level

A level is a command evaluation context.

Description

For the evaluation of most commands, a new level is created and the command evaluated at that level. A level maintains the following four notions:

The current namespace

The current variable table

For most commands, the current variable table is the local variable table that exists only for the duration of the command, and is only accessible to commands in scripts evaluated at the current level. For a few commands such as namespace eval the variable table of the current namespace is the current variable table.

The current command

The sequence of previous levels

A level could be called a stack frame, but that term used when discussing constructs in the implementation of Tcl, while level is used to discuss the behaviour of a Tcl interpreter.

Commands Related to Levels

Some commands manipulate the current level or reference resources at other levels. Here is a brief description of such commands:

info level

Provides information about the current and previous levels.

namespace eval

Creates a new level, associates that level with the indicated namespace, and evaluates a script at that level.

proc

Arranges for a level to be created, arguments to the command to be assigned in the current variable table, and for the body of the procedure to be evaluated at the new level.

set

Modifies the current variable table. At a level associated with a namespace if the specified variable doesn't exist at that level, but does exist at the global level, modifies the global variable table instead. This behaviour is now considered a mistake to be fixed in some future version of Tcl.

uplevel

Arranges for a script to be evaluated at some level, which is resolved relative to the current level.

upvar and namespace upvar

Each of these commands modifies a variable table, which is resolved relative to the current level.

variable

Modifies the variable table of the current level.

See Also

Many ways to eval

Whenever a script is evaluated, a level is involved.

downlevel

An Eagle-only extension to uplevel.