This weekend I read the R5RS spec. Before then I hadn't heard anything
about promise and force, in other words, they don't get blogged about
or written up much. Why not? Are they important? For what kinds of
things are they used?

For example, if you try to implement if as a function (instead of
syntax or macro):

(define (if test then else)
   (cond (test then)
         (#t   else)))

you get the problem that the three arguments are evaluated before the
if function is called:

(if #t (display "yes ") (display "no "))

will print:

    yes no

or:

    no yes

depending on the implementation, while we expected it to print only:

    yes

To avoid the evaluation branches before calling if, we can embed them
in promises:

(if test (delay (display "yes ")) (delay (display "no ")))

with if implemented as:

(define (if test then else)
   (cond (test (force then))
         (#t   (force else))))

then:

(if #t (delay (display "yes ")) (delay (display "no ")))

will print only:

   yes

--
__Pascal Bourguignon__                     http://www.informatimago.com/