TIP:		372
Title:		Multi-argument Yield for Coroutines
Version:	$Revision: 1.3 $
Author:		Colin McCormack <colin@chinix.com>
State:		Draft
Type:		Project
Vote:		Done
Created:	11-Aug-2010
Post-History:
Tcl-Version:	8.6
Discussions-To: http://wiki.tcl.tk/26006
Keywords:	coroutine, yield
Obsoleted-By:	396

~ Abstract

This TIP proposes a command that allows a coroutine to accept multiple
arguments being passed in on return from yield.

~ Rationale

The coroutine mechanism of [328] has given Tcl 8.6 a new primitive which
permits light-weight mulitasking.  The mechanism as specified by [328] lacks
the ability to emulate general-purpose procedural invocation through the
arbitrary limitation of coroutine calls to a single argument (except on the
initial call).

The ability of coroutines to emulate general-purpose procedural invocation is
of great utility, the lack of which leads to stilted verbiage, and diverges
unnecessarily from Tcl's other functionals.

The '''yieldm''' command, as currently implemented by
'''::tcl::unsupported::yieldm''', extends coroutines to permit general
emulation, and it is the proposal of this TIP that yieldm be moved to the
global namespace, or such other namespaces as may be used for coroutine
functionality in the future.

~ Syntax and Semantics of yieldm

 > '''yieldm''' ?''value''?

This is just like '''yield''', except that it causes the coroutine command to
accept arbitrarily many arguments and will return the list of those arguments
(i.e., an empty list if there are no arguments, a single-element list if there
is one, a pair for two, etc.)

~ Detailed Rationale

~~ Utility

Restatement-in-summary of the arguments in http://wiki.tcl.tk/26006:

 1. Coroutines should be able to simulate any command, not just any
    single-arg'd command. [[Argument: generality]]

 2. To implement single-arg's coroutines in multi-arg'd coroutines is trivial
    - nothing needs to be done. The converse (implementing multi-arg'd
    coroutines under coroutine is inefficient and difficult. [[Argument:
    increased expressive power]]

 3. There is no sound reason that the invocation of a coroutine should not
    resemble that of any other command. [[Argument: principle of minimal
    surprise]]

 4. It is impossible to construct forms like '''interp alias''' to address a
    coroutine in the common case that the caller is expecting to pass more
    than one argument.

~~ Consistency

Restatement-in-summary of the arguments in http://wiki.tcl.tk/26056:

The following commands create new commands: '''proc''', '''coroutine''',
'''namespace ensemble''', '''class create''', ''$cls'' '''create''', '''interp
alias''', '''interp create''', '''::thread::create''', '''apply'''.

In each case except '''coroutine''', the created command permits multiple
arguments (arguably this being a key feature of Tcl in practice).  There is no
sound reason for this disparity, and for consistency the disparity should be
removed, which the proposed '''yieldm''' does.

~ Implementation

Simply rename the '''::tcl::unsupported::yieldm''' into the global namespace,
and extend the coroutine documentation to refer to the new command.

~ Copyright

This document has been placed in the public domain.