How &: Works in Ruby

Chances are you've seen and written code like in ruby or rails applications many times. If you're new to Ruby, you may have wondered about this strange syntax. Let's demysify &: and explain exactly how it works.

First we start with blocks and how to pass block arguments to methods.

Passing block arguments to methods and the &

We can think of a Block as a chunk of ruby code associated with method invocation. We know that any method invocation may be followed by a block and the method can invoke the code in that block with a yield statement.

Here is an example. This method generates a sequence of n numbers offset by a constant c and passes them to the block.

def seq(n, c)
  i = 0
  while (i < n)
    yield i + c
    i += 1

And we can invoke that method like this

  seq(5, 2) { |x| puts x }

In this case the block we passed to the method is anonymous. What if we wanted to explicitly refer to the block within the method? We can add a named block argument and precede it with an &. If we do this, the block will be converted to a Proc object. Then we can use the call method of the Proc object instead of the keyword yield in this case (although yield works too).

def seq(n, c, &b)
  i = 0
  while (i < n) + c)
    i += 1

This changes the method definition. We still invoke the method the same way as before seq(5,2) { |x| puts x }. So when would we use the & with method invocation as well?

When & is used before a Proc object in a method invocation, it treats the Proc as if it was an ordinary block following the invocation. Consider the following code that sums up the numbers in an array.

a = [1, 2, 3, 4, 5]
a.inject(0) { |total, n| total + n }

We could be more explicit about using a Proc object for the block.

a = [1, 2, 3, 4, 5]
summation = { |total, n| total + n }
a.inject(0, &summation)

And that code snippet is equivalent to the one above.

One more thing to note. In a method invocation, an & usually appears before a Proc object. However, it's allowed before any object that has a to_proc method. For example Method object and Symbol both have to_proc after Ruby 1.9. So that is why we see code like

Now let's work out exactly how &:first_name above works.

How does the &: thing work?

This is where to_proc method of Symbol class comes in. So translates to { |u| u.first_name }

We can see that that translation is done with a combination of a symbol, the method Symbol#to_proc, implicit class casting, and & operator.


  1. If we put & in front of a Proc instance in the argument position, that will be interpreted as a block.
  2. If we put something other than a Proc instance with &, implicit class casting will try to convert that to a Proc instance using to_proc method defined on that object if there is any. In case of a Symbol there is a to_proc method.

One last thing. A subtle point to note: all method names are available by their Symbol names as well. You can call a method on an object like 1.to_s and 1.send(:to_s). So really (1..10).each(&:to_s) is equivalent to (1..10).each { |x| x.send(:to_s) }. The symbol is passed as an argument to the send() method.

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