When you place a colon in front of a simple word, you get a symbol. Symbols are cheaper than strings (in terms of computer memory.) If you use a word over and over in your program, use a symbol. Rather than having thousands of copies of that word in memory, the computer will store the symbol only once.
Symbols are immutable. Mutable objects can be changed after assignment while immutable objects can only be overwritten.
Comparing identical strings is much slower since the string values need to be compared instead of just the object ids.
as you can see:
2.1.1 :009 > :abc.object_id => 544168 2.1.1 :010 > :abc.object_id => 544168
both are same objects.
2.1.1 :011 > "abc".object_id => 6023220 2.1.1 :012 > "abc".object_id => 5905340
both are different objects.
As Symbols stay in memory throughout the programs operation, we can quickly snag them from memory instead of instantiating a new copy every time. In fact, Symbols are not only stored in memory, they are also keep track of via an optimized Symbols dictionary. You can see it by running the next example.
puts Symbol.all_symbols.inspect # => A long list of Symbols...
This being the case, what can’t Symbols do? Well, they can’t change.
2.1.1 :014 > puts "hello" << " world" hello world => nil 2.1.1 :015 > puts :hello << :" world" NoMethodError: undefined method `<<' for :hello:Symbol