What is the return value of the following Ruby code?

def sum
  n = 0

  n += yield
  n += yield

  n
end

sum { 42 } # => ???

The correct answer is

0

It raises SyntaxError

84

42

Explanation

The yield keyword — in association with a block — allows the passing of a set of additional instructions during a method invocation.

This mechanism allows you to customize a method depending on your needs.

What’s a block?

A block is part of the Ruby method syntax.

This means that when a block is recognized by the Ruby parser it’ll be associated with the invoked method and replace yields in the method

def multiple_yields
  yield
  yield
end

multiple_yields { puts "multiple yields" }

produces

multiple yields
multiple yields

The code inside the block will be "called" for each yield in the method definition.
Whether there is one or more calls.

So 

def sum
  n = 0

  n += yield # => n = 0 + 42
  n += yield # => n = 42 + 42

  n # => 84
end

sum { 42 }

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