Updated 2016-02-25 05:09:42 by mistachkin

A level is a command evaluation context.

Description  edit

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  edit

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.
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.
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.
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.
Modifies the variable table of the current level.

See Also  edit

Many ways to eval
Whenever a script is evaluated, a level is involved.
An Eagle-only extension to uplevel.