What makes Ruby's blocks great2022-02-10

In Ruby, we often talk about "blocks". While not unique to Ruby, few mainstream languages have something I would consider equivalent and, to be frank, I think they're missing out.

Just explaining to someone what's special about them can be tricky. I think many rubyists use blocks without realizing that they are different from what's possible in other mainstream languages.

This post is an attempt to explain what's great about blocks without being a verbose in-depth guide. It's written with non-rubyists in mind. I hope it can pique your interest.

Anonymous function with a nicer syntax

That's the first thing one may say to describe them, or the first thing that someone hearing about them will think of, and... that's a start. The nicer syntax is certainly an important part of what makes Ruby's blocks feel great to use, but there is more than just that.

Compared to anonymous functions, blocks are more powerful and more consistent with other standard constructs such as for loops. So much so that they have entirely replaced for loops in Ruby; you instead call a method and pass it a block.

Blocks managed to do that because they have more control over the flow of code than your regular anonymous functions, giving them as much power as a for loop.

To explain what I mean, let's work with a simple "for each" loop. Here is what a block looks like in Ruby.

To make it easier for non-rubyists, I'm using a style closer to other languages.

What we call a "block" goes from do to end.

Quick code explanation:

[1, 2, 3, 4].each() do |v|
  puts(v)
end

An Array literal, it "gives" an Array with those 4 values. Same as JavaScript

[1, 2, 3, 4]

Calls the method each (on the Array), passing it the block that follows the parentheses

.each() do ... end

The parameters the block accepts: one parameter called "v".

|v|

The content of the block, what will run when it is called. This prints the value to the console.

puts(v)

Other than the clean syntax for passing the block ("anonymous function"), this is nothing special. In javascript, this would be:

// Another syntax exists, but I think this is more universally recognizable
[1, 2, 3, 4].forEach(function(v) {
  console.log(v);
})

From this common ground, let's see how using blocks compare to anonymous functions by thinking of loops.

break

Most languages with loops allows you to exit early from them. Usually with a break keyword.

Think of the previous examples, but stopping (like break) after v == 3.

How do you do this with the forEach method in JavaScript? The simple answer is that you can't do it simply. You either switch to an actual for loop or use exceptions or some other workaround.

break cannot be used to exit an anonymous function. But in Ruby, in a block...

[1, 2, 3, 4].each() do |v|
  puts(v)
  if v == 3
    break
  end
end

break just works.

break exits from the function to which the block was passed (each in this case), so the block can stop early just like in regular for loop.

We'll talk more about break in a bit; it can do more, but I want to go over the simpler cases first.

return

How about returning from a function from within a loop? That's pretty common.

You, again, can't do this using anonymous functions in most languages. The return keyword would only return from the anonymous function and its caller would call it again for the next iterations.

But it's easy in Ruby with its blocks:

def my_func() # Just defining a function to return out of
  [1, 2, 3, 4].each() do |v|
    puts(v)
    if v == 3
      return 42
    end
  end
end

return just works, again!

When the code reaches the return, it's not the block which returns 42, it's actually my_func. return leaves the block, the call to each, and returns from my_func with the specified value. Just like return does in actual loops.

return can still useful be useful in an anonymous functions. It can return a value to be used, such as a map function or it can stop the current iteration to go to the next one. This is the equivalent of continue and next. So let's move over to these.

continue / next

In loops, we sometimes want to skip an iteration, just go to the next one. Most languages will have a continue or next keyword that can be used in loops.

Instead of stopping, let's skip the iteration where v == 3.

As mentioned in the previous section, JavaScript and most other languages can achieve this in anonymous functions using return, but they cannot use the standard continue or next:

[1, 2, 3, 4].forEach(function(v) {
  if (v == 3) {
    return;
  }
  console.log(v);
})

Ruby has next and it works in blocks exactly as one would expect in a loop.

[1, 2, 3, 4].each() do |v|
  if v == 3
    next
  end
  puts(v)
end

Pretty neat.

This is where the parallels with for loops end.

next 42

Blocks are used for more than just replacing a simple for loop. Let's switch to using map for our examples. For those unfamiliar, map is a for loop which also saves a return value for each iteration in an array and returns that array.

As an example, squares gets set to an array of the square (2nd power) of each number.

squares = [1, 2, 3, 4].map() do |v|
  # Ruby detail: the `next` is optional for the last expression, the `next` is implicit
  # Leaving it in for clarity
  next v ** 2
end

next can receive a value. It's what it will "return" to the method that called the block (map in this case). It really is just a return that stays at the block's level.

In JavaScript with an anonymous function, you would use return with a value.

squares = [1, 2, 3, 4].map(function(v) {
  return v ** 2;
})

break 42

In Ruby, break can also receive a value. Break-ing news, right? :)

# We add a secret override code... it's an example ok! suggest a better one in the comments
squares = [1, 2, 666, 4].map() do |v|
  if v == 666
    break [-666]
  end
  v ** 2
end

In addition to breaking out of the function that received the block (the map in this example), the function will return the value given to break. So in the above example, squares will be set to [-666].

Honestly, this is rarely used. Again, no equivalent here in JavaScript and most other languages.

These were the main things that make blocks in Ruby more than just anonymous functions. Still with me? Here is a little wrap up.

Building an intuition

With the right perspective, blocks' behaviors are quite intuitive.

When dealing with loops, you have 3 nested constructs interacting: a wrapping function, a loop statement and the loop's body; and you have 3 keywords to choose where the flow of the code goes.

  • return returns from the wrapping function
  • break leaves the loop statement
  • next / continue leaves loop's body

When dealing with blocks or anonymous functions, it's instead 3 nested "functions" that are interacting: a wrapping function, a called function and an anonymous functions (or block).

Ruby's blocks, let you use the same 3 keywords to choose where the flow of the code goes.

  • return returns from the wrapping function (ex: my_func)
  • break returns from the called function (ex: each, map)
  • next returns from the block

Quite consistent. But since we are talking about functions instead of statements (loop), return values are also involved. Allowing both break and next to provide a return value fits well in that model and is quite useful. The 3 keywords are basically return, but they have different targets.

More than just loops

I've been focusing on looping structures because those are common and simple to understand, but blocks are used for lots of things.

File.open("my_file.txt", "w") do |f|
  f.puts('x')
  f.puts('y')
  f.puts('z')
  # It is automatically closed when leaving the block
  # Including if you use return/break/next within
end

thr = Thread.new() do
  # Do stuff in your thread
end

Timeout.timeout(5) do
  # Code that will be interrupted if it takes more than 5 seconds
end

# (on a Hash, often called a Map or a Dictionary in different languages)
# The block is only called if "my_key" is not in the hash.
value = my_hash.fetch("my_key") do |key|
  # generate default value, maybe set it in the hash
end

# Ok, another loop because it's cute
5.times() do |i|
  # This block gets called 5 times, with `i` going from 0 to 4
end

Tool making

Programming is all about making tools from other tools. To me, blocks are a very neat one that fit seamlessly in the Ruby language and handle many roles. They really feel like a generalization of anonymous functions and I wish more mainstream languages would have them.

Try Ruby out

You can try Ruby from your browser at https://try.ruby-lang.org/. There is a small Ruby lesson built into the site, but it may feel too simple for more advanced programmers. Anyhow, it will let you type some Ruby and see the result.