The great thing about languages that implement HM types is that they allow you work as quickly or as carefully as you'd like without needing to change languages.

If you've done something a million times before and are familiar with how it works, you can leave off all the type hints, and it'll figure them out for you and stay out of your way so that you can work more quickly. If you make a mistake somewhere, the compiler will still tell you.

If you're doing something new, or don't quite understand something, you're free to add type hints wherever you need them to gain clarity. You can even do stuff like:

    let var: () = ...;

if you have no idea what type something is and just want to ask the compiler for help.

Once you're done, you can tidy up if you think it makes your code more readable. You can still leave top level hints to be kind to your coworkers or future you.

I think the ability to dynamically shift between "cautious exploratory mode" and "reckless I-know-what-I'm-doing mode" at the drop of a hat is why you hear a lot of anecdotes about people using languages like OCaml or Rust as if they were scripting languages.