[GPS] Oct 16, 2003 - It occurred to me that I might accidentally set the wrong variable (it has happend in the past), and it could cause me to have a nasty debug session, so I decided to come up with a proc that sets a variable but returns an error if it doesn't exist. 

 proc ?set {v_name val} {
  upvar $v_name v
  if {![info exists v]} {
   return -code error "expected $v_name to exist!"
  }
  set v $val
 } ;# GPS

Here is a log demonstrating usage:
 % ?set foo bar
 expected foo to exist!
 % set foo abc
 abc
 % ?set foo bar
 bar
 % set foo
 bar
 % 

See also [?append]

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