If your language is wholly type inferred, and the type system is expressive enough to deal with whatever one wishes to write, it is indeed "free". If you ignore

If your language is wholly type inferred, and the type system is expressive enough to deal with whatever one wishes to write, it is indeed "free". If you ignore compile time performance, run time performance, and that you're writing in something between Idris and vapourware. Otherwise the cost is merely being unable to write anything that the type checker does not understand, and however long your compiler takes to do the checks on what it does understand. Also a fair chance the whole program must type check before you can see the results of changing a subset. In the worst case you get to hunt down all the unused variables before it'll run the test suite. I prefer dynamic languages. That makes me a heathen on these boards, to be ignored or chastised for my stupidity. Regardless, those benefits do not come for free.