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.

- having a compile step in between writing and running your code. This drastically slows down the feedback loop of development. The more your compiler has to check for you, the slower it gets.

- being able to run your program in a half-broken state. This may not seem like much of an advantage, but sometimes it is good to just be able to run a broken piece of code to see how it crashes. This is especially important when learning to program, but is still very beneficial when learning a new language or framework, or sometimes for debugging.

- As someone who has contributed to the main Haskell compiler, I can definitely confirm that a more complicated type system can slow down development of the language itself too. It takes significantly more effort to grok all the possible interactions as the codebase of the compiler grows.

Don't get me wrong, I think encoding and enforcing program properties with types is a great idea that will grow further in the future. But the dynamic languages gained popularity for good reasons, and some of those reasons are still valid today.