bug#57635: 29.0.50; Look into using generator.el's CPS machinery to replace 'eshell-do-eval'

[Jim Porter <jporterbugs@gmail.com>]

On 9/7/2022 5:45 AM, Stefan Monnier via Bug reports for GNU Emacs, the 
Swiss army knife of text editors wrote:
> Indeed, the idea would be to replace the commands that launch processes
> with "yields" to an external loop (that just launches the next process
> and then calls the generator again).

That part should be fairly straightforward; there's already 
'eshell-eval-command' and friends ('eshell-resume-command', 
'eshell-resume-eval', and 'eshell-do-eval'), which work pretty much like 
this already. Looking through this, I see a few main differences between 
how Eshell does things and how generator.el does things:

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1) Eshell uses (throw 'eshell-defer) to yield

This is probably easy to fix, since it's only used in a couple places, 
and it looks like it's purely an implementation detail.

2) Eshell doesn't have anything like 'iter-defun'

Eshell just evaluates a Lisp form directly (you can see this by tracing 
'eshell-eval-command' and then running some external process in Eshell). 
I'm not sure how hard it would be to get generator.el to be ok with this.

3) Eshell allows replacing a form by throwing 'eshell-replace-command'

This is the trickiest part (to me, anyway). It's typically used by 
Eshell built-in commands when they want to give up and run an external 
process, though it can be used for a bunch of other things. I'm not sure 
what the best way to handle this via generator.el would be. Is there any 
straightforward way to do this?

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If anyone has ideas, I'm all ears. This is pretty far down into the 
weeds of Lisp metaprogramming, so I'm just making some educated guesses 
here. I think doing this would be a big improvement for Eshell though, 
since I've already bumped up against a few cases that really look like 
they're just bugs in 'eshell-do-eval'. Moving to generator.el would 
hopefully fix those and make Eshell easier to maintain/improve.