From Newsgroup: comp.lang.tcl


The man page of variable is in some things not very explicit.
I have some questions.

Question 1:

What is the difference between:

namespace eval foo {variable bar 12345}

and

namespace eval foo {set bar 12345}

Do not create both namespace variable?

Is "variable" about allocating space for the variable if there is
no value in the declaration?


Question 2:

I read "If the variable command is executed inside a Tcl procedure,
it creates local variables linked to the corresponding namespace
variables"

Does it also create the namespace variable? Or I need also to
execute it outside the procedure?


Question 3

I want to source a script, perhaps inside a name space, perhaps not,
and I want instead of global, variables only visible in the name space.

Is the solution to change "global var" with "namespace var"?

Thanks for any hint!

Rod.
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From Newsgroup: comp.lang.tcl

Roderick <hruodr@gmail.com> wrote:

The man page of variable is in some things not very explicit.
I have some questions.

Question 1:

What is the difference between:

namespace eval foo {variable bar 12345}

and

namespace eval foo {set bar 12345}

When executed like that, both create the variable ::foo::bar

Is "variable" about allocating space for the variable if there is
no value in the declaration?

It is about linking proc variables to namespace variables. It can also
be used outside a proc within a namespace, but then it is not so
different from 'set', One difference is 'variable bar' makes the
variable exist, but leaves it undefined (no value).

Question 2:

I read "If the variable command is executed inside a Tcl procedure,
it creates local variables linked to the corresponding namespace
variables"

Does it also create the namespace variable? Or I need also to
execute it outside the procedure?

The docs answer that question for you. Note last sentence in the quote
below from the namespace manpage.:

You can access a namespace variable from a procedure in the same
namespace by using the variable command. Much like the global
command, this creates a local link to the namespace variable. If
necessary, it also creates the variable in the current namespace and
initializes it.

Question 3

I want to source a script, perhaps inside a name space, perhaps not,
and I want instead of global, variables only visible in the name space.

Is the solution to change "global var" with "namespace var"?

"namespaced variables" are effectively "global like" within the
namespace itself (they exist outside of proc's, just like globals exist outsite of proc's). So changing "global var" to "variable var" would
link each of those to a namespaced variable instead of a global.

Whether it works for a given bit of code will depend upon what the rest
of that code attempts to do when accessing the variable. If it
accesses globals as "set ::gvar 2" or "set ::gvar" then just changing
"global" to "variable" won't help, because the code forces those
variables to be in the global namespace by the explit paths.

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From Newsgroup: comp.lang.tcl


Thanks a lot, Rich!

On Sun, 22 Feb 2026, Rich wrote:

The docs answer that question for you. Note last sentence in the quote
below from the namespace manpage.:

You can access a namespace variable from a procedure in the same
namespace by using the variable command. Much like the global
command, this creates a local link to the namespace variable. If
necessary, it also creates the variable in the current namespace and
initializes it.

Very interesting!

Also the two sentences that follow:

"Note that the global command only creates links to variables in
the global namespace. It is not necessary to use a variable command
if you always refer to the namespace variable using an appropriate
qualified name."

I suppose the phrase "links to variables in the global namespace"
in the first sentence means "links to variables relative to
the global namespace".

If I have a variable v in the namespace name, then

"global ::name::v" and "variable v" inside a procedure in
"namespace eval name" will do the same.

It seems the command "variable" is necessary because "global" is
unable to deal with paths relative to the namespace.

Am I wrong?

Rod.
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From Newsgroup: comp.lang.tcl

Roderick <hruodr@gmail.com> wrote:

Thanks a lot, Rich!

On Sun, 22 Feb 2026, Rich wrote:

The docs answer that question for you. Note last sentence in the quote
below from the namespace manpage.:

You can access a namespace variable from a procedure in the same
namespace by using the variable command. Much like the global
command, this creates a local link to the namespace variable. If
necessary, it also creates the variable in the current namespace and
initializes it.

Very interesting!

Also the two sentences that follow:

"Note that the global command only creates links to variables in
the global namespace. It is not necessary to use a variable command
if you always refer to the namespace variable using an appropriate
qualified name."

I suppose the phrase "links to variables in the global namespace"
in the first sentence means "links to variables relative to
the global namespace".

Well, given that the "global namespace" (::) is also the root for all
the other namespaces, yes.

If I have a variable v in the namespace name, then

"global ::name::v" and "variable v" inside a procedure in
"namespace eval name" will do the same.

The point of "global" is to make a "local proc variable" link to a global
of the same name. ::name::v is not a "local proc variable". Trying to
do that won't make the link.

You do realize you can test some of these ideas out yourself to see
what happens, right:

$ rlwrap tclsh
% info globals
tcl_rcFileName tcl_version argv0 argv tcl_interactive auto_path
auto_index env tcl_pkgPath tcl_patchLevel argc tcl_library tcl_platform
% namespace eval ::newnamespace { namespace current }
::newnamespace
% proc ::newnamespace::abc {} { global ::abc:pdq ; set pdq 1 }
% ::newnamespace::abc
1
% namespace children ::
::zlib ::oo ::newnamespace ::tcl
% info globals
tcl_rcFileName tcl_version argv0 argv tcl_interactive auto_path
auto_index env tcl_pkgPath tcl_patchLevel argc tcl_library tcl_platform
%

"global ::abc::pdq" did not create a new namespace, nor did it actually
set a global variable.

It seems the command "variable" is necessary because "global" is
unable to deal with paths relative to the namespace.

'variable' is necessary because the semantics of 'global', which were
set years before Tcl grew namespaces, is incompatible with linking to namespace variables from inside procs. "global" is defined as always
linking to the global namespace.

Variable is "similar to 'global', but works within a namespace".

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From Newsgroup: comp.lang.tcl

On 22/02/2026 23:57, Roderick wrote:

What is the difference between:

namespace eval foo {variable bar 12345}

and

namespace eval foo {set bar 12345}

Do not create both namespace variable?

On Tcl 8 it depends. If there is already a global variable "bar", your
second command will change the value of the global variable, rather than create a namespace variable:

% set bar 42
42
% namespace eval foo {set bar 12345}
12345
% set bar
12345
% info exists ::foo::bar
0

See tip #278: https://core.tcl-lang.org/tips/doc/trunk/tip/278.md


Schelte.

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From Newsgroup: comp.lang.tcl


On Mon, 23 Feb 2026, Rich wrote:

If I have a variable v in the namespace name, then

"global ::name::v" and "variable v" inside a procedure in
"namespace eval name" will do the same.

The point of "global" is to make a "local proc variable" link to a global
of the same name. ::name::v is not a "local proc variable". Trying to
do that won't make the link.

I meant this:

% namespace eval nn {proc pp {} {global ::nn::v; set v 1}}
% nn::pp
1
% set nn::v
1

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From Newsgroup: comp.lang.tcl


Terrible, that looks like a bug.

On Mon, 23 Feb 2026, Schelte wrote:

On 22/02/2026 23:57, Roderick wrote:

What is the difference between:

namespace eval foo {variable bar 12345}

and

namespace eval foo {set bar 12345}

Do not create both namespace variable?

On Tcl 8 it depends. If there is already a global variable "bar", your second
command will change the value of the global variable, rather than create a namespace variable:

% set bar 42
42
% namespace eval foo {set bar 12345}
12345
% set bar
12345
% info exists ::foo::bar
0

See tip #278: https://core.tcl-lang.org/tips/doc/trunk/tip/278.md

Schelte.

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From Newsgroup: comp.lang.tcl

Roderick <hruodr@gmail.com> wrote:

On Mon, 23 Feb 2026, Rich wrote:

If I have a variable v in the namespace name, then

"global ::name::v" and "variable v" inside a procedure in
"namespace eval name" will do the same.

The point of "global" is to make a "local proc variable" link to a global
of the same name. ::name::v is not a "local proc variable". Trying to
do that won't make the link.

I meant this:

% namespace eval nn {proc pp {} {global ::nn::v; set v 1}}
% nn::pp
1
% set nn::v
1

Interesting, so within the same namespace, applying global to a fully qualified variable name looks to do the same thing as 'variable'.

But, it does not work for namespaces that do not yet exist:

$ rlwrap tclsh
% namespace eval nn {proc pp {} {global ::nn::v; set v 1}}
% set nn::v
can't read "nn::v": no such variable
% nn::pp
1
% set nn::v
1

Continue with a test of a non-existant namespace:

% namespace eval nn2 {proc pp2 {} {global ::nn3::v; set v 1}}
% set nn3::v
can't read "nn3::v": no such variable
% nn2::pp2
can't access "::nn3::v": parent namespace doesn't exist
%

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From Newsgroup: comp.lang.tcl


On Mon, 23 Feb 2026, Rich wrote:

Continue with a test of a non-existant namespace:

% namespace eval nn2 {proc pp2 {} {global ::nn3::v; set v 1}}
% set nn3::v
can't read "nn3::v": no such variable
% nn2::pp2
can't access "::nn3::v": parent namespace doesn't exist

# tclsh
% global ::nn3::v
% % set ::nn3::v 1
invalid command name "%"
% variable ::nn3::v
can't define "::nn3::v": parent namespace doesn't exis

I think, that is a difference between global and variable.
The last do allocate memory, the first only signalizes that
a local var in procedure is bound to other var that perhaps
does not exist yet.


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From Newsgroup: comp.lang.tcl

Roderick <hruodr@gmail.com> wrote:

On Mon, 23 Feb 2026, Rich wrote:

Continue with a test of a non-existant namespace:

% namespace eval nn2 {proc pp2 {} {global ::nn3::v; set v 1}}
% set nn3::v
can't read "nn3::v": no such variable
% nn2::pp2
can't access "::nn3::v": parent namespace doesn't exist

# tclsh
% global ::nn3::v
% % set ::nn3::v 1
invalid command name "%"
% variable ::nn3::v
can't define "::nn3::v": parent namespace doesn't exis

I think, that is a difference between global and variable.
The last do allocate memory, the first only signalizes that
a local var in procedure is bound to other var that perhaps
does not exist yet.

Small syntax error on line 3:

$ rlwrap tclsh
% global ::nn3::v
% set ::nn3::v 1
can't set "::nn3::v": parent namespace doesn't exist
% variable ::nn3::v
can't define "::nn3::v": parent namespace doesn't exist
%

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