#stackless #python dis- #continuations

11.20: news.adda/stackless python/dis-continuations:
Christian Tismer 2000:

. this paper presents an implementation
of "Stackless Python" (a Python which
does not keep state on the C stack) .
By decoupling the frame stack from the C stack,
we now have the ability to keep references to frames
and to do non-local jumps.
This turns the frame stack into a tree,
and every leaf of the tree can now be a jump target.
While exploring this idea,
we will recognize ordinary function calls and returns
to be just special cases of so-called continuations, ... .

why should stackless be popular?

. the maximum recursion level would be a user option
and not a limit set by C's stack .
There would be pickleable execution states;
that means you can save your running program
and send it in a file to another computer
where it can continue running .

mem'mgt for a top-down stack layout

9.4: adda/mem'mgt/top-down stack layout:
[11.23: intro:
. the adda`oop system (unlike other oop impl's)
seeks to avoid generating garbage,
and that requires that when a function returns an object,
the return-obj's needed memory was allocated by the caller,
and then the function was given a pointer to that memory .
. that means assignments and function calls
are fused into one operation;
y`= f(x) is implicitly:
y`= f(x, ptr to y),
and this is true even when y is a parameter:
g( f(x) ) is implicitly:
stack g(y); stack f(x, ptr to y); call f; call g .

stackless mem'mgt

9.3: adda/mem'mgt/stackless:
[11.20: what stackless python does:
. a tasklet is like a separate program:
it has its own stack,
but for multi-threading it also
shares some state with some other program  .
. pickling is the saving of a running program
into a file until needed,
when the file is unpickled to become
the same running program it was before .
pickling stackless python's tasklets:
This allows you to take a tasklet mid-execution,
serialise it to a chunk of data
and then unserialise that data at a later point,
creating a new tasklet from it
that resumes where the last left off.]

. stackless python allows simulation of the stack,
so it can freeze your program's long operation,
and restart it when you have the computer back .
. even though we cannot freeze the C`stack,
we can impl' our own stack .
. see also notes on a system for ensuring
user interaction was not interrupted by
a long operation from an uncooperative subprogram
(in our system for ensuring this,
the compiler is rewriting each subprogram
by sprinkling it with calls to the gui;
so that no subprogram causes the gui to be
starved for cpu time).