How useful is the feature of having an atom data type in a programming language?
A few programming languages have the concept of atom or symbol to represent a constant of sorts. There are a few differences among the languages I have come across (Lisp, Ruby and Erlang), but it seems to me that the general concept is the same. I am interested in programming language design, and I was wondering what value does having an atom type provide in real life. Other languages such as Python, Java, C# seem to be doing quite well without it.
I have no real experience of Lisp or Ruby (I know the syntaxes, but haven't used either in a real project). I have used Erlang enough to be used to the concept there.
A short example that shows how the ability to manipulate symbols leads to cleaner code: (Code is in Scheme, a dialect of Lisp).
(define men '(socrates plato aristotle))
(define (man? x)
(contains? men x))
(define (mortal? x)
(man? x))
;; test
> (mortal? 'socrates)
=> #t
You can write this program using character strings or integer constants. But the symbolic version has certain advantages. A symbol is guaranteed to be unique in the system. This makes comparing two symbols as fast as comparing two pointers. This is obviously faster than comparing two strings. Using integer constants allows people to write meaningless code like:
(define SOCRATES 1)
;; ...
(mortal? SOCRATES)
(mortal? -1) ;; ??
Probably a detailed answer to this question could be found in the book Common Lisp: A Gentle Introduction to Symbolic Computation.