This is documentation for Mathematica 6, which was
based on an earlier version of the Wolfram Language.
View current documentation (Version 11.2)

On

On[symbol::tag]
switches on a message, so that it can be printed.
On[s]
switches on tracing for the symbol s.
On[m1, m2, ...]
switches on several messages.
On[]
switches on tracing for all symbols.
  • Some messages for built-in functions are off by default, and must explicitly be switched on using On.
  • When tracing is switched on, each evaluation of a symbol, on its own or as a function, is printed, together with the result.
  • Note that the tracing information is printed when a function returns. As a result, traces of recursive functions appear in the opposite order from their calls.
  • On[s] is equivalent to On[s::trace].
  • On[] is equivalent to On[s::trace] for all symbols.
Turn on a message that is issued whenever a new symbol is created:
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Click for copyable input
Now when a new symbol is created the message is issued:
In[2]:=
Click for copyable input
Out[2]=
This turns the message back off:
In[3]:=
Click for copyable input
New in 1