To some extent, this is rather a general problem with quality of code in dynamically typed languages. Public functions should return predictable results. This i

To some extent, this is rather a general problem with quality of code in dynamically typed languages. Public functions should return predictable results. This is a matter of testing. In my experience, packages on CRAN are well tested. With respect to dealing with return values, you can circumvent some pain points by using identical(), isTRUE(), isFALSE() in if conditions instead of, e.g., `==` which many people use because this is what they know from other languages. The assertive package is also nice.