问题
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.
回答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.
回答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
回答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"
}
回答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.
回答5:
In addition to In Ruby what does "=>" mean and how does it work?:
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.
来源：https://stackoverflow.com/questions/4663074/in-ruby-what-does-mean-and-how-does-it-work