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 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.

Answer 1

Atoms are literals, constants with their own name for value. What you see is what you get and don't expect more. The atom cat means "cat" and that's it. You can't play with it, you can't change it, you can't smash it to pieces; it's cat. Deal with it.

I compared atoms to constants having their name as their values. You may have worked with code that used constants before: as an example, let's say I have values for eye colors: BLUE -> 1, BROWN -> 2, GREEN -> 3, OTHER -> 4. You need to match the name of the constant to some underlying value. Atoms let you forget about the underlying values: my eye colors can simply be 'blue', 'brown', 'green' and 'other'. These colors can be used anywhere in any piece of code: the underlying values will never clash and it is impossible for such a constant to be undefined!

^{taken from http://learnyousomeerlang.com/starting-out-for-real#atoms}

With this being said, atoms end up being a better semantic fit to describing data in your code in places other languages would be forced to use either strings, enums or defines. They're safer and friendlier to use for similar intended results.