I agree that'd be better, but that ship has long since sailed. I for one specifically don't wish for that because a breaking change like that would create a new python 2/3 situation.

Doesn't fit with the Typescript philosophy though: It should be possible to gradually add Typecript annotations to existing JavaScript code. It wouldn't work with 3rd party libraries either.

I agree that it’s not practical. Ship has sailed a long time ago.

But one solution might be a “noImplicitImpure” option.

Still wouldn't work, since there is no way for the compiler to distinguish between a pure function and an impure function which just don't have an annotation - e.g. a third-party library written before "impure" was introduced to the language.

If pure functions were explicitly marked as "pure" the compiler could just conservatively assume all other functions were impure. But the opposite does not work.

Oh. You’re right. The moment you turn that flag on, it can’t complain the away implicitAny can. Therefore you need “pure” rather than “impure.”