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bug#57635: 29.0.50; Look into using generator.el's CPS machinery to repl

From: Jim Porter
Subject: bug#57635: 29.0.50; Look into using generator.el's CPS machinery to replace 'eshell-do-eval'
Date: Sun, 11 Sep 2022 18:01:04 -0700

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:


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?


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.

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