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namespace(n)			      Tcl Built-In Commands			     namespace(n)

_________________________________________________________________________________________________

NAME
       namespace - 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  names-
       paces.  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  names-
	      pace.   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  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 argu-
	      ments 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 names-
	      paces contained in the namespace are deleted.  If a procedure is currently  execut-
	      ing  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 exists namespace
	      Returns 1 if namespace is a valid namespace in the current context, returns 0  oth-
	      erwise.

       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 pre-
	      viously 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  quali-
	      fiers.   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 simple or
	      qualified name such as x, foo::x or a::b::p*.   Qualified  names	contain  ::s  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 com-
	      mands.  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 nor-
	      mally  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 sec-
	      ond 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 com-
	      mand 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 names-
	      pace.  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 names-
	      pace.  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 vari-
	      able 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 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  proce-
       dures 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 com-
       mands.  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 con-
       tained 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 trail-
       ing :: 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, con-
       sider 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 refer-
       ence.  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  names-
       pace  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  fre-
       quently	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
       variable(n)

KEYWORDS
       exported, internal, variable

Tcl					       8.0				     namespace(n)
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