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

As a C programmer I had a problem with understanding what Ruby symbols really are. I was enlightened after I saw how symbols are implemented in the source code.

Inside Ruby code, there is a global hash table, strings mapped to integers. All ruby symbols are kept there. Ruby interpreter, during source code parse stage, uses that hash table to convert all symbols to integers. Then internally all symbols are treated as integers. This means that one symbol occupies only 4 bytes of memory and all comparisons are very fast.

So basically you can treat Ruby symbols as strings which are implemented in a very clever way. They look like strings but perform almost like integers.

When a new string is created, then in Ruby a new C structure is allocated to keep that object. For two Ruby strings, there are two pointers to two different memory locations (which may contain the same string). However a symbol is immediately converted to C int type. Therefore there is no way to distinguish two symbols as two different Ruby objects. This is a side effect of the implementation. Just keep this in mind when coding and that's all.