WOLFRAM

Wolfram Language & System Documentation Center

Add Messages to Functions

You can set your own custom functions to issue messages where appropriate.

    
Define a function to issue a message and return unevaluated upon an error, which in this case occurs when the argument is not greater than or equal to zero:
Wolfram Language code: rsqrt[x_] /; If[TrueQ[x ≥ 0], True, Message[rsqrt::nnarg, x];False] := Sqrt[x]
Define the message:
Wolfram Language code: rsqrt::nnarg = "The argument `1` is not greater than or equal to zero.";
Wolfram Language code: rsqrt[2.25]
Wolfram Language code: rsqrt[-2.25]