[NEM] 2009-07-31: Here's a simple interactive [tclsh]-style application, with the
difference that it runs inside a [coroutine]. This allows you to have complex event-driven
interactions appear as if they are happening in a blocking, non-event-driven manner. For
example, you can have [Tk] [GUI] applications responding to events while you are apparently
in a blocking [HTTP] request.

======
# corotcl --
#
#       A coroutine-enabled tclsh
#
package require Tk
proc lambda {params body args} {
    list ::apply [list $params $body] {*}$args
}
proc prompt p {
    puts -nonewline "$p "
    flush stdout
    fileevent stdin readable [lambda return {
        $return [gets stdin]
    } [info coroutine]]
    yield
}
proc get-command {} {
    set cmd [prompt %]
    while {![info complete $cmd]} {
        append cmd \n [prompt >]
    }
    return $cmd
}
proc repl {} {
    while 1 {
        set cmd [get-command]
        set code [catch { uplevel #0 $cmd } result opts]
        if {$code == 1} {
            puts [dict get $opts -errorinfo]
        } else { puts $result }
    }
}
coroutine main repl
#vwait forever ;# if no Tk
======

An example of use. First, we bring up a simple Tk interface to check that
everything keeps running in the background:
======
% pack [button .b -text Test -command {puts Test}]
.b
======
Testing this will show that it does print "Test" in the console when clicked. So far,
so normal. Now, we create a coroutine-enabled version of [http::geturl]:
======
% package require http
2.7.2
% proc fetch url {
>   http::geturl $url -command [info coroutine]
>   yield
> }
======
We can now use this to fetch data from the web, just like the usual geturl. The difference
is that this works in the background: you can still click on the Tk button and it will
still print to the console even while the fetch is still in progress:
======
% set t [fetch http://wiki.tcl.tk/4]
Test
Test
Test
::http::1
======
There we go: an interactive Tcl interpreter that is fully coroutine enabled.

Next step? [Futures]?

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