Check if an argument is a list or an atom

scheme racket
Koen picture Koen · Mar 23, 2011 · Viewed 24k times · Source

How do I check if something is an atom? I'm looking for something like number? or list?.

Answer

Eli Barzilay picture Eli Barzilay · Mar 23, 2011

Usually, you'll want to exclude the empty list too:

(define (atom? x) (not (or (pair? x) (null? x))))

or, if you want to be more pedantic, then forbid vectors too:

(define (atom? x) (not (or (pair? x) (null? x) (vector? x))))

And of course you can add much more here -- since it's marked as a racket question, you might want to add hash tables, structs, etc etc. So it can just as well be easier to specify the kinds of values that you do consider as atoms:

(define (atom? x)
   (ormap (lambda (p) (p x)) (list number? symbol? boolean? string?)))

or using the racket contract system:

(define atom? (or/c number? symbol? boolean? string?))

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