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Phantom Error Messages in v9

• To: mathgroup at smc.vnet.net
• Subject: [mg128978] Phantom Error Messages in v9
• From: James Stein <mathgroup at stein.org>
• Date: Thu, 6 Dec 2012 05:00:53 -0500 (EST)
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• Delivered-to: l-mathgroup@wolfram.com
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*Mathematica* v9 can produce error messages during the evaluation of the *
next* line to be evaluated!

Yes, that statement seems nonsensical. Read on:

(*Mathematica* v9.0.0.0 running on MacBook Pro with OS X 10.6.8)

I have placed a notebook with instructions and code to demonstrate  these
errors at https://www.dropbox.com/s/3w5e1fb834av5wp/RETURN%20BUG.nb

otherwise you may observe them from scratch with these instructions:

Enter the following four lines into a new cell:

   Clear[sum2];

   sum2::usage="f[*a,b*] returns *a+b*";

   sum2[a_,b_]:=Module[{},a+b];

   sum2[1,2]

The use of some italics in line 2 is important!


1. Evaluate that cell.

     the output value, 3, appears;

     note the number of the Output Cell;

     if this is the first input cell to be evaluated in your session,

     then the output cell withh be Out[4].

2. Double click on "sum2".

3. Press the forward arrow key,

4. Seven error messages now appear! These messages, in the
*Messages*window, are:

StringMatchQ::strse: String or list of strings expected

     at position 1 in StringMatchQ[$Failed,sum2*]. >>

StringMatchQ::strse: String or list of strings expected

    at position 1 in StringMatchQ[$Failed,sum2*]. >>

StringMatchQ::strse: String or list of strings expected

    at position 1 in StringMatchQ[$Failed,sum2*]. >>

General::stop: Further output of StringMatchQ::strse

    will be suppressed during this calculation. >>

(Then, after a slight pause without user doing anything):

The String "f[

The string "FontSlant->"Italic"]

The string "FontSlant->"Italic"]


You will notice that the first four messages occurred "During evaluation of
In[5], despite the fact that there is not yet any Input cell with that
number (which you can confirm by clicking in some empty place in the
notebook, entering a simple expression (such as 1+1) and evaluating).