async.el: A simple asynchronous framework for Emacs

[John Wiegley <jwiegley@gmail.com>]

Hello,

I think we've all wanted threading and asynchronicity in Emacs for some time
now (Gnus, anyone?), and there have been many attempts to provide it.  I
propose a module for inclusion in Emacs, async.el, which offers a very ease to
use model for asynchronicity, without the need for any threading (and its
attendant complexities).  It should work on every platform that supports
asynchronous processes using `start-process'.

The whole interface is two functions: `async-start' and `async-get' (of which
the latter is optional).  Here is the basic form of use:

    (async-start (lambda () ...)
                 'function-to-call-when-done)

This will execute the lambda (which must *not* be byte-compiled -- in other
words, don't use `function' or #') in a child Emacs asynchronously.  When it's
done, the return value is passed to `function-to-call-when-done' as an
argument.  (If you don't care about the return value, pass the `ignore' symbol
as the second argument).

If you pass no second argument, a future is returned.  You can later call
`async-get' on this future to obtain the value, blocking if necessary.

That's it.  All you need to do asynchronous computation within Emacs.

Since it's likely that you'll want the child Emacs to heavy lifting based on
the parent Emacs' configuration, you can use `async-inject-environment' to
pass variable definitions across the process boundary:

    (async-start (lambda ()
                   (require 'some-module)
                   (async-inject-environment "\\`some-module-")
                   ...))

The variable definitions from the module "some-module" will be passed into the
child.

Using these facilities, here's all it takes to send e-mail asynchronously with
smtpmail.el:

    (defun async-smtpmail-send-it ()
      (async-start
       `(lambda ()
          (require 'smtpmail)
          (with-temp-buffer
            (insert ,(buffer-substring-no-properties (point-min) (point-max)))
            ;; Pass in the variable environment for smtpmail
            ,(async-inject-environment "\\`\\(smtpmail\\|\\(user-\\)?mail\\)-")
            (smtpmail-send-it)))
       'ignore))

I've also written dired-async.el, which performs copies, moves and deletes
asynchronously.  It works great with Tramp.

The files are hosted on GitHub presently:

    https://github.com/jwiegley/emacs-async

One thing I would love to do is to work with the authors of other modules --
such as one of my all-time favorites, Gnus -- to see how a facility like this
can help improve user experience.

Comments welcome,
  John

p.s. This e-mail sent asynchronously with smtpmail.el :)