[kripke]

There is a balance to find between not enough annotations and too much annotations that one has to find. My algorithm is to write few annotations by default, and my answer to error messages I do not understand is to add "obvious" annotations (e.g. type of arguments) to tell type inference what I expect. Rarely does that solve the underlying issue, but it often makes the error message much clearer.

With experience, I am more or less able to preemptively add the annotations that will probably help (e.g. aiming avoid accidentally polymorphic functions, the return type of mutually recursive functions, etc.).

I don't think I have ever gotten "This expression is of type X but an expression was expected of type X" outside of the toplevel with type re-definitions, and recent OCaml versions have a nice error message for this now:

    Error: This expression has type t/1 but an expression was expected of type
             t/2
           Hint: The type t has been defined multiple times in this toplevel
             session. Some toplevel values still refer to old versions of this
             type. Did you try to redefine them?

In general the error messages have been continuously improving in recent years.