"nan" v. try-catch for arithmetic

Graham Samuel livfoss at mac.com
Tue Jun 24 14:02:56 EDT 2014


I have a bit of code that looks for input errors where the user is supposed to write an arithmetic expression into a field. The relevant fragment looks like this

  put value(fld "myField") into testResult
  answer "oops, that's not a valid real number. Try again"
  exit mouseUp
 end try

Now I input a legitimate expression like 3+sin(pi/2)

 the catch doesn't fire, as expected. If I put in something which isn't a well-formed expression, like

 ardvaark + 92

it does fire, as expected. If I put


that fires too. But if I put


I get a result, "nan", and the catch doesn't fire. Why not? Maybe someone can explain what the engine is doing here - or is it just an obscure bug?



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