[SkyMarshal]

Being able to see a function's entire type signature at a glance is the single most useful way of documenting code I've ever come across. No alt-tabbing to docs or relying on an IDE to pop up hints, etc., it's just right there.

Haskell is the ideal here, since it separates the type signature into its own separate line at the top of the function declaration. That confers readability and reduces cognitive overhead more than any other language I've used.

It also changes the programming thought process. You can pseudo-code an entire program just with function type definitions. Then test that the type defs compile without error, and go back and implement the function definitions. As long as the function definitions adhere to their type signature, the program almost always works (barring some I/O errors).

Not all statically typed languages are created equal. The ones fundamentally oriented around function type signatures, rather than variable types, are the ones I think parent had in mind in his comment.