, for completeness.
* scheme-evaluation.texi (Delayed Evaluation): Add delay, reword
promise? and force a bit, describe recursive forcing of a promise by
its own code.
I'm assuming the R5RS sample code for promises is to be read as a
specification for what should happen in a recursive force. It's the
way guile works at least.
Promises are a convenient way to defer a calculation until its result
is actually needed, and to run such a calculation only once.
- syntax: delay expr
Return a promise object which holds the given EXPR expression,
ready to be evaluated by a later `force'.
- Scheme Procedure: promise? obj
- C Function: scm_promise_p (obj)
Return true if OBJ is a promise.
- Scheme Procedure: force p
- C Function: scm_force (prom)
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
During a `force', an EXPR can call `force' on its own promise,
resulting in a further 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 value.
Description: Text document
- doco delay,
Kevin Ryde <=