[vidarh]

That's true, but it can also add unnecessary constraints if done thoughtlessly.

E.g. if you require an input to be StringIO, instead of requiring an object that responds to "read".

Too often I see people add typing (be it with a project like this, or with is_a? or respond_to?) that makes assumptions about how the caller will want to use it, rather than state actual requirements.

That is why I prefer projects to be very deliberate and cautious about how they use types, and keep it to a minimum.