Continuations and threads (was Re: Iterators & generators)

[Harald Hanche-Olsen]

+ "Samuel A. Falvo II" <kc5tja at armored.net>:

| On Fri, 18 Feb 2000, Moshe Zadka wrote:
| 
| > with the value "x". On the other hand, if the function which was given to
| > call/cc returns with the value "x", the function which called call/cc also 
| > thinks call/cc returned with value "x". 
| 
| OK, so it's _exactly_ like setjmp/longjmp then -- a threads package.  :)

No, no, no, NO!  Oh well, OK, maybe a bit, if you were allowed to
longjmp() to someplace *after* the function doing the corresponding
setjmp() had exited, thus magically reviving the function doing the
longjmp() in the first place.

But you have caught on to an important point: Call/cc has uses which
generalize those of setjmp/longjmp, or throw/catch in Lisp.  This use
is called backward continuations, and are easy to implement.  If you
want to implement the full power of continuations in a stack based
language, you wind up doing dirty tricks like storing away a copy of
the call stack whenever someone does a call/cc.

possible-but-expensive-ly y'rs,
-- 
* Harald Hanche-Olsen     <URL:http://www.math.ntnu.no/~hanche/>
- "There arises from a bad and unapt formation of words
   a wonderful obstruction to the mind."  - Francis Bacon