Ruby/Ruby on Rails ampersand colon shortcut

Question

In Ruby (actually only in Rails, apparently), I know that if I do:

some_objects.each(&:foo)

It's the same as

some_objects.each { |obj| obj.foo }

That is -- &foo creates the block { |obj| obj.foo }, turns it into a Proc, and passes it to each. Why does this work? Is it just a Ruby special case, or is there reason why this works as it does?

Answer 1

Just found the answer after a bit of searching. This article by Jarkko Laine does a great job explaining it: http://jlaine.net/2008/5/8/amp-lified

Basically this is not a Ruby shortcut, but a clever hack done by Rails. Rails adds a to_proc method to Symbol. The ampersand calls to_proc on the symbol to turn into a proc, and the handy to_proc method is used:

class Symbol
  def to_proc
    lambda {|i| i.send(self)}
  end
end

Answer 2

Your question is wrong, so to speak. What's happening here isn't "ampersand and colon", it's "ampersand and object". The colon in this case is for the symbol. So, there's & and there's :foo.

The & calls to_proc on the object, and passes it as a block to the method. In Rails, to_proc is implemented on Symbol, so that these two calls are equivalent:

something {|i| i.foo }
something(&:foo)

Also, to_proc on Symbol is implemented in Ruby 1.8.7 and 1.9, so it is in fact a "ruby thing".

So, to sum up: & calls to_proc on the object and passes it as a block to the method, and Rails implements to_proc on Symbol.

Answer 3

I actually blogged about this very thing last week:

http://swaggadocio.com/post/287689063