Ruby inherited this from Scheme (and I prefer empty? over isempty or emptyp or variations thereof). They return truthy or falsy values and not true or false (the is* macros/functions in C only guarantee zero and nonzero as return value either).

>AFAIK the general practice is to use ! for methods that perform a mutation in-place The ! suffix is rather loosely defined as it just means the user should be cautious/the method might have "dangerous"/surprising behavior.

Your description is usually correct but only when a non-suffixed version exists and not even then the behavior has to match your description. There are also a lot unsuffixed mutating methods. See also Kernel#exit vs. Kernel#exit! (immediate exist/exit handlers aren't called) or Tempfile#close vs. Tempfile#close! (close and unlink).