http://www.purl.org/tcl/home/man/tcl8.4/TclCmd/trace.htm 
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What are some of the uses for trace?  Well, see [Traces] for some wiki info.

   * Communication between parts of a GUI and the internal state of the app. (Simplified MVC, observer). In general communication between different parts of an app without coupling them strongly.

   * Simple constraint computation for a number of variables ("if this flag is and that one, then no other is allwed to set", and some such).
   * [Canvas] text items bound to a variable, dynamically updating

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''A small example to illustrate the order of processing -- [JCW]''

  proc tellme {id a e op} {
    puts "  $id a=$a e=$e op=$op\
                ax=[info exists ::$a] ex=[info exists ::${a}($e)]"
  }

  proc do {args} {
    puts "<$args>"
    uplevel $args
  }

  trace var a wu {tellme array}
  trace var a(1) wu {tellme element}

  puts [trace vinfo a]
  puts [trace vinfo a(1)]

  do set a(0) zero
  do set a(1) one
  do set a(2) two
  do unset a(0)
  do unset a(2)
  do unset a

  # output is:
  #
  #   {wu {tellme array}}
  #   {wu {tellme element}}
  #   <set a(0) zero>
  #     array a=a e=0 op=w ax=1 ex=1
  #   <set a(1) one>
  #     array a=a e=1 op=w ax=1 ex=1
  #     element a=a e=1 op=w ax=1 ex=1
  #   <set a(2) two>
  #     array a=a e=2 op=w ax=1 ex=1
  #   <unset a(0)>
  #     array a=a e=0 op=u ax=1 ex=0
  #   <unset a(2)>
  #     array a=a e=2 op=u ax=1 ex=0
  #   <unset a>
  #     array a=a e= op=u ax=0 ex=0
  #     element a=a e=1 op=u ax=0 ex=0

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On news:comp.lang.tcl, [CLN] answers [Erik Leunissen]'s question:
Erik Leunissen wrote:
 > The following passage from the man page to the trace command is
 > mystifying to me:
 > 
 > "If an unset occurs because of a procedure return, then the trace will
 > be invoked in the variable context of the procedure being returned to:
 > the stack frame of the returning procedure will no longer exist."
 > 
 > I've read it again and again; I can't imagine how a procedure return
 > affects/causes an unset.
 > ...


    proc foo { } { 
       set x 1
       trace variable u x {some code here}
    }

When foo is invoked, x is created (locally), has a trace associated with
it, then becomes unset as foo exits.
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