Tcl provides a number of commands for introspection - commands that tell what is going on in your program, what the implementation is of your procedures, which variables have been set and so on.

The info command allows a Tcl program to obtain information from the Tcl interpreter. The next three lessons cover aspects of the info command. (Other commands allowing introspection involve: traces, namespaces, commands scheduled for later execution via the after command and so on.)

This lesson covers the info subcommands that return information about which procs, variables, or commands are currently in existence in this instance of the interpreter. By using these subcommands you can determine if a variable or proc exists before you try to access it.

The code below shows how to use the info exists command to make an incr that will never return a no such variable error, since it checks to be certain that the variable exists before incrementing it:

proc safeIncr {val {amount 1}} {
    upvar $val v
    if { [info exists v] } {
        incr v $amount
    }  else {
        set v $amount
    }
}

Info commands that return lists of visible commands and variables.

Almost all these commands take a pattern that follow the string match rules. If pattern is not provided, a list of all items is returned (as if the pattern was "*").

info commands ?pattern?: Returns a list of the commands, both internal commands and procedures, whose names match pattern.
info exists varName: Returns 1 if varName exists as a variable (or an array element) in the current context, otherwise returns 0.
info functions ?pattern?: Returns a list of the mathematical functions available via the expr command that match pattern.
info globals ?pattern?: Returns a list of the global variables that match pattern.
info locals ?pattern?: Returns a list of the local variables that match pattern.
info procs ?pattern?: Returns a list of the Tcl procedures that match pattern.
info vars ?pattern?: Returns a list of the local and global variables that match pattern.

Example

if {[info procs safeIncr] eq "safeIncr"} {
     safeIncr a
}

puts "After calling SafeIncr with a non existent variable: $a"

set a 100
safeIncr a
puts "After calling SafeIncr with a variable with a value of 100: $a"

safeIncr b -3
puts "After calling safeIncr with a non existent variable by -3: $b"

set b 100
safeIncr b -3
puts "After calling safeIncr with a variable whose value is 100 by -3: $b"

puts "\nThese variables have been defined: [lsort [info vars]]"
puts "\nThese globals have been defined:   [lsort [info globals]]"

set exist [info procs localproc]
if {$exist == ""} {
    puts "\nlocalproc does not exist at point 1"
}

proc localproc {} {
    global argv

    set loc1 1
    set loc2 2
    puts "\nLocal variables accessible in this proc are: [lsort [info locals]]"
    puts "\nVariables accessible from this proc are:     [lsort [info vars]]"
    puts "\nGlobal variables visible from this proc are: [lsort [info globals]]"
}

set exist [info procs localproc]
if {$exist != ""} {
    puts "localproc does exist at point 2"
}

localproc

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