| The variable command defines a variable inside a namespace. It is like the set command because it can define a value for the variable. You can declare several namespace variables with one variable command. The general form is: variable name ?value? ?name value? ... If you have an array, do not assign a value in the variable command. Instead, use regular Tcl commands after you declare the variable. You can put any commands inside a namespace block: namespace eval foo { variable arr array set arr {name value name2 value2} } A namespace variable is similar to a global variable because it is outside the scope of any procedures. Procedures use the variable command or qualified names to reference namespace variables. For example, the random procedure has a variable command that brings the namespace variable into the current scope: variable seed If a procedure has a variable command that names a new variable, it is created in the namespace when it is first set.  | Watch out for conflicts with global variables. |
You need to be careful when you use variables inside a namespace block. If you declare them with a variable command, they are clearly namespace variables. However, if you forget to declare them, then they will either become namespace variables, or latch onto an existing global variable by the same name. Consider the following code: namespace eval foo { variable table for {set i 1} {$i <= 256} {incr i} { set table($i) [format %c $i] } } If there is already a global variable i, then the for loop will use that variable. Otherwise, it will create the foo::i variable. I found this behavior surprising, but it does make it easier to access global variables like env without first declaring them with global inside the namespace block. Qualified Names A fully qualified name begins with ::, which is the name for the global namespace. A fully qualified name unambiguously names a procedure or a variable. The fully qualified name works anywhere. If you use a fully qualified variable name, it is not necessary to use a global command. For example, suppose namespace foo has a namespace variable x, and there is also a global variable x. The global variable x can be named with this: ::x The :: syntax does not affect variable substitutions. You can get the value of the global variable x with $::x. Name the namespace variable x with this: ::foo::x A partially qualified name does not have a leading ::. In this case the name is resolved from the current namespace. For example, the following also names the namespace variable x: foo::x You can use qualified names with global. Once you do this, you can access the variable with its short name: global ::foo::x set x 5 | Declaring variables is more efficient than using qualified names. |
The Tcl byte-code compiler generates faster code when you declare namespace and global variables. Each procedure context has its own table of variables. The table can be accessed by a direct slot index, or by a hash table lookup of the variable name. The hash table lookup is slower than the direct slot access. When you use the variable or global command, then the compiler can use a direct slot access. If you use qualified names, the compiler uses the more general hash table lookup. | 
