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Updated: Wednesday, February 9, 2022

namespace (1t)


namespace - create and manipulate contexts for commands and variables


namespace ?subcommand? ?arg ...?


namespace(1t)                Tcl Built-In Commands               namespace(1t)


       namespace - create and manipulate contexts for commands and variables

       namespace ?subcommand? ?arg ...?

       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 values of
       subcommand are listed below.  Note that you can abbreviate the  subcom-

       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 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  [names-
              pace  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 nor-
              mally 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, 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 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  argu-
              ments are concatenated together with a space between each one in
              the same fashion as the eval command, and the result  is  evalu-

              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 is not 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 double colons (::) and qual-
              ify a name with the name of one or more namespaces.  Each "qual-
              ified pattern" is qualified with the name of an exporting names-
              pace 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 com-
              mand deletes the matching commands of the current namespace that
              were imported from a different namespace.  For  "qualified  pat-
              terns", 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

              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  names-
              pace  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  cur-
              rently  exported  from  their namespace are added to the current
              namespace.  This is done by creating a new command in  the  cur-
              rent namespace that points to the exported command in its origi-
              nal 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  com-
              mand  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

                     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  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 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 reso-
              lution  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.  Qual-
              ifiers 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

       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 names-
              pace 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 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.

       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  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.

       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 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

       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 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.

       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 basic rules
       for looking it up:

       o      Variable names are always resolved by looking first in the  cur-
              rent namespace, and then in the global namespace.

       o      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 pro-
              cessed by the appropriate namespace unknown handler.)

       o      Namespace names are always resolved by looking in only the  cur-
              rent 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

              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 ::.

       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

       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.

       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  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 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 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

       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
                      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.

       The namespace code command is the means by which a script may be  pack-
       aged  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"
                  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

       The  namespace  ensemble is used to create and manipulate ensemble com-
       mands, 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  com-
              mand.   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 com-
              mand exists and is an ensemble command.  This command only  ever
              returns  an  error  if the number of arguments to the command is

       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.

       The following options, supported by the namespace ensemble  create  and
       namespace  ensemble configure commands, control how an ensemble command

       -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.

              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.

              This  option  (which is enabled by default) controls whether the
              ensemble command recognizes unambiguous prefixes of its  subcom-
              mands.   When  turned  off,  the ensemble command requires exact
              matching of subcommand names.

              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.

              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  BEHAV-
              IOUR for more details.

       The following extra option is allowed by namespace ensemble create:

              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-qual-
              ified name of the namespace in which the namespace ensemble cre-
              ate command is invoked.

       The following extra option is allowed by namespace ensemble configure:

              This  read-only  option allows the retrieval of the fully-quali-
              fied name of  the  namespace  which  the  ensemble  was  created

       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 condi-
       tions 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 speci-
       fied -unknown option and appends each argument of the attempted  ensem-
       ble   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  subcom-
       mand  (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  ensem-
       ble,  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.

       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

              # Use the command resolution path to find the name
              namespace eval boo {
                  namespace path ::foo

              # Import into current namespace, then call local alias
              namespace import foo::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 -parame- |
       ters option to allow the operator to be put between the first and  sec- |
       ond 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 attributes(7) for descriptions of the following attributes:

       |Availability   | runtime/tcl-8    |
       |Stability      | Uncommitted      |

       interp(n), upvar(n), variable(n)

       command, ensemble, exported, internal, variable

       Source  code  for open source software components in Oracle Solaris can
       be found at https://www.oracle.com/downloads/opensource/solaris-source-

       This     software     was    built    from    source    available    at
       https://github.com/oracle/solaris-userland.   The  original   community
       source was downloaded from  http://prdownloads.sourceforge.net/tcl/tcl-

       Further information about this software can be found on the open source
       community website at https://www.tcl.tk/.

Tcl                                   8.5                        namespace(1t)