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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	NAMES‐
       PACE? 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 names‐
	      pace  namespace.	 If namespace is not specified, then the chil‐
	      dren 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 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 [names‐
	      pace 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 com‐
	      mand captures a command together with its namespace context in a
	      way  that allows it to be executed properly later.  See the sec‐
	      tion SCOPED VALUES for some examples of how this is used to cre‐
	      ate 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,	proce‐
	      dures,  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 argu‐
	      ments are concatenated together with a space between each one in
	      the  same	 fashion as the eval command, and the result is evalu‐
	      ated.

	      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 con‐
	      text, 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  com‐
	      mands 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 com‐
	      mand 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 names‐
	      pace'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 names‐
	      pace'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	names‐
	      pace  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 com‐
	      mand 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  correspond‐
	      ing  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 com‐
	      mand.  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 names‐
	      pace 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 names‐
	      pace 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 names‐
	      paces 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.	 Qual‐
	      ifiers  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 variable command or through a
	      trace on the variable, this command will return the fully-quali‐
	      fied 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  com‐
       mand  named  bump in the global namespace, for example, it will be dif‐
       ferent from the command bump in the Counter namespace.

       Namespace variables resemble global variables in Tcl.  They exist  out‐
       side  of	 the procedures in a namespace but can be accessed in a proce‐
       dure 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 hierar‐
       chically.  A nested namespace is encapsulated inside its parent	names‐
       pace 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  com‐
       mands,  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 names‐
       pace 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 quali‐
       fier 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  sup‐
       port  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 names‐
       pace.  Namespace names, on the other hand, are always resolved by look‐
       ing 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 res‐
       olution 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 names‐
       paces 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 names‐
       pace 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  com‐
       mands are used so frequently that it is a nuisance to type their quali‐
       fied 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 spe‐
       cific 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 dif‐
       ferent 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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