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Union Type-Checking Problems

• Problem with Pascal's design: type checking is ineffective

• Reasons:
  • User can create inconsistent unions (because the tag can be individually assigned)

      var blurb : intreal;
              x : real;
      blurb.tagg := true;  { it is an integer }
      blurb.blint := 47;   { ok }      
      blurb.tagg := false  { it is a real
      x := blurb.blreal;   { oops! }
  

  • Also, the tag is optional!