I was noticing a discussion on [comp.lang.tcl] about variable
scope and when I searched the wiki, I didn't notice quickly
any place where the topic of scope was centrally discussed.

Tcl has several layers of scoping - from an article by [Bryan Oakley],
we read:

 Sektor van Skijlen wrote:
 > Could someone explain me, what is the proper syntax with correct scope for
 > variables, which are:
 > 
 >  - declared inside in a namespace

[variable] varname ?initValue?

 >  - globally

[global] varname ?varname ...?

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[LV] With the advent of namespaces, one can provide a more consistent
variable usage by using ::varname as opposed to invoking the global
command with varname.

[RHS] However, if you're accessing the variable a lot, its worth using the
global command, since it is faster to access the variable that way. Assuming
performance matters in what you are doing.
 % proc p1 {} { global v1 ; for {set i 0} {$i <1000} {incr i} { set v1 } }
 % proc p2 {} {for {set i 0} {$i <1000} {incr i} { set ::v2 } }
 % time { p1 } 100
 1802 microseconds per iteration
 % time { p2 } 100
 4520 microseconds per iteration

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[LV] Curious - is it due to additional parsing?  Or is tcl just missing
an optimization?

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>  - accessed in a proc defined in the same namespace

use variable if you wish to reference the [namespace] variable, global if 
you want to reference the global variable. Use neither to reference a 
local variable.

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Please add other information, discussion, corrections to this page.

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Note that merely declaring a variable with the global or variable
command does '''NOT''' create the variable.  It just tells Tcl
where to go looking for the variable if there's a reference to it.

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[Category Tutorial]