Globals are expensive if you call them only a few times in the proc - the cost of the initial linking to a local variable weighs heavily; they are quite fast if you use them a lot (the breakeven seems to be around 15 reads ...). For fewer calls, use fully qualified variable names (::name) instead. (from: Can you run this benchmark 10 times faster)

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Globbing globals: Want to import several globals in one go, with glob wildcards (similar to the public statement in VB)? This comes from David Cuthbert (mailto:dacut@kanga.org):

    proc globalpat {args} {
        foreach pattern $args {
            set varnames [info globals $pattern]
            if {[llength $varnames] != 0} {
               uplevel 1 global $varnames
            }
        }
    }

To use:

    proc hello {} {
        globalpat *tcl*
        puts $tcl_patchLevel
    }
  % hello
  8.2.2

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Using the variable command in place of [global] is often good practice, even when the variable is global. If you later decide that you want to encapsulate a group of globals in a namespace, you can do so easily. If you have something like:

   proc myProc {} {
       global myvar1
       global myvar2
       # ...
   }

you have to go off and edit a bunch of code if you later decide that myProc and its associated variables should be in a namespace. If instead you say:

   proc myProc {} {
       variable myvar1
       variable myvar2
       # ...
   }

the variables are still global, but the procedure becomes easy to encapsulate: just wrap it in [namespace eval]:

   namespace eval myNamespace {
       proc myProc {} {
           variable myvar1
           variable myvar2
           # ...
       }
   }

and now myvar1 and myvar2 are namespace variables. In general, you should use [global] only when you have a particular reason to make a variable global -- such as making it the text variable of a widget. Even then, consider something like:

    label .path.to.label -textvariable [namespace current]::varName

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LV July 7, 2003 In the above discussion, we see variable recommended over global. Is variable better to use than the '::name'' notation as well?

DGP July 7, 2003 For the purpose we are discussing, yes. If you want to keep a body of code free to move from one namespace to another, you should use relative variable names, and make use of [variable] and [namespace which -variable] to set up scope and generate fully qualified names as needed.