Stackless & String-processing

[Greg Ewing]

Neel Krishnaswami wrote:
> 
> What's the difference, exactly? AFAICT you need to save the execution
> state when suspending both coroutines and generators, but I might be
> missing something obvious....

The crucial difference is that a generator is never resumed
after its caller has returned. This means that the generator's
state can be pushed onto the same stack as the caller's.

A coroutine, on the other hand, can outlive the context in
which it was created, and therefore needs a stack all of
its own.

Another way to think about it is that a generator call is
equivalent to an ordinary call where one of the parameters
is a procedure. For example, where in "generator python"
you might write

  def even_numbers_up_to(n):
    for i in range(n):
      if i % 2 == 0:
        yield(i)

  for e in even_numbers_up_to(42):
    print e, "is an even number!"

you could do the same thing in ordinary python as

  def for_each_even_number_up_to(n, do_it):
    for i in range(n):
      if i % 2 == 0:
        do_it(i)

  def print_it(e):
    print e, "is an even number!"

  for_each_even_number_up_to(42, print_it)

which clearly can be executed quite happily using 
a single stack.

If Python had something akin to Smalltalk code blocks,
generators wouldn't be needed. It would be nifty to
be able to write something like

  for_each_even_number_up_to(42) -> (e):
    print e, "is an even number!"

Greg