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On

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On
switches on a message, so that it can be printed.
On
switches on a named group of messages.
On[s]
switches on tracing for the symbol s.
On
switches on several messages or message groups.
  • Some messages for built-in functions are off by default, and must explicitly be switched on using On.
  • In On, possible named message groups include:
"Compiler"warnings associated with compiled code
"Graphics"warnings associated with graphics generation
"Packing"notifications associated with packed arrays
"Spelling"spell-checking messages for symbol names
"Symbolics"warnings associated with symbolic computation
  • 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[Assert] switches on messages associated with assertions specified by Assert.
  • On[s] is equivalent to On.
  • On is equivalent to On for all symbols.
Turn on a message that is issued whenever a new symbol is created:
Now when a new symbol is created the message is issued:
This turns the message back off:
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
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