[George Peter Staplin]: July 10, 2004 - It sometimes amazes me how beautiful some aspects of Tcl are.  I've been using proc unknown args {expr $args} for a while for doing calculations and tests when designing software.  Today it occured to me that I could make life easier and avoid so many parentheses by extending that simple unknown procedure to this:

 proc unknown args {set ::that [expr $args]}

Now I can do:

 $ echo "proc unknown args {set ::that [expr \$args]}" > .tclshrc
 $ tclsh8.4 
 % 1000 * 256
 256000
 % $that / 8
 32000

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[Scott Nichols] Really cool. Would there be any issues adding your calculator proc to existing Tcl applications? This would be helpful and save typing time for programmers.

[George Peter Staplin]: Error handling and auto-loading/lazy-loading might not work the same.  You could do it this way: 

 rename unknown _unknown
 proc unknown args {
  global that
  if {[catch {expr $args} that]} {
   uplevel 1 [concat _unknown $args]
  }
 }

[MG] That introduces some bugs for me on Win XP; when you enter a command that can't be parsed by [expr], the ''$that'' variable is overwritten with an error message. Also, math commands handled by unknown no longer return the answer. Changing it like this works for me. . .
  rename unknown _unknown
  proc unknown {args} {
  global that
  if { [catch {expr $args} temp] } {
       uplevel 1 [concat _unknown $args]
     } else {
       set that $temp
     }
  }

[George Peter Staplin]: That is better! :)
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Should the ''that'' variable move into a namespace to protect it from
accidental overwrites?
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[GN] what was the idea of "that"? A slightly improved version:
  rename unknown _unknown
  proc unknown {args} {
    if {! [catch {set result [expr $args]}] } {
      return $result
    }
    uplevel 1 [concat _unknown $args]
  }

[MG] - GPS explained it up the top; it's so you can continue calculations. For instance, and very basically,

 % 1 + 2
 3
 % $that * 2
 6
 

Also, in yours, the 'set result ..' isn't necessary, because of the way [catch] works, that line could've been
 if { ![catch {expr $args} result] } {
with the call to [catch] automatically setting $result for you, with the return value of the [expr] call.
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[RS] Here's a simple one-liner macro for zsh (on bash it didn't work) to start a short-lived tclsh as calculator:
 $ tc () { echo puts '[expr' $@ ']' | tclsh }
Testing:
 $ tc acos(-1)*2              
 6.28318530718
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[Category Mathematics]