You are only describing a small part. Here's another example. You are using a library and call a function that expects a List[Foo] as argument. Now you check i

You are only describing a small part. Here's another example. You are using a library and call a function that expects a List[Foo] as argument. Now you check in the same library if there is a function that returns a List[Foo] or at least a Foo. So you find it and are good. Without types, now you have to read the documentation which is much much slower than having your IDE help you out. And yeah, sometimes variable names and function names are sufficient, but in my experience they are very often not. Also, what happens when you update your library to the next version and it changed what it expects and returns? Now you either have a typechecker that checks _all_ the code, even the one you haven't written... or you have to do it yourself. For each version. And each library.