TIP #375: Symmetric Coroutines and Yieldto
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TIP:	375
Title:	Symmetric Coroutines and Yieldto
Version:	$Revision: 1.3 $
Authors:	Miguel Sofer <msofer@users.sourceforge.net>
State:	Draft
Type:	Project
Tcl-Version:	8.6
Vote:	Done
Created:	Thu Aug 12 00:00:00 UTC 2010
Keywords:	coroutine, yield
Abstract
The new command yieldto allows a coroutine to suspend its execution and tailcall into an arbitrary command. If the new command is another coroutine's resume command we obtain symmetric coroutines.
Rationale
The current yield and proposed yieldm commands suspend a coroutine and return the control to the coroutine's caller: they implement asymmetric coroutines.
It is well known that symmetric and asymmetric coroutines have equal power, in that each can be implemented on top of the other. In Tcl8.6 as of today, symmetric coroutines can be implemented by coding a scheduler, which may but doesn't have to use the event loop. The new command yieldto implements symmetric coroutines directly.
The ability of yieldto to yield to an arbitrary command also provides new possibilities. For instance, it allows to return a non-ok code to the caller without terminating the coroutine:
 yieldto return -level 0 -code 1 ERROR
Proposal for "yieldto"
The new command
yieldto cmd ?arg1 ...?
causes:
1.	the currently executing coroutine to suspend its execution (yield),
2.	the command built from the arguments to yieldto, as resolved in the coroutine's context, to be run in the coroutine's caller scope,
3.	from the point of view of the coroutine's caller, the return value and options of the new command is what the coroutine returned on yielding.
In other words, yieldto implements "suspend yourself and tailcall the new command"; yieldto is to yield as tailcall is to return.
Implementation of "yieldto"
Simply rename the ::tcl::unsupported::yieldTo into the global namespace while getting rid of the camelCase, and extend the coroutine documentation and test-suite to refer to the new command.
Copyright
This document has been placed in the public domain.
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