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 some languages, associative array literals have keys that behave like symbols.

In Python[1], a dictionary.

d = dict(foo=1, bar=2)

In Perl[2], a hash.

my %h = (foo => 1, bar => 2);

In JavaScript[3], an object.

var o = {foo: 1, bar: 2};

In these cases, foo and bar are like symbols, i.e., unquoted immutable strings.

[1] Proof:

x = dict(a=1)
y = dict(a=2)

(k1,) = x.keys()
(k2,) = y.keys()

assert id(k1) == id(k2)

[2] This is not quite true:

my %x = (a=>1);
my %y = (a=>2);

my ($k1) = keys %x;
my ($k2) = keys %y;

die unless \$k1 == \$k2; # dies

[1] In JSON, this syntax is not allowed because keys must be quoted. I don't know how to prove they are symbols because I don't know how to read the memory of a variable.