Well, the post shows that the given function is a functor. However, I might argue that calling the function a "parser" is a bit of a stretch. In a toy, abstrac

Well, the post shows that the given function is a functor. However, I might argue that calling the function a "parser" is a bit of a stretch. In a toy, abstract sense, sure, and maybe there's a formal definition of "parser" that it conforms to, but just because it's named a "parser" doesn't really mean that's all a "parser" is when we talk about it in other contexts. Parsers, to me, in a professional software engineering sense, need to have error handling, performance and memory guarantees, the ability to do "best case" parsing and return a number of errors all at once, and so on. So one interpretation of "No, a parser is not a functor", despite the proof here, is "I reject that the toy example here being called a parser is sufficiently close to what I would call a 'parser', and what a 'parser' is to me is not a functor [because of these other things not captured by the example]". I tend to agree with that. Haskellers don't have an exclusive claim to terminology, and I don't think "parser" is defined formally enough that we all must agree on it.