Question

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Best practices in error reporting (Mathematica)

Assume that I have a function

func[xNumber_,str_,listOfReals_]:=Module[{},
....
]

and I want to test that the user gave input expressions of the correct type. I know I can do something like xNumber_?NumberQ but then I don’t seem to find how to generate some informative error message. Therefore, currently, in the the module’s code I inserts If‘s where I test the arguments’ types and in case test fails I print some message and return False.

What should be considered smarter/more canonical/better practices to achieve this?

Answer 1

Built-in function Message is what you need. The following example is from the documentation.

First define a function that issues an error message and returns unevaluated upon error

 rsqrt[x_] /; If[TrueQ[x >= 0], True, Message[rsqrt::nnarg, x]; False] := Sqrt[x]

Next, define the message

 rsqrt::nnarg = "The argument `1` is not greater than or equal to zero.";

With correct input the function evaluates as defined

 rsqrt[2.25]

returns 1.5. But, with incorrect input, you get

 In[4]:= rsqrt[-2.25]

 During evaluation of In[4]:= rsqrt::nnarg: The argument -2.25 is not greater than or equal to

 zero.

 Out[4]= rsqrt[-2.25]