Ruby blocks made easy, part I, methods and procs

Blocks in Ruby are powerful structures that are part of our daily basis as Ruby developers. We see them being used across a variety of standard classes as well as in almost every popular gem.

A few examples:

# print each number within an array
[1, 2, 3].each { |number| puts number }

# usage in some ActiveRecord models 
scope :published, -> { where(published: true) }
has_many :users, -> { order(:name) }

# usage in Devise configuration
Devise.setup do |config|
  # config.encryptor = :sha512
  # ...
end

# usage in Rails views
<%= form_for @user do |form| %>
  // do something with the `form` object
<% end %>

The examples could go beyond.

Blocks enable flexibility and extensibility. It's practically impossible for any Ruby application to exist without blocks.

There are lots of blogposts explaining the differences between blocks, procs and lambdas, but in this guide which is a series of articles, I'll explain the fundamentals and try to present, in baby-steps, some common problems and how Ruby blocks can help to solve them.

How Ruby evaluates expressions

Everything in Ruby is an expression, so an expression results in a value:

# ruby evaluates the expression, which results in a value `2`
1 + 1

# ruby evaluates the expression, which results in a value `2`, and stores the result in the variable `number`
number = 1 + 1

Same as calling any method in an Object:

# ruby evaluates the method `.now` in the class `Time`,
#  resulting in a value which is the current time,
#  and stores the result in the variable `current_time` 
current_time = Time.now

Everything in Ruby is an object

For those who are already know that everything in Ruby is an object, the following expression is familiar:

1.+(2)

• 1 is an instance object of the class Integer
• The class Integer defines a special method, called +
• This method takes an argument, which should be an instance of the class Integer, in this case, 2
• The method results in a value, which is 3

Note that, once:

• everything is object
• objects have methods
• and everything is an expression which results in a value

Then, expressions are all about calling methods. Hence, we can come to the conclusion that expressions are evaluated immediately.

def fetch_current_time
  Time.now
end

current_time = fetch_current_time

In the above example, Ruby will evaluate the expression fetch_current_time, which is a method, then resulting in the value immediately, storing the result in the variable current_time.

current_time # produces 2021-04-10 17:22:06
current_time # produces 2021-04-10 17:22:06
current_time # produces 2021-04-10 17:22:06
current_time # produces 2021-04-10 17:22:06

No matter how many times we call the variable: anytime the variable current_time is called, it will produce the same value that was already evaluated previously. Expressions are evaluated only immediately.

Evaluating expressions later

What if we wanted to evaluate an expression, not immediately, but later? Sometimes, we have the need to pass through some variable that contains an expression to be evaluated in another context, not in the current one.

Let's see the implementation for the fetch_current_time method:

def fetch_current_time
  Time.now
end

Let's suppose we want to store this method in some variable, but we want to send this variable to another component, class, whatever context, but later.

Potential solution

This is not Ruby code, just a PSEUDO CODE for didatic purposes

current_time = later(fetch_current_time)

Then, we could have the ability to call current_time, producing a different result (time) everytime it is called:

current_time.evaluate # produces 2021-04-10 17:22:15
current_time.evaluate # produces 2021-04-10 17:22:16
current_time.evaluate # produces 2021-04-10 17:22:17

Note that, everytime time the variable is called, the expression within is evaluated again.

Ruby solution

The Ruby standard API provides a method called method which takes an argument which is the method name:

method(:fetch_current_time)

#<Method: main.fetch_current_time() (irb):91>

Yes, this is weird at first but it is part of the metaprogramming Ruby API. Metaprogramming in Ruby is really powerful. We should learn and use its capabilities consciously and with moderation.

meth = method(:fetch_current_time)
meth.class # => Method

Basically, this is the representation of a method.
It is literally the instance of the class Method (remember everything in Ruby is object? Methods included!).
And as such, it has a method that transforms the method into an structure that can be evaluated later. This method is called .to_proc:

method(:fetch_current_time).to_proc

#<Proc:0x00007fca441d4ed8 (lambda)>

• it returns an instance of the class Proc
• the class Proc is used to include expressions that will be evaluated later
• the lambda indicates that this proc is a lambda type. Then, we can say that method procs are lambdas

Once we know that, how to use the proc to solve the problem of evaluating the current_time variable many times later?

current_time = method(:fetch_current_time).to_proc

current_time.call # produces 2021-04-10 17:42:00
current_time.call # produces 2021-04-10 17:42:01
current_time.call # produces 2021-04-10 17:42:02

Conclusion

In this post we learned that Ruby methods can be transformed into Procs to be evaluated later.

In the upcoming series, we will keep learning the fundamentals of Ruby blocks and see how to use procs as arguments to methods.