Function parameters in SmileBASIC are not type-checked - a parameter can have any type, regardless of the variable name, depending only on what value was passed to the function.
If you want to treat different types of parameters differently, it is useful to be able to identify the type of a parameter. Currently there is no known way to identify arrays, but it is possible to differentiate between real number type, integer type, and string type. Consider the following function:
'Returns 0 for real, 1 for integer, 2 for string
DEF TYPEOF(X)
IF X*0 && X*0==X*0 THEN RETURN 2
RETURN X*0-&H80000000<0
END
So, how does this work? First, let's consider differentiating strings from numbers. The critical point of this is that all strings are considered to be True in IF statements or the && and || operators. So if you multiply a string by 0, you get the empty string, which is considered True. On the other hand, if you multiply a number by 0, usually you get 0, which is False.
I say "usually" because there are some special cases as far as real numbers are concerned. Multiplying Infinity or NaN (Not a Number) by 0 will produce NaN, which is considered non-zero, or True. So if we check only X*0, then strings, Infinity, and NaN will be detected. The key to solving this problem is that NaN is considered to be not equal to itself, while on the other hand the empty string is always equal to itself. So X*0==X*0 out of these possible values will be True only if X is a string.
After strings are ruled out, it is fairly simple to differentiate between reals and integers. We will detect an instance of integer overflow - calculating (0 - (-2^31)) should return 2^31, but if we are working with integers, that overflows and becomes negative, so we return True or 1. On the other hand, if X is real it will return 2^31 as expected, so we return False or 0. Note also that if X*0 is our good friend NaN, comparing NaN<0 will also return False. All possible cases are handled.
