Hi! The explanations are on
https://okmij.org/ftp/Scheme/macros.html#macro-symbol-p Maybe this version will be easier to understand (we don't really need continuation-passing style here): (define-syntax symbol?? (syntax-rules () ((symbol?? maybe-symbol) (let-syntax ((test (syntax-rules () ((test maybe-symbol) #t) ((test _) #f)))) (test abracadabra))))) (symbol?? foo) ⇒ #t (symbol?? (a . b)) ⇒ #f (symbol?? 5) ⇒ #f (symbol?? "a") ⇒ #f (symbol?? #(1 a)) ⇒ #f Basically: the macro call (symbol?? <foobar>) expands to a macro definition of test as (syntax-rules () ((test <foobar>) #t) ((test _) #f)) and a call (test abracadabra). Now, observe that if <foobar> is a symbol, then it's a catch-all pattern when inserted in the syntax-rules definition of `test`, so it will match abracadabra (because it matches anything). On the other hand, if it's not a symbol, then it won't match abracadabra, by case analysis: if it's a number it will only match that number; booleans, strings and characters likewise; if it's a pair it can only match pairs; if it's a vector it can only match vectors; etc. I'm not exactly sure why Oleg Kiselyov included special cases for a pair and a vector, but my guess is that not all Scheme implementations support vectors in syntax-rules patterns (and the pair check is necessary because the car or cdr could contain a vector). The Scheme standards certainly have their opinion on this, and I knew that stuff by heart at some point (when I implemented a syntax-rules/syntax-case expander for a university project), but I don't remember, and it's too late for scouring the standards... Best, Jean
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