About coroutines and light threads

[Edgar Toernig <froese@...>]

Hi,

Just for your information.  When Lua 4.0 is out, I will make a
version of Lua that will contain (beside other stuff) coroutines.

Coroutines are all you need to implement a lightweight cooperative
multitasking environment.  I'll not take the route to make a
stackless Lua.  IMHO that would cripple Lua.  The tight coupling
between C and Lua will be lost.  I'll take coroutines for C and
export them to Lua.

There's one downside of this:  The coroutines will be optional
because they can't be implemented in a portable way.  It requires
a couple of lines of assembler code.  Currently supported are
x86 Linux and FreeBSD.

The upside: you not only get coroutines in Lua but also in C.
And, no feature of Lua will be lost.  You just get a new one.

If you want concurrent/preemptive multitasking this will not
help.  Then you better stick with POSIX threads or similar
stuff.  Good luck.

setjmp/longjmp was mentioned to help with this and some people
were asking how?  It's possible to implement coroutines with
the combination of setjmp/longjump and sigaltstack.  But, it's
not much easier to port than a version with about a dozen lines
of assembler.

Btw, Python was mentioned several times.  Don't only look at
stackless Python.  There's also a Python-coro.  eGroups.com is
completely written in it.  It uses the same C coroutine library
that I'll take for Lua.

Ciao, ET.

PS: IMHO, coroutines are the most underrated programming technique.
They are more limited than continuations (Scheme) but normally,
continuations are only used to implement coroutines ;)  And,
coroutines are much much lighter.  They cost nothing during normal
program execution and only as much as a subroutine call to switch
from one coroutine to another.
  And, they are _really_ easy to implement.  Unfortunately only
in a system dependent way.  But, putting it into the standard
C library would have increased its size by about 0.1%.  Seems,
that was too much *ig*