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Practical Programming in Tcl & Tk, Third Edition
By Brent B. Welch

Table of Contents
Chapter 14.  Namespaces


Nested Namespaces

Namespaces can be nested inside other namespaces. Example 14-3 shows three namespaces that have their own specific variable x. The fully qualified names for these variables are ::foo::x, ::bar::x, and ::bar::foo::x.

Example 14-3 Nested namespaces.
namespace eval foo {
   variable x 1      ;# ::foo::x
}
namespace eval bar {
   variable x 2      ;# ::bar::x
   namespace foo {
      variable x 3   ;# ::bar::foo::x
   }
   puts $foo::x      ;# prints 3
}
puts $foo::x         ;# prints 1

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Partially qualified names can refer to two different objects.


In Example 14-3 the partially qualified name foo::x can reference one of two variables depending on the current namespace. From the global scope the name foo::x refers to the namespace variable x inside ::foo. From the ::bar namespace, foo::x refers to the variable x inside ::bar::foo.

If you want to unambiguously name a variable in the current namespace, you have two choices. The simplest is to bring the variable into scope with the variable command:

variable x
set x something

If you need to give out the name of the variable, then you have two choices. The most general solution is to use the namespace current command to create a fully qualified name:

trace variable [namespace current]::x r \
     [namespace current]::traceproc

However, it is simpler to just explicitly write out the namespace as in:

trace variable ::myname::x r ::myname::traceproc

The drawback of this approach is that it litters your code with references to ::myname::, which might be subject to change during program development.


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