[zahlman]

> it is simply more enjoyable to design the types in your program than to write unit tests.

I have tried both and I have no idea what you're talking about.

> Making yourself think about “for all x” rather than a concrete x forces your brain to consider deeply the properties of x being used.

The entire point of dynamic typing is that you can think about interfaces rather than concrete types, which entails deep consideration of the properties of the object (semantics of the provided interface).

[array_key_first]

That's not the entire point of dynamic typing, because all the interface stuff comes from statically typed languages. Some* dynamic languages borrowed it, but most use "implicit" interfaces - where the interface is whatever kind of works, I guess.

[zahlman]

> because all the interface stuff comes from statically typed languages.

No, it doesn't. It comes from theory that came after the languages.

> Some* dynamic languages borrowed it, but most use "implicit" interfaces

An implicit interface is an interface, and is exactly the sort of thing I'm talking about in GP. The point is that you think about the object in terms of its capabilities, rather than some proven-up-front categorization that it fits into. What it does, not what it is.

[maleldil]

You can achieve this with structural subtyping, such as Go interfaces and Python protocols. Whether that is desirable is a different question.