[Coroutine%|%coroutines] and the [event loop] are a powerful combination.
Together they allow multiple control flows to run in a single interpreter and
to cooperate to achieve the objective of the program.  In a Tcl script, it's a
good idea to place the main script into a procedure, and many programmers are
in the habit of doing something like this:

======
proc main {argv0 argv} {
    # The main script
}
main $argv0 $argv
======

As coroutines take deeper root in the Tcl ecosystem, if third-party commands
can assume they're called in a coroutine context, they can take advantage of
that fact to `[yield]` from the coroutine at opportune moments, allowing other
activities in the system to continue on in the meantime.  This development will
be in large part transparent to the script that considers itself the only
control flow in the program.  All that's required to use such third-party
commands is that the main script be structured like this: 

======
after 0 [list coroutine main apply [list {argv0 argv} {
    # the main script
    exit 0 
} [namespace current]] $argv0 $argv]
vwait ::nameofsomevariablethatwillneverexist
======

[APN] My understanding is a little different. As I think miguel pointed out
some time back, in addition to the above, the Tcl event loop also has to be
modified to invoke event handlers via something similar to the above. The
issue with this are that although the change would not be hard, it has
to be done at the C level in the Tcl core and cannot be done via script.

[aspect]:  Another wrinkle is that not all coroutines can [yield] to the event loop!
For instance, [Generators in Tcl] must yield values, and only be resumed by their consumer.
Inadvertently calling code that assumes [yield] means suspend from within a generator is bad news.


** See Also **

   [update considered harmful]:   Wisdom on working in a concurrent world.  coroutines can help avoid the issue of entering the event loop multiple times, but the issue of critical sections is one the programmer will need pay careful attention to in order to maintain sanity.
 

<<categories>> coroutine | event loop