I'm not sure about this. I get what he's saying and agree that singleton methods are nothing but a naming convention for the more fundamental/atomic construct called instance methods (which indeed are the only kind of method that exist in Ruby, depending how you look at it), but I think I would actually say that singleton methods are language concepts because those methods like Object#define_singleton_method, ... are always available in Ruby (without needing to require a standard library first, for example). In other words, I would argue that something belonging in the Ruby core "library" (?) by definition makes it part of the language -- even if it in turn builds on even lower-level Ruby language features/constructs.

Wow, that's a lot of different ways.

The inline_form way in particular seems interesting to me, though it's worth noting that that method is just an example, not actually part of this project's code, so it's not really a first-class option like the other options.

1. Ironic that this reference is ostensibly about the usage of "first-class citizen" in society, yet it links to a seemingly-mismatched (by name only, that is) article, entitled "second-class citizen".

2. Ironic that the first-class (unqualified) article is about the figurative meaning of "citizen" used in computer science, and that the page describing first-class and second-class status of the more literal citizens in society is relegated to what I kind of think is a second-class position in the encyclopedia (because it takes the #2 position numerically, even though it is (at least as is implied in this reference) also about first-class citizens (though the word "first-class" does not appear a single time in that article, so maybe this reference is the one that is more ironic/incorrect).