Ruby Blocks : Procs and Lambdas

Sixth article in the Prepare for a Ruby job interview series. In this article, we’re going to talk about the Blocks in Ruby : Procs and Lambas.

Proc and Lambda are anonymous function that can be moved around in your code as variable.

pr = Proc.new { |a, b| a + b }
p pr.call(1, 2)

l = lambda { |a, b| a + b }
p l.call(1, 2)

The main difference between those is that a return statement in a proc will behave like a part of the calling method (and return from it) :

def test_proc
  Proc.new { return 1 }.call
  return 0
end

def test_lambda
  lambda { return 1 }.call
  return 0
end

p test_proc # => 1
p test_lambda # => 0

As you can see, the proc returned from the test_proc method.

Passing a block to a method can be done in different ways.

With the & (ampersand) symbol :

def super_method(arg1, &block)
  p arg1
  block.call
end

super_method('Hi!') { p "I'm a proc!" }

# => "Hi!"
# => "I'm a proc!"

With yield :

def super_method(arg1)
  p arg1
  yield
end

super_method('Hi!') { p "I'm a proc!" }

# => "Hi!"
# => "I'm a proc!"

You can also simply pass the block as an argument :

def super_method(arg1, pr)
  p arg1
  pr.call
end

pr = Proc.new { p "I'm a proc!" }
super_method('Hi!', pr)

# => "Hi!"
# => "I'm a proc!"