doco futures

[Kevin Ryde]

A small start at getting some stuff out of the NEWS file and into the
manual.

        * scheme-scheduling.texi (Futures): New section.

Dunno if make-future or the C functions should appear too.  I'll leave
that to an executive decision.


Futures
=======

Futures are a convenient way to run a calculation in a new thread, and
only wait for the result when it's actually needed.

   Futures are similar to promises (*note Delayed Evaluation::), in that
they allow mainline code to continue immediately.  But `delay' doesn't
evaluate at all until forced, whereas `future' starts immediately in a
new thread.

 - syntax: future expr
     Begin evaluating EXPR in a new thread, and return a "future"
     object representing the calculation.

 - Scheme Procedure: future-ref f
     Return the value computed by the future F.  If F has not yet
     finished executing then wait for it to do so.

scheme-scheduling.texi.futures.diff
Description: Text document

NEWS.futures.diff
Description: Text document