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