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

Question

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.

Solution 1

=> 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.

Solution 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

Solution 3

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"
}

Solution 4

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.

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