In Tcl, every statement is a command - with the parameters separated by spaces. Elements which are written in square brackets are commands which are evaluated before the rest of the statement, and elements which are written in curly braces are deferred - held back - and are only evaluated (if at all) after the statement itself is has been initiated.

Let's see what that means for a while command - full source [here]:

  while {$hunger > 0} {
The test (deferred block) is passed into the while command and will be evaluated each time the condition needs to be checked as the program loops. An expr is assumed by while, so there's no need for you to add one yourself. You could have written
  while {[expr $hunger > 0]} {
and that would function in the same way.

However - if you use square brackets in the condition of a while command, without deferring them, the condition will be evaluated once only before the loop block is evaluated for the first time. And if that evaluation is true, you'll then have a block which repeats forever - an infinite loop - which you'll have to break out of with a break command, or a return:
  while [expr $hunger > 0] {

Once I've explained this to you, it's probably quite clear - but the code in that last example looks confusing and I would advise against using the structure for a while loop. If you're writing a conditional such as an if, you may use square brackets, since the condition is only going to be evaluated once anyway.

Example written on yesterday's Tcl course.

(written 2011-03-02, updated 2011-03-04)

 T203 - Tcl/Tk - Conditionals and Loops
  [210] Joining lists in Tcl. Indirect variables in Tcl. - (2005-02-12)
  [1401] Tcl - using [] or {} for conditions in an if (and while) - (2007-10-23)
  [1477] Decisions - small ones, or big ones? - (2007-12-18)
  [1696] Saying NOT in Perl, PHP, Python, Lua ... - (2008-07-04)
  [2261] Tcl - nice and nasty - (2009-06-29)
  [2471] A short form of if ... then ... else - (2009-10-23)
  [2681] Tcl - a great engineering language - (2010-03-17)
  [3397] Does a for loop evaluate its end condition once, or on every iteration? - (2011-08-18)
  [3570] Trapping errors in Tcl - the safety net that catch provides - (2012-01-06)
  [3571] Comparing loop commands in Tcl - (2012-01-06)
  [4322] Learning to Program - the conditional statement (if) - (2014-11-21)
  [4323] Learning to program - Loop statements such as while - (2014-11-22)
  [4455] Working out distance between places, using OS grid references and a program in Tcl - (2015-03-11)

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