What I mean is that it is objectively difficult to know that you understand A. How do you know that understanding B wouldn't give you a greater understanding of A?

He didn't claim that category theory couldn't possibly give him a greater understanding of functional programming.

If he'd claimed "I understand CT and it didn't help my understanding of FP in the slightest", I'd have no qualms. The problem I have is, what he's saying is essentially "I don't understand CT, but I'm having no problems with FP without it".

Here's an analogy. There's an apprentice carpenter who has been given some tools and as far as he is concerned the tools he has been given are all of the tools in existence. It's quite clear that if he was given a new tool, he could immediately establish whether it was useful or not. He could not establish, however, that he has access to every useful tool.

From his account, I don't know whether CT is like a hammer that he is using upside-down (needs explanation before it's useful) or a banana (never going to be useful to construct anything).

>what he's saying is essentially "I don't understand CT, but I'm having no problems with FP without it."

Saying that understanding category theory can give you a greater understanding of functional programming doesn't imply that a lack of understanding of category will cause problems in the understanding of functional programming.

Our difference is that I do not see anything in the thread parent that contradicts the article, and I don't think he's trying to convince you that CT is either a hammer or a banana, just that professional carpentry work is possible without it.

Then a greater understanding of FP is possible through understanding of CT.

That does not follow from my comment, which takes no stance, but is the thesis of the article.

You could also have some anonymous understanding of B. This reminds me of Richard Feynman's story about his dad and the bird.