scheme - Check if an argument is a list or an atom

Question

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

Answer 1

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?))