Concurrency support?

John Cowan cowan at ccil.org
Mon Jun 26 07:41:35 PDT 2006


Brendan Eich scripsit:

> So far, the bugs or limitations combined with the promise of call/cc  
> generality seem to me to indicate overkill, or overreach.  If anyone  
> on this list has an example use-case of call/cc in JS that you think  
> is important, and that can't be mapped to coroutine-generators  
> (Python 2.5, PEP 342), please post it.

I believe that the facilities of PEP 342, while necessary, is
insufficient, as it does not allow subroutines invoked by a coroutine
to yield for it, where some of the subroutines on the dynamic chain are
coroutine-blind (or if it does, it's too subtle for me to see how).

Lua (http://www.lua.org ) provides a particular flavor of coroutines
as its sole nonlinear control abstraction.  Lua coroutines unite
generators, user-level multitasking, and backtracking into a single
fairly straightforward construct; I urge that they be considered.

Lua coroutines are asymmetric; that is, each coroutine returns only
to its caller, not to some arbitrary coroutine (though a coroutine
trampoline can overcome this restriction).  Furthermore, a yield
can happen after arbitrarily many nested subroutine calls within
a coroutine.  There's a short paper, which I strongly recommend, at
http://www.inf.puc-rio.br/~roberto/docs/corosblp.pdf that explains the
issues, the Lua coroutine syntax, a formal semantics, and points to some
interesting literature, including a demonstration that coroutines are
equivalent to one-shot delimited continuations; as is well known, call/cc
cannot provide arbitrary delimited continuations without recompiling
all uses.

Lua uses a slick implementation of its coroutines in pure C: Lua is
properly tail-recursive, and each Lua coroutine keeps its own stack
in the heap, but since coroutine invocation is entirely stack-like,
invocation is a recursive call into the Lua interpreter, and yield is a
return from the interpreter.  The only restriction is that a C routine
called from Lua cannot yield.

-- 
You escaped them by the will-death              John Cowan
and the Way of the Black Wheel.                 cowan at ccil.org
I could not.  --Great-Souled Sam                http://www.ccil.org/~cowan



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