In Ruby what does "=>" mean and how does it work?

While learning Ruby I've come across the "=>" operator on occasion. Usually I see it in the form of

:symbol => value

and it seems to be used frequently when passing values to functions. What exactly is that operator called? What does it do/mean? Is it built into Ruby or is it something that different frameworks like Rails and DataMapper add to the symbol class? Is it only used in conjunction with the symbol class? Thanks.

=> separates the keys from the values in a hashmap literal. It is not overloadable and not specifically connected to symbols.

A hashmap literal has the form {key1 => value1, key2 => value2, ...}, but when used as the last parameter of a function, you can leave off the curly braces. So when you see a function call like f(:a => 1, :b => 2), f is called with one argument, which is a hashmap that has the keys :a and :b and the values 1 and 2.

You might hear this operator referred to as a "hash rocket," meaning you use it when defining a ruby hash.

This is the Ruby Hash documentation, if you're not familiar: http://www.ruby-doc.org/core/classes/Hash.html

Note that in Ruby 1.9, if you're defining a hash that uses symbols as keys, there's now an alternative syntax available to you: http://blog.peepcode.com/tutorials/2011/rip-ruby-hash-rocket-syntax

Tip: if you're using it in a hash like {:a => "A", :b => "B"}, in Ruby 1.9, you can use it like a JSON hash:

{
  a: "A",
  b: "B"
}

If you want to do any further Googling, => is sometimes called a hashrocket, because it looks like a rocket (in the same sense that <=> looks like a spaceship), and it's used in hashes.

Or you could use SymbolHound.

In addition to https://stackoverflow.com/questions/4663074/in-ruby-what-does-mean-and-how-does-it-work/4663089#4663089:

You mostly will see the => to define parameters for a function. Think of this as a nice convenience: You need not remember the right order of your parameters, as all parameters are wrapped into a giant hash. So if you have a simple helper method like

link_to "My link", my_path, :confirm => "Are you sure?"

this is way better than

link_to "My link", my_path, null, null, null, null, "Are you sure?"

just because you want to use a rarely used parameter. So passing parameters with a hash is just a convention in Ruby/Rails to make life easier.