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, it's 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

[schofield]
Consider the above procs. Both p1 & p2 access the variable v1 & v2, respectively, many times. Should you only need to access the variable once or maybe twice, it is faster to fully qualify the variable name and pay the price of having tcl look up the variable, which is cheaper than the "global" call.
 
 %  proc p3 {} {global v3; set v3}
 %  proc p4 {} {set ::v4}
 %  set v3 1
 1
 %  set v4 1
 1
 % time p3 100
 41 microseconds per iteration
 % time p4 100
 33 microseconds per iteration
 % proc g1 {} {global v3}
 % time g1 100
 51 microseconds per iteration

What I'd like to know is why is it more expensive to "global v3" than it is to "global v3; set v3"? This happened consistently for with many "time" invocations.

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

[RHS] My guess (and it's just a guess) is that, when it goes to look up the variable:
   * For the one linked via global, it's now a local variable so it's in the local table. This means that it finds the variable immediately
   * For the qualified name, it has to parse the name, recognize the namespace it's in, and then go look in that namespace's table to get the value
As a sidenote, the bytecode is pretty much the same for them (other than the global command part), so the issue isn't there.
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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]