Re: continuation passing in Emacs vs. JUST-THIS-ONE

[Jim Porter <jporterbugs@gmail.com>]

On 3/13/2023 8:58 PM, Richard Stallman wrote:
> [[[ To any NSA and FBI agents reading my email: please consider    ]]]
> [[[ whether defending the US Constitution against all enemies,     ]]]
> [[[ foreign or domestic, requires you to follow Snowden's example. ]]]
> 
>    > TL;DR: (Why) is there no standard way for continuation passing
>    > style[1] ("event driven") programming in Emacs?
> 
> I implemented Emacs Lisp using simple, natural C data structures
> including the C call stack.  This does not lend itself to implementing
> continuations.
> 
> To change that would be enormous trouble, and i expect it would cause
> a big slowdown too.  In my opinion, continuation-passing style is not
> worth that downside.

There's already some support in Emacs for coroutines: generator.el 
provides, well... generators, which should allow for most (all?) of what 
you can normally do with coroutines, albeit with syntax that might not 
be as fluent as we might like. This is implemented entirely in Lisp, so 
I wouldn't be surprised if the performance suffers, but for certain 
kinds of tasks where Emacs isn't CPU-bound, even that could be a 
significant improvement for overall responsiveness.

For this thread in particular, I believe it's inspired by some issues 
with Tramp, where (if I understand correctly), process filters from 
relatively-long network operations are causing hangs (and also the 
dreaded "forbidden reentrant call to Tramp" error). In these cases, I 
think it's at least reasonably likely that the operations in question 
are network/IO-bound, so slicing them up into continuations might be 
good enough, even if those continuations have a performance penalty in 
terms of CPU use.

Of course, without at least a simple proof of concept, it's hard to say 
what the pros and cons look like. I'm hoping to test something like this 
out in Eshell by using/adapting generator.el, since Eshell already 
effectively contains its own CPS transformer called 'eshell-do-eval'.