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