Extensible Data Types

• Parser assigns no meaning to arguments (quoting by default, evaluation is special):

  C: x = 4; y = x+10
  
  y is 14
  Tcl: set x 4; set y x+10
  y is "x+10"

• Different commands assign different meanings to their arguments:

  set a 122
  
  expr 24/3.2
  eval "set a 122"
  button .b -text Hello -fg red
  string length Abracadabra

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