One very interesting feature of Tcl is that you can write your own control structures, essentially extending the language.
The ''uplevel'' command, which allows you to write a procedure
which executes code in the context of the caller, is the trick.

For example, if we want to augment Tcl's standard ''for''
and ''while'' looping constructs with

  do body while condition

we would simply write a ''do'' proc which executes the code in the
caller's context.  In its simplest form, it would look like this:

  proc do {body condition} {
      while {1} {
          uplevel $body
          if {![uplevel "expr $condition"]} {break}
      }
  }

We can make this a little more robust by specifying the exact number
of levels to ''uplevel'', which may prevent 
''$body'' from being misinterpreted.  We should also use
''list'' to construct the conditional command.

  proc do {body condition} {
      while {1} {
          uplevel 1 $body
          if {![uplevel 1 [list expr $condition]]} {break}
      }
  }

Just to make things look prettier, we can add an exra argument to
''do'' that expects the word ''while''.  You could probably get 
fancy and make the ''while'' optional 
(like the ''else'' is optional for the
''if'' command), but this should be good enough for our purposes.

  proc do {body whileword condition} {
      if {![string equal $whileword while]} {
          error "should be \"do body while condition\""
      }
      while {1} {
          uplevel 1 $body
          if {![uplevel 1 [list expr $condition]]} {break}
      }
  }

This procedure does not handle errors well, though.  For a good discussion of the issues involved, take a look at the exception handling chapter in John Ousterhout's book 
[BOOK Tcl and the Tk Toolkit].
But the primary issue is that we want to ''catch'' exceptional
conditions when executing the ''$body'' code, and correctly
throw errors upwards.  This error handling makes our new
procedure really appear to be part of the language.

Recall that the error codes are

   * 0 OK
   * 1 error (should be thrown upwards)
   * 2 return (throw that upwards, too)
   * 3 break {stop execution of our loop)
   * 4 continue {just continue)
   * anything else is a user defined code.

  proc do {body whileword condition} {
      global errorInfor errorCode
      if {![string equal $whileword while]} {
          error "should be \"do body while condition\""
      }
      while {1} {
          set code [catch {uplevel 1 $body} message]
          switch -- $code {
            1 { return  -code      error \
                        -errorinfo $errorInfo \
                        -errorcode $errorCode $message }
            2 { return -code return $message }
            3 { return {} }
            4 { }
            default { return -code $code $message }
          }
          if {![uplevel 1 [list expr $condition]]} {break}
      }
  }

To make this really really really robust, you should consider
adding the same error handling to the ''uplevel condition'',
but that is left as an exercise for the reader.

RWT (with a lot of help from comp.lang.tcl) Feb 4, 2000
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Some pages with various new control structures are:
   * [return]
   * [returneval]
   * [try ... finally ...]
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[Category Concept]