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

Atoms are like an open enum, with infinite possible values, and no need to declare anything up front. That is how they're typically used in practice.

For example, in Erlang, a process is expecting to receive one of a handful of message types, and it's most convenient to label the message with an atom. Most other languages would use an enum for the message type, meaning that whenever I want to send a new type of message, I have to go add it to the declaration.

Also, unlike enums, sets of atom values can be combined. Suppose I want to monitor my Erlang process's status, and I have some standard status monitoring tool. I can extend my process to respond to the status message protocol as well as my other message types. With enums, how would I solve this problem?

enum my_messages {
  MSG_1,
  MSG_2,
  MSG_3
};

enum status_messages {
  STATUS_HEARTBEAT,
  STATUS_LOAD
};

The problem is MSG_1 is 0, and STATUS_HEARTBEAT is also 0. When I get a message of type 0, what is it? With atoms, I don't have this problem.

Atoms/symbols are not just strings with constant-time comparison :).