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Diffstat (limited to 'doc/ref/srfi-modules.texi')
-rw-r--r-- | doc/ref/srfi-modules.texi | 57 |
1 files changed, 22 insertions, 35 deletions
diff --git a/doc/ref/srfi-modules.texi b/doc/ref/srfi-modules.texi index 347b3de1f..c99905c7d 100644 --- a/doc/ref/srfi-modules.texi +++ b/doc/ref/srfi-modules.texi @@ -3845,58 +3845,45 @@ words, no program that uses the R5RS definitions of delay and force will break if those definition are replaced by the SRFI-45 definitions of delay and force. -Guile compatibly extends SRFI-45 to support multiple values. It also -adds @code{promise?} to the list of exports. - -@deffn {Scheme Procedure} promise? obj -Return true if @var{obj} is an SRFI-45 promise, otherwise return false. -@end deffn - @deffn {Scheme Syntax} delay expression -Takes an expression and returns a promise which at some point in the -future may be asked (by the @code{force} procedure) to evaluate the -expression and deliver the resulting value(s). +Takes an expression of arbitrary type @var{a} and returns a promise of +type @code{(Promise @var{a})} which at some point in the future may be +asked (by the @code{force} procedure) to evaluate the expression and +deliver the resulting value. @end deffn @deffn {Scheme Syntax} lazy expression -Takes an expression (which must evaluate to a promise) and returns a -promise which at some point in the future may be asked (by the -@code{force} procedure) to evaluate the expression and deliver the -resulting promise. +Takes an expression of type @code{(Promise @var{a})} and returns a +promise of type @code{(Promise @var{a})} which at some point in the +future may be asked (by the @code{force} procedure) to evaluate the +expression and deliver the resulting promise. @end deffn -@deffn {Scheme Procedure} force promise -Takes a promise and returns the associated value(s) as follows: If -value(s) have been computed for the promise, these value(s) are -returned. Otherwise, the promise is first evaluated, then overwritten -by the obtained promise or value(s), and then force is again applied -(iteratively) to the promise. +@deffn {Scheme Procedure} force expression +Takes an argument of type @code{(Promise @var{a})} and returns a value +of type @var{a} as follows: If a value of type @var{a} has been computed +for the promise, this value is returned. Otherwise, the promise is +first evaluated, then overwritten by the obtained promise or value, and +then force is again applied (iteratively) to the promise. @end deffn -@deffn {Scheme Procedure} eager obj ... -Takes any number of argument(s) and returns a promise. As opposed to -@code{delay}, the argument(s) are evaluated eagerly. Semantically, -writing @code{(eager expression)} is equivalent to writing +@deffn {Scheme Procedure} eager expression +Takes an argument of type @var{a} and returns a value of type +@code{(Promise @var{a})}. As opposed to @code{delay}, the argument is +evaluated eagerly. Semantically, writing @code{(eager expression)} is +equivalent to writing @lisp (let ((value expression)) (delay value)). @end lisp However, the former is more efficient since it does not require -unnecessary creation and evaluation of thunks. For expressions that -return a single value, we also have the equivalence +unnecessary creation and evaluation of thunks. We also have the +equivalence @lisp (delay expression) = (lazy (eager expression)) @end lisp - -More generally, the following equivalence holds: - -@lisp -(delay expression) = (lazy (call-with-values - (lambda () expression) - eager)) -@end lisp @end deffn The following reduction rules may be helpful for reasoning about these @@ -3906,7 +3893,7 @@ usage semantics specified above: @lisp (force (delay expression)) -> expression (force (lazy expression)) -> (force expression) -(force (eager obj ...)) -> (values obj ...) +(force (eager value)) -> value @end lisp @subsubheading Correct usage |