[BoppreH]

I've never used OCaml, so I'm curious to what exactly happens, and if language design can prevent that.

If I download a random project and delete the interface files, will that be enough to see issues, or is it something that happens when writing new code?

[nu11ptr]

If you delete your interface files and then change the type used when calling a function it can cascade through your program and change the type of the function parameter. For this reason, I generally feel function level explicit types are a fair compromise. However, making that convention instead of required (so as to allow fast prototyping) is probably fine.

[iopq]

Just require it for public functions. Your own code can be as messy as you want unser the hood