I've watched Marcel's efforts with admiration for his commitment for a long time.
I looked at this and browsed some of the examples, and something clicked for me finally. I did a good solid 20 years of dedicated Smalltalk. When I started doing Objective-C, I was glad I could send messages, but noticed a number of differences. One of the things that you don't see just comparing syntax between Smalltalk and Objective-C is some of the deeper differences. Objective-S, like Objective-C, has type annotations. In ObjectiveC, there is a preference for void return types. Smalltalk, has no annotations, but all functions methods return. If you don't specifically return something from a method, self is the implicit return type. For any closure, it's always the last value of the expressions. It made cascading messages really easy, because methods tended to return the thing you wanted most of the time. When I started doing Objective-C, I commented on this lack of object return. I don't recall what the answers were, but it was clear that ship had sailed. :)
Since then, as I've sailed with Swift and Kotlin and Python, it's clear that that is the dominant pattern. Python, which doesn't have types, still returns None when nothing is specified. And then last year, I did some work on Erlang/Elixir. Like Smalltalk, all functions return something. You can return nil if you want, but you have to make the effort to do so. Otherwise, you just get the result of the last expression. And I loved it. It makes it so you can compose functional chains really easy and idiomatic.
It's led me to wonder why any language that wants to be more "functional" has a Void return type. And once again impressed that Smalltalk, for all of its OOness, is really quite functional in some ways. Functional programming, is of course, many things. But one of them is obviously the good old f(x) -> y thing we learn in high school. Functions take input, and return output. A function that returns nothing, isn't that useful. But we also have computerese "functions" that are more "subroutines". An imperative sequence of steps designed to produce a certain side effect. When one makes a Void function, it seems one is essentially saying "not interested in functional here, just want side effects." I'm not as opposed to side effect programming as some, but I guess I prefer the approach like Smalltalk and Elixir which strongly encourages returns of some value (even if ignored) so that one can be simultaneously functional and side-affecty.
I'm disappointed that Objective-S didn't embrace the no-such-thing-as-a-void-return philosophy of Smalltalk, even while lifting so much from Smalltalk.