Question (short version): Should a function, which requires numerical arguments, validate its inputs? If so, how?

Question (long version): Assuming you write a function, which takes a numeric argument. Should this function validate its input and warn/fail if a non-numeric value is provided?

The built-in functions do not do this - they rely on implicit numeric conversion. For instance, sin("foo") evaluates to 0 (=sin(0)), and cos("3.1415bar") evaluates to -0.99999 (ie to -1 =cos(pi)).

This may be ok for functions that return immediately, and without side effects. But what about an object, that collects values, to perform some calculation later:

for( @data ) {
  $s->add_data( $_ );
}
print $s->average();

Would it not be useful to be warned early (ie. as the data is added) if garbage is passed in?

The argument can be put forward that implicit typ conversion by the function is a good thing, and that it is up to the calling code to perform any validation, if desired. However, what makes this question tricky is that there is a very similar problem, where input validation must occur, namely if the function is only defined for a certain parameter range. For example, any implementation of the logarithm must validate that its input is strictly greater than 0. But this seems inconsistent: we validate that the input is legal, but after relying on implicit type conversion. Would it not be better if both forms of validation (input is formally correct as "number") and is mathematically correct (within the range of validity) are performed together? In particular for an accumulating object, similar to the code snippet above?

I am really quite uncertain - I can find reasons to go with either version. I have not found any discussion/recommendation on this point elsewhere. What do others think?

(The strongest argument I can think of is to "do as the built-ins do", that is: convert implicitly, and only then validate the parameter range if necessary. Is this the general consensus?)

In reply to Parameter validation for numeric function arguments? by janert

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