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

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)))))