> I think that I don't understand the definition. Isn't encountering a runtime error precisely getting 'stuck' (in the "opposite-of-progress" sense)? Sorry, pe

> I think that I don't understand the definition. Isn't encountering a runtime error precisely getting 'stuck' (in the "opposite-of-progress" sense)? Sorry, perhaps I should have said "exception" rather than "error". Raising an exception is absolutely "safe"/progress-obeying, as long as your language specifies the behavior of exceptions. That the exception happens to be called a "TypeError" doesn't change anything. Getting "stuck" means being in a state such that there's no defined next state - a state which the language spec doesn't cover. So most "dynamically-typed" languages are in fact safe in progress-and-preservation terms, because they have well-defined behavior in all cases. C, otoh, is "statically-typed" but unsafe, because you can write C code with undefined behavior. (That dynamically typed languages can be (statically) typesafe should be unsurprising, since they're just statically-unityped languages. Nothing says a unityped language can't be typesafe, it's just a very uninteresting property!)