Ruby Case Statement and the `threequals`

Vinicius Negrisolo Vinicius Negrisolo Ruby

Ruby case/when/else statement uses the Ruby operator ===, also known as threequals. There are some nice implementations of === that we have to know. So let’s see some of them.

Ruby Threequals Operator

The first of all, === is a Ruby operator, and this means that it can be called in this two distinct ways:

'hello' === 'hello'
#=> true

#=> true

And that’s why the order (left/right) matters. See this example:

Integer === 4
#=> true
4 === Integer
#=> false

Additionally ruby-object implementation is the same as calling ==. So let’s see the cases that they differ.

Class and Module implementation

The most used === implementation comes from ruby-module that’s the same for Ruby Modules and Ruby Classes. This implementation checks if the argument is an instance or descendant of a Class/Module.

Integer === 4
#=> true

Numeric === 5
#=> true

Float === 6
#=> false

Range implementation

ruby-range implementation of === verifies if the range contains the argument.

(1..10) === 7
#=> true
(1..10) === 15
#=> false

Regexp implementation

ruby-regexp === implementation evaluates the regexp with the argument and returns if the argument matched the regexp or not:

/^[a-z]+$/ === 'hello'
#=> true

/^[a-z]+$/ === 'HELLO'
#=> false

Proc and Lambda implementation

Another interesting implementation of === is in ruby-proc. It evaluates the block passing the argument to the Proc or Lambda.

even = ->(n) { n % 2 == 0 }
even === 5
#=> false
even === 4
#=> true
odd = lambda { |n| n % 2 != 0 }
odd === 5
#=> true
odd === 4
#=> false

Ruby Case Statement

And finally we know that the Ruby case/when/else statement uses === method to evaluate which clause matches the argument. Putting all together, let’s see this example and how the syntax is short and nice to read:

def find_out(obj = nil)
  even = ->(n) { Integer === n && n % 2 == 0 }
  odd = lambda { |n| Integer === n && n % 2 != 0 }

  case obj
  when Symbol; 'This is a Symbol'
  when 1..10;  'This is a small Integer'
  when /^9/;   'Starts with 9'
  when even;   'This is an even Integer'
  when odd;    'This is an odd Integer'
  else;        'I could not identify it'

#=> "This is a Symbol"
#=> "This is a small Integer"
#=> "Starts with 9"
#=> "This is an even Integer"
#=> "This is an odd Integer"
find_out(foo: :bar)
#=> "I could not identify it"


The Ruby operator === threequals allow us to define custom implementations and then use it in the case statement. The main goal here is to let code more readable. So let’s use the threequals operator more often.

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