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On[symbol::tag]

switches on a message, so that it can be printed.

On["name"]

switches on a named group of messages.

On[s]

switches on tracing for the symbol s.

On[m1,m2,…]

switches on several messages or message groups.

Details
Details and Options Details and Options
Examples  
Basic Examples  
Scope  
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On

On[symbol::tag]

switches on a message, so that it can be printed.

On["name"]

switches on a named group of messages.

On[s]

switches on tracing for the symbol s.

On[m1,m2,…]

switches on several messages or message groups.

Details

  • Some messages for built-in functions are off by default, and must explicitly be switched on using On.
  • In On["name"], 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
  • $MessageGroups gives a complete list of message groups recognized by On.
  • On[General::name] switches on all messages derived from General::name.
  • 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[s::trace].

Examples

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Basic Examples  (2)

Turn on a message that is issued whenever a new symbol is created:

Wolfram Language code: On[General::newsym]

Now when a new symbol is created the message is issued:

Wolfram Language code: a

This turns the message back off:

Wolfram Language code: Off[General::newsym]

Messages about unpacking are off by default:

Wolfram Language code: f@@Range[10]

Turn them on:

Wolfram Language code: On["Packing"]
Wolfram Language code: f@@Range[10]

Restore the default behavior:

Wolfram Language code: Off["Packing"]

Scope  (2)

Operations on symbols normally are done silently:

Wolfram Language code: a = 1;a++

Use On to enable tracing of the symbol:

Wolfram Language code: On[a]
Wolfram Language code: a = 1;a++

Restore the default behavior:

Wolfram Language code: Off[a]

Turn on multiple messages:

Wolfram Language code: On[a, b]
Wolfram Language code: a = 1;a++ b = 1;b++

Restore the default behavior:

Wolfram Language code: Off[a, b]
Wolfram Research (1988), On, Wolfram Language function, https://reference.wolfram.com/language/ref/On.html (updated 2010).

Text

Wolfram Research (1988), On, Wolfram Language function, https://reference.wolfram.com/language/ref/On.html (updated 2010).

CMS

Wolfram Language. 1988. "On." Wolfram Language & System Documentation Center. Wolfram Research. Last Modified 2010. https://reference.wolfram.com/language/ref/On.html.

APA

Wolfram Language. (1988). On. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/On.html

BibTeX

@misc{reference.wolfram_2026_on, author="Wolfram Research", title="{On}", year="2010", howpublished="\url{https://reference.wolfram.com/language/ref/On.html}", note=[Accessed: 12-June-2026]}

BibLaTeX

@online{reference.wolfram_2026_on, organization={Wolfram Research}, title={On}, year={2010}, url={https://reference.wolfram.com/language/ref/On.html}, note=[Accessed: 12-June-2026]}