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Continuations and threads (was Re: Iterators & generators)

Harald Hanche-Olsen hanche at math.ntnu.no
Thu Feb 17 17:36:48 EST 2000 
+ Mike Fletcher <mfletch at tpresence.com>:

| Not being a CS person, maybe I can confuse the issue :) ...

I'm not one either, so surely I can confuse it further!

| A continuation is a suspended execution context, you get to a point
| and say "can't/won't finish this now, I'll store the state (frame)
| and let other execution contexts call me when I should continue".
| When you want to resume the context, you "call" the continuation,
| which resumes running the suspended execution context at the next
| line after the call to suspend.

So far, this is not all that different from
coroutines/generators/threads/whatever.  To my mind, one of the
mindboggling things about continuations is that you can call the same
continuation multiple times, whereas when you call a
coroutine/generator, its program counter advances, so that the next
time you call it, you are really calling a new continuation.

| If I'm reading things right, there are methods on continuations
| which allow for updating variables while not running the context, so
| you can pass information into the context while it is suspended.

Well, the global environment can always change between calls, but you
may even have two continuations sharing a lexical environment (i.e.,
they were both generated within the same procedure call), so calling
one can modify the local environment of the other.

maybe-posting-some-Scheme-code-could-obfuscate-the-issue-furhter-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
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