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based on an earlier version of the Wolfram Language.
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MessageName (::)

symbol::tag
is a name for a message.
  • You can specify messages by defining values for symbol::tag.
  • symbol::tag is converted to MessageName[symbol, "tag"]. tag can contain any characters that can appear in symbol names. symbol::"tag" can also be used.
  • Assignments for s::tag are stored in the Messages value of the symbol s.
  • The f::usage message is typically defined for functions intended for general use, giving a description of how to use the function.
  • ?f prints out the message f::usage.
  • When ?form finds more than one function, only the names of the functions are printed.
  • You can switch on and off messages using On[s::tag] and Off[s::tag].
  • MessageName[symbol, "tag", "lang"] or symbol::tag::lang represents a message in a particular language.
Define a usage message for a function:
Show the usage message:
Process messages generated during an evaluation:
Define a usage message for a function:
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Show the usage message:
 
Process messages generated during an evaluation:
In[1]:=
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Out[1]=
In[2]:=
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Out[2]=
In[3]:=
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Out[3]=
Define a function that will test another function and give the argx message if needed:
Define g to use the test and return unevaluated when it fails:
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