One of the really neat things about mojo is, as a superset of python, things like static typing, and ownership/moving can be gradually adopted. It encourages value semantics, and is borrow-by-default. So you can literally just write pythonic looking `def` functions, but then gradually opt in to the more performant features, e.g. with `fn` functions.

In my mind this is kinda similar to what TypeScript does as a super-set of JavaScript- except that was not about performance, it was about purely about typing.