Version 49 of Creating Commands

Updated 2010-04-20 13:55:36 by CMcC

Tcl provides several commands which create commands and script evaluation contexts.

The purpose of this page is to enumerate and classify those commands.

• [proc] creates a Tcl script based procedure (function) which can be defined to have a fixed or variable number arguments, with default values for the fixed arguments.
• [apply] creates an anonymous command
• [interp create] creates a named slave interp command
• [interp alias] creates a named alias to another command
• [thread create] creates an opaque token associated with a thread
• [namespace ensemble] creates a namespace command from a namespace
• [class create] creates a named class command
• [$class create] creates an opaquely named object instance command
• [coroutine] creates a named coroutine command
• Tcl_CreateObjCommand is the C API for creating a command object in a specific Tcl interpreter.

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generator	cmd?	#args	invocation	destructor	rename?
proc	yes	any	by name		yes
coroutine	yes	1	by name		yes
Tcl_CreateObjCommand	yes	any	by name		yes
namespace ensemble	yes	any	by name	namespace delete	yes
class create	yes	any	by name	$class destroy	yes
$class create	yes	any	by name	$obj destroy	yes
interp alias	yes	any	by name	interp alias	yes
interp create	yes	any	$interp eval	$interp destroy	yes
thread::create	no	any	thread::send	thread::release	no
apply	no	any	by reference	implicit	no

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key
generator	what command generates an instance of this form?
cmd?	does this form construct a command?
#args	how many args does the constructed form take?
invocation	how is this form invoked?
destructor	what explicit destructor disposes of this form?
rename?	does [rename $name {}] destroy this form?

Observations

The generation of commands appears to be a means of invoking the form and of controlling the resources associated with the form.

There are two exceptions: In the case of apply, resources are associated with a value, and the lifetime of the generated form is tied to that of the value. In the case of thread, explicit refcounting is emplyed. In all other cases [rename $name {}] destroys both the command and its associated form.

Irregularities

interp-created commands duplicate functionality available through interp, to control interp resources. It would be more consistent with the other forms if $interp args were passed directly into the interp as if [interp eval]'d.

coroutine-created commands are the only forms restricted to single args.

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