>It's like code expecting rectangles wouldn't mind to get squares, but a code expecting squares will definitely do mind getting rectangles.

It is actually not that obvious since in OOP neither a square or a rectangle type is properly a subtype of the other. [0]

I understand how implementation-wise it could be convenient to treat a function as a closure. Although, I would prefer some other type, eg:

    functoid := closure | function

[0] - https://stackoverflow.com/questions/1030521/is-deriving-squa...