Saying that e.g. Ruby is "vacuously type safe" strikes me as fairly misleading. Consider an extension to Ruby which defines the ++ operator as casting its argum

Saying that e.g. Ruby is "vacuously type safe" strikes me as fairly misleading. Consider an extension to Ruby which defines the ++ operator as casting its argument as an integer (e.g. Fixnum) and incrementing it. "Properly" defined, this operator would break type safety in a way that __can not__ currently be done in Ruby (i.e. modifying a string from "abcdefgh" to "bbcedefgh", "abcdefgi", or some other nonsense). This is clearly a type error, and it is one that Ruby does not allow, so describing Ruby's type _safety_ as vacuous strikes me as wrong. Additionally, Ruby prevents e.g. calling on an undefined method on an object. Given the way the interpreter works this comes trivially, but that does not mean that it does not affect type safety. To summarize: there are certain classes of type errors related to e.g. casting which would be dumb to allow in a (non-toy) dynamically typed language and others which come with no particular cost. This does not mean the type safety of the language is "vacuous" (though it may negate the need for an explicit "type checker").