Provided by: tcl8.6-doc_8.6.1-4ubuntu1_all 
NAME
namespace - create and manipulate contexts for commands and variables
SYNOPSIS
namespace ?subcommand? ?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 values of subcommand are listed below. Note that you can
abbreviate the subcommands.
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 a double
colon (::). 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 double colon (::) is used directly, otherwise
the namespace namespace (or the fully-qualified name of the current namespace) is
prepended onto 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 inscope
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 [list 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 SCRIPTS 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 does not exist, this command returns an
error. If no namespace names are given, this command does nothing.
namespace ensemble subcommand ?arg ...?
Creates and manipulates a command that is formed out of an ensemble of subcommands.
See the section ENSEMBLES below for further details.
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 exists namespace
Returns 1 if namespace is a valid namespace in the current context, returns 0
otherwise.
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 is not 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 simple or
qualified name such as x, foo::x or a::b::p*. Qualified names contain double
colons (::) and qualify a name with the name of one or more namespaces. Each
“qualified 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. For each “simple
pattern” this command deletes the matching commands of the current namespace that
were imported from a different namespace. For “qualified patterns”, this command
first finds the matching exported commands. It then checks whether any of 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, or queries the set of imported commands in a
namespace. When no arguments are present, namespace import returns the list of
commands in the current namespace that have been imported from other namespaces.
The commands in the returned list are in the format of simple names, with no
namespace qualifiers at all. This format is suitable for composition with
namespace forget (see EXAMPLES below).
When pattern arguments are present, 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. When the namespace name is not
fully qualified (i.e., does not start with a namespace separator) it is resolved as
a namespace name in the way described in the NAME RESOLUTION section; it is an
error if no namespace with that name can be found.
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 script ?arg ...?
Executes a script in the context of the specified 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 the namespace must already
exist, and namespace inscope appends additional args as proper list elements.
namespace inscope ::foo $script $x $y $z
is equivalent to
namespace eval ::foo [concat $script [list $x $y $z]]
thus additional arguments will not undergo a second round of substitution, as is
the case with namespace eval.
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 path ?namespaceList?
Returns the command resolution path of the current namespace. If namespaceList is
specified as a list of named namespaces, the current namespace's command resolution
path is set to those namespaces and returns the empty list. The default command
resolution path is always empty. See the section NAME RESOLUTION below for an
explanation of the rules regarding name resolution.
namespace qualifiers string
Returns any leading namespace qualifiers for string. Qualifiers are namespace
names separated by double colons (::). 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 double colons (::). 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 upvar namespace ?otherVar myVar ...?
This command arranges for zero or more local variables in the current procedure to
refer to variables in namespace. The namespace name is resolved as described in
section NAME RESOLUTION. The command namespace upvar $ns a b has the same
behaviour as upvar 0 ${ns}::a b, with the sole exception of the resolution rules
used for qualified namespace or variable names. namespace upvar returns an empty
string.
namespace unknown ?script?
Sets or returns the unknown command handler for the current namespace. The handler
is invoked when a command called from within the namespace cannot be found in the
current namespace, the namespace's path nor in the global namespace. The script
argument, if given, should be a well formed list representing a command name and
optional arguments. When the handler is invoked, the full invocation line will be
appended to the script and the result evaluated in the context of the namespace.
The default handler for all namespaces is ::unknown. If no argument is given, it
returns the handler for the current namespace.
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 the variable has been created but not defined, such as with the
variable command or through a trace on the variable, this command will return the
fully-qualified name of the variable. 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 will not 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 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 colons in any
separator part of a qualified name are ignored; i.e. two or more colons 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 basic rules for looking it up:
• Variable names are always resolved by looking first in the current namespace, and
then in the global namespace.
• Command names are always resolved by looking in the current namespace first. If not
found there, they are searched for in every namespace on the current namespace's
command path (which is empty by default). If not found there, command names are
looked up in the global namespace (or, failing that, are processed by the
appropriate namespace unknown handler.)
• Namespace names 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 ::.
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 do not 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
a 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.
SCOPED SCRIPTS
The namespace code command is the means by which a script may be packaged for evaluation
in a namespace other than the one in which it was created. It is used most often to
create event handlers, Tk bindings, and traces for evaluation in the global context. For
instance, the following code indicates how to direct a variable trace callback into the
current namespace:
namespace eval a {
variable b
proc theTraceCallback { n1 n2 op } {
upvar 1 $n1 var
puts "the value of $n1 has changed to $var"
return
}
trace add variable b write [namespace code theTraceCallback]
}
set a::b c
When executed, it prints the message:
the value of a::b has changed to c
ENSEMBLES
The namespace ensemble is used to create and manipulate ensemble commands, which are
commands formed by grouping subcommands together. The commands typically come from the
current namespace when the ensemble was created, though this is configurable. Note that
there may be any number of ensembles associated with any namespace (including none, which
is true of all namespaces by default), though all the ensembles associated with a
namespace are deleted when that namespace is deleted. The link between an ensemble
command and its namespace is maintained however the ensemble is renamed.
Three subcommands of the namespace ensemble command are defined:
namespace ensemble create ?option value ...?
Creates a new ensemble command linked to the current namespace, returning the fully
qualified name of the command created. The arguments to namespace ensemble create
allow the configuration of the command as if with the namespace ensemble configure
command. If not overridden with the -command option, this command creates an
ensemble with exactly the same name as the linked namespace. See the section
ENSEMBLE OPTIONS below for a full list of options supported and their effects.
namespace ensemble configure command ?option? ?value ...?
Retrieves the value of an option associated with the ensemble command named
command, or updates some options associated with that ensemble command. See the
section ENSEMBLE OPTIONS below for a full list of options supported and their
effects.
namespace ensemble exists command
Returns a boolean value that describes whether the command command exists and is an
ensemble command. This command only ever returns an error if the number of
arguments to the command is wrong.
When called, an ensemble command takes its first argument and looks it up (according to
the rules described below) to discover a list of words to replace the ensemble command and
subcommand with. The resulting list of words is then evaluated (with no further
substitutions) as if that was what was typed originally (i.e. by passing the list of words
through Tcl_EvalObjv) and returning the result of the command. Note that it is legal to
make the target of an ensemble rewrite be another (or even the same) ensemble command.
The ensemble command will not be visible through the use of the uplevel or info level
commands.
ENSEMBLE OPTIONS
The following options, supported by the namespace ensemble create and namespace ensemble
configure commands, control how an ensemble command behaves:
-map When non-empty, this option supplies a dictionary that provides a mapping from
subcommand names to a list of prefix words to substitute in place of the ensemble
command and subcommand words (in a manner similar to an alias created with interp
alias; the words are not reparsed after substitution); if the first word of any
target is not fully qualified when set, it is assumed to be relative to the current
namespace and changed to be exactly that (that is, it is always fully qualified
when read). When this option is empty, the mapping will be from the local name of
the subcommand to its fully-qualified name. Note that when this option is non-
empty and the -subcommands option is empty, the ensemble subcommand names will be
exactly those words that have mappings in the dictionary.
-parameters
This option gives a list of named arguments (the names being used during generation │
of error messages) that are passed by the caller of the ensemble between the name │
of the ensemble and the subcommand argument. By default, it is the empty list.
-prefixes
This option (which is enabled by default) controls whether the ensemble command
recognizes unambiguous prefixes of its subcommands. When turned off, the ensemble
command requires exact matching of subcommand names.
-subcommands
When non-empty, this option lists exactly what subcommands are in the ensemble.
The mapping for each of those commands will be either whatever is defined in the
-map option, or to the command with the same name in the namespace linked to the
ensemble. If this option is empty, the subcommands of the namespace will either be
the keys of the dictionary listed in the -map option or the exported commands of
the linked namespace at the time of the invocation of the ensemble command.
-unknown
When non-empty, this option provides a partial command (to which all the words that
are arguments to the ensemble command, including the fully-qualified name of the
ensemble, are appended) to handle the case where an ensemble subcommand is not
recognized and would otherwise generate an error. When empty (the default) an
error (in the style of Tcl_GetIndexFromObj) is generated whenever the ensemble is
unable to determine how to implement a particular subcommand. See UNKNOWN HANDLER
BEHAVIOUR for more details.
The following extra option is allowed by namespace ensemble create:
-command
This write-only option allows the name of the ensemble created by namespace
ensemble create to be anything in any existing namespace. The default value for
this option is the fully-qualified name of the namespace in which the namespace
ensemble create command is invoked.
The following extra option is allowed by namespace ensemble configure:
-namespace
This read-only option allows the retrieval of the fully-qualified name of the
namespace which the ensemble was created within.
UNKNOWN HANDLER BEHAVIOUR
If an unknown handler is specified for an ensemble, that handler is called when the
ensemble command would otherwise return an error due to it being unable to decide which
subcommand to invoke. The exact conditions under which that occurs are controlled by the
-subcommands, -map and -prefixes options as described above.
To execute the unknown handler, the ensemble mechanism takes the specified -unknown option
and appends each argument of the attempted ensemble command invocation (including the
ensemble command itself, expressed as a fully qualified name). It invokes the resulting
command in the scope of the attempted call. If the execution of the unknown handler
terminates normally, the ensemble engine reparses the subcommand (as described below) and
tries to dispatch it again, which is ideal for when the ensemble's configuration has been
updated by the unknown subcommand handler. Any other kind of termination of the unknown
handler is treated as an error.
The result of the unknown handler is expected to be a list (it is an error if it is not).
If the list is an empty list, the ensemble command attempts to look up the original
subcommand again and, if it is not found this time, an error will be generated just as if
the -unknown handler was not there (i.e. for any particular invocation of an ensemble, its
unknown handler will be called at most once.) This makes it easy for the unknown handler
to update the ensemble or its backing namespace so as to provide an implementation of the
desired subcommand and reparse.
When the result is a non-empty list, the words of that list are used to replace the
ensemble command and subcommand, just as if they had been looked up in the -map. It is up
to the unknown handler to supply all namespace qualifiers if the implementing subcommand
is not in the namespace of the caller of the ensemble command. Also note that when
ensemble commands are chained (e.g. if you make one of the commands that implement an
ensemble subcommand into an ensemble, in a manner similar to the text widget's tag and
mark subcommands) then the rewrite happens in the context of the caller of the outermost
ensemble. That is to say that ensembles do not in themselves place any namespace contexts
on the Tcl call stack.
Where an empty -unknown handler is given (the default), the ensemble command will generate
an error message based on the list of commands that the ensemble has defined (formatted
similarly to the error message from Tcl_GetIndexFromObj). This is the error that will be
thrown when the subcommand is still not recognized during reparsing. It is also an error
for an -unknown handler to delete its namespace.
EXAMPLES
Create a namespace containing a variable and an exported command:
namespace eval foo {
variable bar 0
proc grill {} {
variable bar
puts "called [incr bar] times"
}
namespace export grill
}
Call the command defined in the previous example in various ways.
# Direct call
::foo::grill
# Use the command resolution path to find the name
namespace eval boo {
namespace path ::foo
grill
}
# Import into current namespace, then call local alias
namespace import foo::grill
grill
# Create two ensembles, one with the default name and one with a
# specified name. Then call through the ensembles.
namespace eval foo {
namespace ensemble create
namespace ensemble create -command ::foobar
}
foo grill
foobar grill
Look up where the command imported in the previous example came from:
puts "grill came from [namespace origin grill]"
Remove all imported commands from the current namespace:
namespace forget {*}[namespace import]
Create an ensemble for simple working with numbers, using the -parameters option to allow │
the operator to be put between the first and second arguments. │
namespace eval do { │
namespace export * │
namespace ensemble create -parameters x │
proc plus {x y} {expr { $x + $y }} │
proc minus {x y} {expr { $x - $y }} │
} │
# In use, the ensemble works like this: │
puts [do 1 plus [do 9 minus 7]] │
SEE ALSO
interp(3tcl), upvar(3tcl), variable(3tcl)
KEYWORDS
command, ensemble, exported, internal, variable