namespace (n)
create and manipulate contexts for commands and variables
SYNOPSIS
namespace ?option? ?arg ...?
DESCRIPTION
The namespace command lets you create, access, and destroy
separate contexts for commands and variables.
See the section WHAT IS A NAMESPACE? below
for a brief overview of namespaces.
The legal option's are listed below.
Note that you can abbreviate the option's.
namespace children ?namespace? ?pattern?
Returns a list of all child namespaces that belong to the
namespace namespace.
If namespace is not specified,
then the children are returned for the current namespace.
This command returns fully-qualified names,
which start with ::.
If the optional pattern is given,
then this command returns only the names that match the glob-style pattern.
The actual pattern used is determined as follows:
a pattern that starts with :: is used directly,
otherwise the namespace namespace
(or the fully-qualified name of the current namespace)
is prepended onto the the pattern.
namespace code script
Captures the current namespace context for later execution
of the script script.
It returns a new script in which script has been wrapped
in a namespace code command.
The new script has two important properties.
First, it can be evaluated in any namespace and will cause
script to be evaluated in the current namespace
(the one where the namespace code command was invoked).
Second, additional arguments can be appended to the resulting script
and they will be passed to script as additional arguments.
For example, suppose the command
set script [namespace code {foo bar}]
is invoked in namespace ::a::b.
Then eval "$script x y"
can be executed in any namespace (assuming the value of
script has been passed in properly)
and will have the same effect as the command
namespace eval ::a::b {foo bar x y}.
This command is needed because
extensions like Tk normally execute callback scripts
in the global namespace.
A scoped command captures a command together with its namespace context
in a way that allows it to be executed properly later.
See the section SCOPED VALUES for some examples
of how this is used to create callback scripts.
namespace current
Returns the fully-qualified name for the current namespace.
The actual name of the global namespace is ``''
(i.e., an empty string),
but this command returns :: for the global namespace
as a convenience to programmers.
namespace delete ?namespace namespace ...?
Each namespace namespace is deleted
and all variables, procedures, and child namespaces
contained in the namespace are deleted.
If a procedure is currently executing inside the namespace,
the namespace will be kept alive until the procedure returns;
however, the namespace is marked to prevent other code from
looking it up by name.
If a namespace doesn't exist, this command returns an error.
If no namespace names are given, this command does nothing.
namespace eval namespace arg ?arg ...?
Activates a namespace called namespace and evaluates some code
in that context.
If the namespace does not already exist, it is created.
If more than one arg argument is specified,
the arguments are concatenated together with a space between each one
in the same fashion as the eval command,
and the result is evaluated.
If namespace has leading namespace qualifiers
and any leading namespaces do not exist,
they are automatically created.
namespace export ?-clear? ?pattern pattern ...?
Specifies which commands are exported from a namespace.
The exported commands are those that can be later imported
into another namespace using a namespace import command.
Both commands defined in a namespace and
commands the namespace has previously imported
can be exported by a namespace.
The commands do not have to be defined
at the time the namespace export command is executed.
Each pattern may contain glob-style special characters,
but it may not include any namespace qualifiers.
That is, the pattern can only specify commands
in the current (exporting) namespace.
Each pattern is appended onto the namespace's list of export patterns.
If the -clear flag is given,
the namespace's export pattern list is reset to empty before any
pattern arguments are appended.
If no patterns are given and the -clear flag isn't given,
this command returns the namespace's current export list.
namespace forget ?pattern pattern ...?
Removes previously imported commands from a namespace.
Each pattern is a qualified name such as
foo::x or a::b::p*.
Qualified names contain ::s and qualify a name
with the name of one or more namespaces.
Each pattern is qualified with the name of an exporting namespace
and may have glob-style special characters in the command name
at the end of the qualified name.
Glob characters may not appear in a namespace name.
This command first finds the matching exported commands.
It then checks whether any of those those commands
were previously imported by the current namespace.
If so, this command deletes the corresponding imported commands.
In effect, this un-does the action of a namespace import command.
namespace import ?-force? ?pattern pattern ...?
Imports commands into a namespace.
Each pattern is a qualified name like
foo::x or a::p*.
That is, it includes the name of an exporting namespace
and may have glob-style special characters in the command name
at the end of the qualified name.
Glob characters may not appear in a namespace name.
All the commands that match a pattern string
and which are currently exported from their namespace
are added to the current namespace.
This is done by creating a new command in the current namespace
that points to the exported command in its original namespace;
when the new imported command is called, it invokes the exported command.
This command normally returns an error
if an imported command conflicts with an existing command.
However, if the -force option is given,
imported commands will silently replace existing commands.
The namespace import command has snapshot semantics:
that is, only requested commands that are currently defined
in the exporting namespace are imported.
In other words, you can import only the commands that are in a namespace
at the time when the namespace import command is executed.
If another command is defined and exported in this namespace later on,
it will not be imported.
namespace inscope namespace arg ?arg ...?
Executes a script in the context of a particular namespace.
This command is not expected to be used directly by programmers;
calls to it are generated implicitly when applications
use namespace code commands to create callback scripts
that the applications then register with, e.g., Tk widgets.
The namespace inscope command is much like the namespace eval
command except that it has lappend semantics
and the namespace must already exist.
It treats the first argument as a list,
and appends any arguments after the first
onto the end as proper list elements.
namespace inscope ::foo a x y z
is equivalent to
namespace eval ::foo [concat a [list x y z]]
This lappend semantics is important because many callback scripts
are actually prefixes.
namespace origin command
Returns the fully-qualified name of the original command
to which the imported command command refers.
When a command is imported into a namespace,
a new command is created in that namespace
that points to the actual command in the exporting namespace.
If a command is imported into a sequence of namespaces
a, b,...,n where each successive namespace
just imports the command from the previous namespace,
this command returns the fully-qualified name of the original command
in the first namespace, a.
If command does not refer to an imported command,
the command's own fully-qualified name is returned.
namespace parent ?namespace?
Returns the fully-qualified name of the parent namespace
for namespace namespace.
If namespace is not specified,
the fully-qualified name of the current namespace's parent is returned.
namespace qualifiers string
Returns any leading namespace qualifiers for string.
Qualifiers are namespace names separated by ::s.
For the string ::foo::bar::x,
this command returns ::foo::bar,
and for :: it returns ``'' (an empty string).
This command is the complement of the namespace tail command.
Note that it does not check whether the
namespace names are, in fact,
the names of currently defined namespaces.
namespace tail string
Returns the simple name at the end of a qualified string.
Qualifiers are namespace names separated by ::s.
For the string ::foo::bar::x,
this command returns x,
and for :: it returns ``'' (an empty string).
This command is the complement of the namespace qualifiers command.
It does not check whether the namespace names are, in fact,
the names of currently defined namespaces.
namespace which ?-command? ?-variable? name
Looks up name as either a command or variable
and returns its fully-qualified name.
For example, if name does not exist in the current namespace
but does exist in the global namespace,
this command returns a fully-qualified name in the global namespace.
If the command or variable does not exist,
this command returns an empty string.
If no flag is given, name is treated as a command name.
See the section NAME RESOLUTION below for an explanation of
the rules regarding name resolution.
WHAT IS A NAMESPACE?
A namespace is a collection of commands and variables.
It encapsulates the commands and variables to ensure that they
won't interfere with the commands and variables of other namespaces.
Tcl has always had one such collection,
which we refer to as the global namespace.
The global namespace holds all global variables and commands.
The namespace eval command lets you create new namespaces.
For example,
namespace eval Counter {
namespace export Bump
variable num 0
proc Bump {} {
variable num
incr num
}
}
creates a new namespace containing the variable num and
the procedure Bump.
The commands and variables in this namespace are separate from
other commands and variables in the same program.
If there is a command named Bump in the global namespace,
for example, it will be different from the command Bump
in the Counter namespace.
Namespace variables resemble global variables in Tcl.
They exist outside of the procedures in a namespace
but can be accessed in a procedure via the variable command,
as shown in the example above.
Namespaces are dynamic.
You can add and delete commands and variables at any time,
so you can build up the contents of a
namespace over time using a series of namespace eval commands.
For example, the following series of commands has the same effect
as the namespace definition shown above:
namespace eval Counter {
variable num 0
proc Bump {} {
variable num
return [incr num]
}
}
namespace eval Counter {
proc test {args} {
return $args
}
}
namespace eval Counter {
rename test ""
}
Note that the test procedure is added to the Counter namespace,
and later removed via the rename command.
Namespaces can have other namespaces within them,
so they nest hierarchically.
A nested namespace is encapsulated inside its parent namespace
and can not interfere with other namespaces.
QUALIFIED NAMES
Each namespace has a textual name such as
history or ::safe::interp.
Since namespaces may nest,
qualified names are used to refer to
commands, variables, and child namespaces contained inside namespaces.
Qualified names are similar to the hierarchical path names for
Unix files or Tk widgets,
except that :: is used as the separator
instead of / or ..
The topmost or global namespace has the name ``'' (i.e., an empty string),
although :: is a synonym.
As an example, the name ::safe::interp::create
refers to the command create in the namespace interp
that is a child of of namespace ::safe,
which in turn is a child of the global namespace ::.
If you want to access commands and variables from another namespace,
you must use some extra syntax.
Names must be qualified by the namespace that contains them.
From the global namespace,
we might access the Counter procedures like this:
Counter::Bump 5
Counter::Reset
We could access the current count like this:
puts "count = $Counter::num"
When one namespace contains another, you may need more than one
qualifier to reach its elements.
If we had a namespace Foo that contained the namespace Counter,
you could invoke its Bump procedure
from the global namespace like this:
Foo::Counter::Bump 3
You can also use qualified names when you create and rename commands.
For example, you could add a procedure to the Foo
namespace like this:
proc Foo::Test {args} {return $args}
And you could move the same procedure to another namespace like this:
rename Foo::Test Bar::Test
There are a few remaining points about qualified names
that we should cover.
Namespaces have nonempty names except for the global namespace.
:: is disallowed in simple command, variable, and namespace names
except as a namespace separator.
Extra :s in a qualified name are ignored;
that is, two or more :s are treated as a namespace separator.
A trailing :: in a qualified variable or command name
refers to the variable or command named {}.
However, a trailing :: in a qualified namespace name is ignored.
NAME RESOLUTION
In general, all Tcl commands that take variable and command names
support qualified names.
This means you can give qualified names to such commands as
set, proc, rename, and interp alias.
If you provide a fully-qualified name that starts with a ::,
there is no question about what command, variable, or namespace
you mean.
However, if the name does not start with a ::
(i.e., is relative),
Tcl follows a fixed rule for looking it up:
Command and variable names are always resolved
by looking first in the current namespace,
and then in the global namespace.
Namespace names, on the other hand, are always resolved
by looking in only the current namespace.
In the following example,
set traceLevel 0
namespace eval Debug {
printTrace $traceLevel
}
Tcl looks for traceLevel in the namespace Debug
and then in the global namespace.
It looks up the command printTrace in the same way.
If a variable or command name is not found in either context,
the name is undefined.
To make this point absolutely clear, consider the following example:
set traceLevel 0
namespace eval Foo {
variable traceLevel 3
namespace eval Debug {
printTrace $traceLevel
}
}
Here Tcl looks for traceLevel first in the namespace Foo::Debug.
Since it is not found there, Tcl then looks for it
in the global namespace.
The variable Foo::traceLevel is completely ignored
during the name resolution process.
You can use the namespace which command to clear up any question
about name resolution.
For example, the command:
namespace eval Foo::Debug {namespace which -variable traceLevel}
returns ::traceLevel.
On the other hand, the command,
namespace eval Foo {namespace which -variable traceLevel}
returns ::Foo::traceLevel.
As mentioned above,
namespace names are looked up differently
than the names of variables and commands.
Namespace names are always resolved in the current namespace.
This means, for example,
that a namespace eval command that creates a new namespace
always creates a child of the current namespace
unless the new namespace name begins with a ::.
Tcl has no access control to limit what variables, commands,
or namespaces you can reference.
If you provide a qualified name that resolves to an element
by the name resolution rule above,
you can access the element.
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.
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.
IMPORTING COMMANDS
Namespaces are often used to represent libraries.
Some library commands are used so frequently
that it is a nuisance to type their qualified names.
For example, suppose that all of the commands in a package
like BLT are contained in a namespace called Blt.
Then you might access these commands like this:
Blt::graph .g -background red
Blt::table . .g 0,0
If you use the graph and table commands frequently,
you may want to access them without the Blt:: prefix.
You can do this by importing the commands into the current namespace,
like this:
namespace import Blt::*
This adds all exported commands from the Blt namespace
into the current namespace context, so you can write code like this:
graph .g -background red
table . .g 0,0
The namespace import command only imports commands
from a namespace that that namespace exported
with a namespace export command.
Importing every command from a namespace is generally
a bad idea since you don't know what you will get.
It is better to import just the specific commands you need.
For example, the command
namespace import Blt::graph Blt::table
imports only the graph and table commands into the
current context.
If you try to import a command that already exists, you will get an
error. This prevents you from importing the same command from two
different packages. But from time to time (perhaps when debugging),
you may want to get around this restriction. You may want to
reissue the namespace import command to pick up new commands
that have appeared in a namespace. In that case, you can use the
-force option, and existing commands will be silently overwritten:
namespace import -force Blt::graph Blt::table
If for some reason, you want to stop using the imported commands,
you can remove them with an namespace forget command, like this:
namespace forget Blt::*
This searches the current namespace for any commands imported from Blt.
If it finds any, it removes them. Otherwise, it does nothing.
After this, the Blt commands must be accessed with the Blt::
prefix.
When you delete a command from the exporting namespace like this:
rename Blt::graph ""
the command is automatically removed from all namespaces that import it.
EXPORTING COMMANDS
You can export commands from a namespace like this:
namespace eval Counter {
namespace export Bump Reset
variable num 0
variable max 100
proc Bump {{by 1}} {
variable num
incr num $by
check
return $num
}
proc Reset {} {
variable num
set num 0
}
proc check {} {
variable num
variable max
if {$num > $max} {
error "too high!"
}
}
}
The procedures Bump and Reset are exported,
so they are included when you import from the Counter namespace,
like this:
namespace import Counter::*
However, the check procedure is not exported,
so it is ignored by the import operation.
The namespace import command only imports commands
that were declared as exported by their namespace.
The namespace export command specifies what commands
may be imported by other namespaces.
If a namespace import command specifies a command
that is not exported, the command is not imported.
SEE ALSO
KEYWORDS
exported, internal, variable
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