Blocks

Like methods, but without a name

Blocks are one of the things programmers absolutely love about Ruby. They are an extremely powerful feature that allows us to write very flexible code. At the same time they read very well, and they are used all over the place.

So, what is a block?

A block, essentially, is the same thing as a method, except it does not have a name, and does not belong to an object.

I.e. a block is an anonymous piece of code, it can accept input in form of arguments (if it needs any), and it will return a value, but it does not have a name.

Moreover, blocks can only be created by the way of passing them to a method when the method is called.

A block is a piece of code that accepts arguments, and returns a value. A block is always passed to a method call.

Let’s jump right in:

5.times do
  puts "Oh, hello from inside a block!"
end

As you can see times is a method that is defined on numbers: 5.times calls the method times on the number 5.

Now, when this method is called the only thing passed is a block: that is the anonymous piece of code between do and end. There are no objects passed as arguments to the method times, instead it just passes a block.

The method times is implemented in such a way that it simply calls (executes) the block 5 times, and thus, when you run the code, it will print out the message "Oh, hello from inside a block!" 5 times.

The code almost reads like an English sentence Five times do output this message, right? It does, and that’s one of the reasons why Rubyists love using blocks.

One of the things that seem rather hard to grasp about blocks is that

• First, they are anonymous chunks of code.
• Second, they are passed to methods just like other objects.
• And third, they still can be called (just like methods), from inside the method that it was passed to.

Does that make sense?

Imagine you are the object that represents the number 5. You are a number and you do know your own value.

Now I hand you a piece of paper saying: Please print the following on the screen: "Oh, hello!", and I ask you to execute this instruction as many times as the value that you know.

You’d go ahead and follow the instructions on the paper, and thus print out the message. You repeat this 5 times, because 5 is the value that you know.

This is pretty much how the method times on numbers works, and how blocks work: times takes the block (the instructions), and runs it as many times as the value of the number.

To summarize: Methods can not only accept input in the form of objects passed as arguments. They can also accept this one special piece of input, which is an anonymous block of code. And they can then call (execute) this block of code in order to do useful things with it.

Let’s look at some other aspects of how blocks work next.