What are considered atoms in Scheme?

Question

Can someone please explain or link me to any helpful resources ( I couldn\'t find any threads on google) that could help me understand what atoms are.

Answer 1

The term "atom" is used by several authors (McCarthy and Friedman/Felleisen, among others) to refer to a datum that is not a "cons" pair. I claim that these days, you'd be more likely to invert that, and test for "cons"-hood rather than "atom"-hood. Where are you seeing the term used?

Answer 2

Nowadays we consider an atom an element that's not a cons-pair and that is not null. That includes:

• Numbers
• Strings
• Symbols
• Booleans
• Characters

This is best expressed with the following procedure, taken from the book The Little Schemer:

(define atom?
  (lambda (x)
    (and (not (pair? x)) (not (null? x)))))