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

[Jim Porter]

On 3/16/2023 2:35 PM, miha@kamnitnik.top wrote:
> There's also the issue that using continuation passing (async-io)
> doesn't auto-magically solve the re-entrancy issues.
>
> Consider the following hypothetical command, written using JS-style
> async/await operators:
>
> (async-defun insert-some-parent-dirs ()
>    (interactive)
>    (insert (await (locate-dominating-file default-directory "go.mod")))
>    (insert "\n")
>    (insert (await (locate-dominating-file default-directory "go.work"))))
>
> If the user executed such a command multiple times in quick succession,
> the executions could happen in parallel and would trample over each
> other.
>
> For each use of "await", the programmer has to think about the
> possibility of other code running "in-between". This style of
> programming may be harder in Elisp which has a lot of global state in
> form of buffer contents, markers and overlays.

Yeah, this isn't easy to fix on its own. The best I can think of (and 
this would take quite a bit of experimentation) would be some way of 
declaring async functions as non-reentrant for certain contexts. So your 
example would be non-reentrant for a given buffer. Some other functions 
might be non-reentrant globally, or for a particular argument to the 
function.

With a declaration like that, we could hopefully go a fair way towards 
solving the problem by serializing any async calls that are non-reentrant.

I hope to test out Stefan's futur.el (and maybe vanilla generator.el) as 
a new iterative evaluation backend for Eshell in the coming months; 
hopefully that will help produce some more concrete information about 
what the pitfalls are.