You’ve probably heard that Ruby is an Object Oriented Language. That’s certainly correct but I find it more useful to think about Ruby as a Message Oriented Language as the two are deeply intertwined.

Most new programmers grasp the idea of sending messages and payloads pretty quickly once it is explained. We can send messages with the send message, or send messages themselves. The following are synonymous:

object.send(:message, payload)
object.message(payload)

"Ruby is awesome!".send(:reverse)
"Ruby is awesome!".reverse

Equally straight forward is how we define how objects respond to messages with def blocks in class statements.

class String
  def reverse_up
    reverse.upcase
  end
end

"hello ruby!".reverse_up
# => "!YBUR OLLEH"

Where things start to get a little fuzzy is when blocks get introduced into the mix. Let’s look at a very common Ruby idiom:

(1..10).each do |number|
  puts number * 2
end
# => 2 4 6 ... 20

What’s going on here? We are obviously sending the message .each to a Range object but what exactly is the block’s relationship with the message & object? It turns out that blocks are just message payloads - optional payloads that are appended to a message.

Blocks are just payloads. Almost

Blocks are optional payloads that are appended to messages - any message. Append a block to any message and see what happens:

"Ruby is awesome!".reverse
# => "!emosewa si ybuR"

"Ruby is awesome!".reverse { puts "I'm in a single line block!" }
# => "!emosewa si ybuR"    

"Ruby is awesome!".reverse do
  puts "I'm in a multi-line block!"
end
# => "!emosewa si ybuR"

See - nothing extra happens. If an object receives a message with a block payload, the block is silently ignored unless the message definition explicitly yields to it. Note however, that the block is appended to the message, not included with the rest of the payload.

"Ruby is sweet!".send(:reverse) { puts "I'm in a block!" }
# => "!teews si ybuR"

"Ruby is sweet!".send(:reverse, { puts "I'm in a block!" })
# => SyntaxError: syntax error, unexpected tSTRING_BEG, expecting keyword_do or '{' or '('

So our new pattern for Ruby messaging looks like this:

object.message(payload) {block}

So far, so good. Let’s see how things work on the message definition side of things by creating a method that responds to messages with a block.

def yield_me
  yield
end

yield_me { p "I'm in a block!" }
# => "I'm in a block!"

Here we send the message yield_me to the object main - (the context that IRB runs in) - along with a block. The method yield_me then just yields control to the block { p "I'm in a block!" } that was appended to the message, before control is returned to the method.

Great. But what can you actually use passing-a-block-with-a-message for? Lots it turns out but lets use it to create a custom enumerator. We will monkey patch the core Enumerable module to create an every_nth method that will iterate over, you guessed it, every nth item in an enumerable.

module Enumerable
  def every_nth(n)
    each_with_index do |enum, index|
      yield enum if (index + 1) % n == 0
    end
  end
end

Now there’s a little Ruby magic at work here. Inside every_nth, we send each_with_index to self with a block that counts the enumerations and passes control to the message block if the enumeration is evenly divisible by n. Yeps, the block we pass to each is yielding to the block passed to every_nth. Ruby is meta.

And because we monkey patched Enumerable, every_nth is available to all objects mixin Enumerable like ranges and arrays.

(1..50).every_nth(5) { |n| p n }
# => 5 10 15 20 ... 50

"ruby is clever".chars.every_nth(2) { |n| p n.upcase }
# => "U" "Y" "I" " " "L" "V" "R"

Et voilà!.