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

In Scheme (and other members of the Lisp family), symbols are not just useful, they are essential.

An interesting property of these languages is that they are homoiconic. A Scheme program or expression can itself be represented as a valid Scheme data structure.

An example might make this clearer (using Gauche Scheme):

> (define x 3)
x
> (define expr '(+ x 1))
expr
> expr
(+ x 1)
> (eval expr #t)
4

Here, expr is just a list, consisting of the symbol +, the symbol x, and the number 1. We can manipulate this list like any other, pass it around, etc. But we can also evaluate it, in which case it will be interpreted as code.

In order for this to work, Scheme needs to be able to distinguish between symbols and string literals. In the example above, x is a symbol. It cannot be replaced with a string literal without changing the meaning. If we take a list '(print x), where x is a symbol, and evaluate it, that means something else than '(print "x"), where "x" is a string.

The ability to represent Scheme expressions using Scheme data structures is not just a gimmick, by the way; reading expressions as data structures and transforming them in some way, is the basis of macros.