Coroutines in C

[der Mouse]

In article <10109 at csli.Stanford.EDU>, poser at csli.Stanford.EDU (Bill Poser) writes:
> In article <5710 at ficc.uu.net> peter at ficc.uu.net (Peter da Silva) writes:
>> Generators in Smalltalk are implemented differently than in Icon.
>> You set up methods for getting the first and next element of the
>> sequence, and use instance variables (if necessary) to maintain
>> state. This is more conventional, and wouldn't require coroutines.

> I'm not sure I understand how this solves the problem.  Using
> instance variables is pretty much like using statics in C to store
> the state information, but the real problem is in implementing the
> method for getting the next element in the sequence.  [...What's
> needed is] the equivalent of the Icon "suspend" primitive, which
> returns a value but saves the stack frame.

Right.

> Isn't the Smalltalk method of implementing generators just doing it
> by hand as in the above C example, with instance variables playing
> the role of statics?

Doing a generator in C this way works fine - provided only one copy of
each generator is active at once.  Suppose we write a generator that
returns numbers in sequence, from 0 up:

generate_Z() { static int i = 0; return(i++); }

This works fine if we use it in simple applications.  But try to use
two copies of it to drive two nested loops, for example, and you'll get
a little surprise....  The `instance variable' approach, while not as
nice as the Icon way, is nicer than using statics in C.  (C can do
this, but as far as I can see you need to fake instances:

typedef struct generator_Z_state { int i; } ZZZ; /* typedef for brevity */
char *new_generator_Z() { struct ZZZ z; /* char * for data hiding */
	z = (ZZZ *)malloc(sizeof(ZZZ)); z->i = 0; return((char *)z); }
int generator_Z(state) char *state; { return(((ZZZ *)state)->i++); }
destroy_generator_Z(state) char *state; { free(state); }

which is a bit ugly, but sometimes necessary.  This would of course be
nicer in some other language, C++ for example, which is better suited
to object-oriented coding.)

					der Mouse

			old: mcgill-vision!mouse
			new: mouse at larry.mcrcim.mcgill.edu