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Re: continuations and threads

This page is part of the web mail archives of SRFI 18 from before July 7th, 2015. The new archives for SRFI 18 contain all messages, not just those from before July 7th, 2015.

Matthias Felleisen <matthias@xxxxxxxxxxx> writes:
> May I ask a heretical question (especially from me): 
>  How many of you use call/cc and continuation objects 
>  (rather than mimicing continuations with lambda's)
>  in large programs? Do "we" really use it to implement 
>  coroutines and backtracking and threads and whatever? 

Well, my current project (which was intended to be a quick-and dirty
hack and is taking over my life :) is a Scheme->Scheme compiler
(motivated by the desire to have a nice source code partitioning
system and SRFI support in Stalin). It is now pushing 3000+ lines of
original code, with an Essence (thanks Mr. Preprocessor) parser and
bits of SLIB tacked on for good measure.

I'm using call/cc for four times internally. Once is to create a
global exception handling environment. Another is to decompose the
loop that grovels over source forms checking for all the different
source partitioning systems out there (I've got Bigloo, Guile, and
SLIB (w/variants) covered). One is to short-circuit the name collision 
detection loop (NB: this code doesn't work right). And one is to tame
the parser (essentially co-routining it).

Of these four uses, only the first is *necessary*, as there is no
standard way to cause a Scheme program to terminate without returning
from the main function [equiv. C exit()]. The second one allows me to
modularize a big ugly loop, yet retain the (theoretical) efficiency of 
implementing it with local tail calls. I could easily code around the
lack of call/cc, and it might even *really* be the right thing
(TM). I'm pretty sure that the third one was just being too clever,
and that there's a better way to do it. I'd have to do a major rewrite 
to get around the fourth (Co-routining the Essence parser). Call/cc
was a major win for me there, as it let me continue to have my code
structured like I was using Scheme's READ while using my own parser
with all of the advantages that brings (for error location, case
sensitivity etc...)

>  Is call/cc necessary for Scheme? 

I would miss it, yes. OTOH, it seems like it could be implemented
entirely as a source transformation. In this opinion I may very well
be displaying my ignorance of some of the deeper issues. On an
informal level, it feels very 'global' in its effects. I do find it
interesting that you introduced expression contexts (which were my
first introduction to call/cc in _Scheme_and_the_Art_of_Programming_)
as part of your explanation of the compatible closure for lambda
expressions in your CS 411 monograph. I'd never thought of it as quite 
so fundamental operation before...

david rush