Check for symbol in syntax-rules


You are writing a syntax-rules macro, and want to check whether or not the value of a pattern variable is a symbol.


syntax-rules doesn't let the programmer directly check data types, but there's a famous trick to do it using nested syntax-rules:

(define-syntax symbol??
  (syntax-rules ()
    ((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
    ((symbol?? maybe-symbol kt kf)
           (syntax-rules ()
             ((test maybe-symbol t f) t)
             ((test x t f) f))))
        (test abracadabra kt kf)))))

The expression (symbol?? expr kt kf) expands to kt if expr is a symbol. Else it expands to kf.

Credit: Oleg Kiselyov (ref:


(define-syntax study
  (syntax-rules ()
    ((study obj)
     (begin (write 'obj)
            (display (symbol?? obj
                               " is a symbol"
                               " is not a symbol"))

(study 78)
;; 78 is not a symbol
(study (kekkonen))
;; (kekkonen) is not a symbol
(study "kekkonen")
;; "kekkonen" is not a symbol
(study 'kekkonen)
;; (quote kekkonen) is not a symbol
(study kekkonen)
;; kekkonen is a symbol

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