[Python-Dev] Stackless Python - Pros and Cons

[Tim Peters]

[Tim]
> IMO, the real reason we don't have generators already is that
> they keep getting hijacked by continuations (indeed, Steven gave
> up on his patches as soon as he realized he couldn't extend his
> approach to continuations).

[esr]
> This report of repeated "hijacking" doesn't surprise me a bit.  In
> fact, if I'd thought about it I'd have *expected* it.  We know from
> experience with other languages (notably Scheme) that call-with-
> current-continuation is the simplest orthogonal primitive that this
> whole cluster of concepts can be based on.  Implementors with good
> design taste are going to keep finding their way back to it, and
> they're going to feel incompleteness and pressure if they can't get
> there.

On the one hand, I don't think I know of a language *not* based on Scheme
that has call/cc (or a moral equivalent).  REBOL did at first, but after Joe
Marshal left, Carl Sassenrath ripped it out in favor of a more conventional
implementation.  Even the massive Common Lisp declined to adopt call/cc, the
reasons for which Kent Pitman has posted eloquently and often on
comp.lang.lisp (basically summarized by that continuations are, in Kent's
view, "a semantic mess" in the way Scheme exposed them -- btw, people should
look his stuff up, as he has good ideas for cleaning that mess w/o
sacrificing the power (and so the Lisp world splinters yet again?)).  So
call/cc remains "a Scheme thing" to me after all these years, and even there
by far the most common warning in the release notes for a new implementation
is that call/cc doesn't work correctly yet or at all (but, in the meantime,
here are 3 obscure variations that will work in hard-to-explain special
cases ...).  So, ya, we *do* have experience with this stuff, and it sure
ain't all good.

On the other hand, what implementors other than Schemeheads *do* keep
rediscovering is that generators are darned useful and can be implemented
easily without exotic views of the world.  CLU, Icon and Sather all fit in
that box, and their designers wouldn't touch continuations with a 10-foot
thick condom <wink>.

> This is why I'm holding out for continuation objects and
> call-with-continuation to be an explicit Python builtin. We're
> going to get there anyway; best to do it cleanly right away.

This can get sorted out in the PEP.  As I'm sure someone else has screamed
by now (because it's all been screamed before), Stackless and the
continuation module are distinct beasts (although the latter relies on the
former).  It would be a shame if the fact that it makes continuations
*possible* were to be held against Stackless.  It makes all sorts of things
possible, some of which Guido would even like if people stopped throwing
continuations in his face long enough for him to see beyond them <0.5
wink -- but he doesn't like continuations, and probably never will>.