Some languages (like Lisp family, Smalltalk, Lua)
contains an interesting feature called ''closures''. Closure represents a block of code with surrounding context, whose execution is deferred.
A block can be executed later (probably more then once) with old context and newly passed arguments.

Closures often used to provide customized behaviour for some algorithm.

Tcl has no direct support for closures, but they can emulated using ''eval'' mechanism, in particular, commands [uplevel] and [upvar].

First example. The following command accept a block of code and executes it in specified file catalog.

 proc temp-chdir {dir block} {
 	set cd [pwd]
 	cd $dir
 	uplevel $block
 	cd $cd
 }

 # sample usage:
 temp-chdir / {puts "I'm in [pwd]"}

With upvar and uplevel it is simple to write custom iterators over some structure, like this:

 proc foreach-pocket {pw block} {
 	upvar 1 $pw pwvar
 	foreach p [get-container-pockets] {
 		set pwvar $p
 		uplevel 1 $block
 	}
 }

 # sample usage
 foreach-pocket p {
  puts "Processing $p"
 }

[mfi]

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Another example of such style of code can be found on
[How do I read and write files in Tcl] page.