isNegativeNumberScalarOrNull(-2)
# returns TRUE (argument is valid)
isNegativeNumberScalarOrNull("X")
# returns FALSE (argument is invalid)
#isNegativeNumberScalarOrNull("X", stopIfNot = TRUE)
# throws exception with message defined by message and argumentName parameters
isNegativeNumberScalarOrNull(-2, default = -1)
# returns -2 (the argument, rather than the default, since it is not NULL)
#isNegativeNumberScalarOrNull("X", default = -1)
# throws exception with message defined by message and argumentName parameters
isNegativeNumberScalarOrNull(NULL, default = -1)
# returns -1 (the default, rather than the argument, since it is NULL)
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