Re: [RFC] Adding threads to Eshell

[Jim Porter <jporterbugs@gmail.com>]

On 12/16/2022 6:05 AM, Stefan Monnier wrote:
>> However, before I go too far, I wanted to check with other, more
>> knowledgeable people: are Emacs threads available on all platforms?
> 
> I'll let others answer this part.  IIUC the answer is not "yes", but
> like you I hope the cases where they're not supported can be simply
> swept under the rug.

I'm not sure it's worth the added complexity (yet?), but maybe it would 
be useful to have some generic way of executing code asynchronously 
(probably using CPS-transformations). Then you could pass this code to a 
thread if those are available, or if not, just run in the main thread 
when Emacs is idle.

>> A second issue I noticed is that Emacs threads have their own,
>> completely separate set of lexical bindings.
> 
> Do you really mean *lexical* bindings?
> If yes, then I don't understand what you mean (I don't understand how
> they could be non-completely separate).

I suppose I mean *local* bindings, really. The manual says, "The new 
thread is created with no local variable bindings in effect." That's 
something that I think would be nice to change in this case, so that a 
user could do something like:

   (let ((some-defvar t))
     (eshell-command "echo $some-defvar"))

Internally, 'eshell-command' would convert "echo $some-defvar" to some 
Lisp form, then call 'make-thread' and eval that form in the thread.

> For dynamically scoped vars, you'd have to do it more explicitly.
> Lisp Machine Lisp had an operation they called `closure` for that
> purpose (they didn't have static scoping at all).
> See https://hanshuebner.github.io/lmman/fd-clo.xml

That sounds like what I want. Thanks for the code snippet.

The only bit I'm still unsure about is if there's a good (and 
performant) way to ask, "What are all the local bindings of dynamic 
variables?" Then, I could use that to feed those bindings to the thread 
and this would work. (I'm still not sure whether this is the right thing 
to do in the first place, but I want to see what it's like. It would 
also make more of the regression tests pass, which would make it easier 
to keep developing this.)