(define-syntax symbol??
(syntax-rules ()
;; The check is done by first pattern matching against some other
;; forms, that are not a symbol.
((symbol?? (x . y) kt kf) kf) ; It's a pair, not a symbol
((symbol?? #(x ...) kt kf) kf) ; It's a vector, not a symbol
The first two cases don’t need any explanation I think.
;; After those things are excluded, the thing might be a symbol.
((symbol?? maybe-symbol kt kf)
(let-syntax ((test
(syntax-rules ()
((test maybe-symbol t f) t)
((test x t f) f))))
(test abracadabra kt kf)))))
If maybe-symbol is a symbol, then (test abracadabra kt kf) matches (test
maybe-symbol t f) (let t=kt, f=kf, maybe-symbol=abracadabra).
If maybe-symbol is not a symbol, for example it is a string “hello” (please
ignore wrong quoting), then
(test maybe-symbol t f) becomes (test “hello” t f). The string “hello” cannot
act as an identifier (because it is a string, not a symbol), so (test
abracadabra kt kf) does not match the first case (test maybe-symbol t f). Only
the second case (test x t f) remains, and (test abracadabra kt kf) matches this
(set x to abracadabra, t to kt, f to kf).
I think the first two cases are superfluous, but perhaps there is a performance
advantage.
(I’m wondering if this still works in the case (symbol? ...), because ... is
special in syntax-rules)
Best regards,
Maxime Devos.
