Why can't I use `defconst` to define a new rx?

Question

I want to define a new rx to use within another rx definition, like so:

(defconst my-word
    (rx (or word "*")))

(defconst my-regex
    (rx (1+ my-word)))

However, this code returns the following error:

Eager macro-expansion failure: (error "Unknown rx symbol 'my-word'")

The solution, as per rx documentation, is to define your rx with rx-define:

(rx-define my-word
    (or word "*"))

(defconst my-regex
    (rx (1+ my-word)))

I suspect this has something to do with the sequence of how Emacs loads symbols, but I was just curious as to the specifics of this.

Answer 1

They're just two utterly different things.

(defconst my-word (rx (or word "*"))) stores the evaluated value "[[:word:]]\\|\\*" in the value slot of the my-word symbol.

(rx-define my-word (or word "*")) sets the symbol property rx-definition on the symbol my-word with the value ((or word "*"))

(We can surmise that symbols which are meaningful to rx therefore have a rx-definition property, and are not evaluated by rx as variables.)

Answer 2

Try

(defconst my-word
    (rx (or word "*")))

(defconst my-regex
    (rx (1+ (regexp my-word))))

This works since Emacs 27.