What is ->() { } in Ruby?

Question

I've just seen this expression in a Ruby/Rails app:

def method(a, b = nil, &c)
   c ||= ->(v) { v }

I understand the first part, but not the ->() { ... } syntax. What does it mean?

The variable names have been changed for briefness. I tried searching, but the non-alphanumeric characters are obviously a nightmare for SEO.

Answer 1

It is a lambda literal. Put the block variables inside () and the body inside {}.

->(x, y){x + y}

In the example, ->(v){v} takes a single argument v and returns it, in other words, it is an identity function. If a block is passed to method, then that is assigned to c. If not, the identity function is assigned to c as default.

Answer 2

That is a lambda literal, introduced in Ruby 1.9:

irb> l = ->(v) { v }
# => #<Proc:0x007f4acea30410@(irb):1 (lambda)>
irb> l.call(1)
# => 1

It is equivalent to write:

irb> l = lambda { |v| v }
# => #<Proc:0x00000001daf538@(irb):1 (lambda)> 

In the example you posted it is used to provide a default block to the method when none is specified, consider this:

def method(a, &c)
  c ||= ->(v) { v }
  c.call(a)
end

method(1)
# => 1
method(1) { |v| v * 2 }
# => 2