Yeah, but "noun" is not a function, it is a type. It's probably hard to communicate this way because those are not formal names in English it would seem, but they are in mine, and the material is pretty strict about the definition.
For example: the test in "Let's test" is a verb by its type, and a predicate by its function. The test in "I took a test" is a noun by its type, and a direct object by its function. We look at them as completely different words that just happen to have the same form - in English, they're different in my language - which is probably the reason why this difference…