Lisp and Erlang Atoms, Ruby and Scheme Symbols. How useful are they?

Question

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 consta

Answer 1

The problem I have with similar concepts in other languages (eg, C) can be easily expressed as:

#define RED 1
#define BLUE 2

#define BIG 1
#define SMALL 2

or

enum colors { RED, BLUE  };
enum sizes  { BIG, SMALL };

Which causes problems such as:

if (RED == BIG)
    printf("True");
if (BLUE == 2)
    printf("True");

Neither of which really make sense. Atoms solve a similar problem without the drawbacks noted above.