💎Ruby Tip💎 Did you know that Ruby supports Pattern Matching?


8 December 2023

💎Ruby Tip💎 Did you know that Ruby supports Pattern Matching? Pattern Matching in Ruby allows for concise data destructuring, making it easy to assign variables with clear syntax. From filtering values in arrays to customizing destructuring in classes, this feature simplifies data manipulation in an elegant way.

Pattern matching is a feature that was introduced in Ruby 2.7. From Ruby 3.0 onwards, it is no longer an experimental feature, and we can start using it without an annoying warning:

(irb):3: warning: Pattern matching is experimental, and the behavior may change in future versions of Ruby!

But what is Pattern Matching?

Pattern matching is a feature that allows you to compare and understand the structure of organized information, such as data or variables. This is done by checking how the information is organized and assigning the matching parts to local variables for later use.

Pattern Matching is supported through the case / in syntax. Important not to confuse with case / when and not to mix. If there is no match with any expression and there is no else defined, then a NoMatchingPatternError exception is raised.

case <expression>
in <pattern1>
  # ...
in <pattern2>
  # ...
  # ...

Patterns can be:

  • Value: Any Ruby object (compared with the === operator, as in ‘when’).

  • Array: Array pattern: [<subpattern>, <subpattern>, <subpattern>, <subpattern>, ...].

  • Find: Search pattern: [*variable, <subpattern>, <subpattern>, <subpattern>, <subpattern>, ..., *variable].

  • Hash: Hash pattern: {key: <subpattern>, key: <subpattern>, ...}.

  • Alternative: Pattern combination with | (vertical bar).

  • Variable capture: <pattern> => variable or variable.

Pattern Matching in practice

Here we have a function that processes data:

# Define a method that uses pattern matching with case/in
def process_data(data)
  case data
  in { type: "number", value: Integer => num }
    puts "Received a number: #{num}"
  in { type: "string", value: String => str }
    puts "Received a string: #{str}"
  in { type: "array", value: Array => arr }
    puts "Received an array: #{arr}"
  in { type: "hash", value: Hash => hash }
    puts "Received a hash: #{hash}"
    puts "Received something else."

# Test the method with different data structures
process_data({ type: "number", value: 42 })               # Output: Received a number: 42
process_data({ type: "string", value: "Hello, Ruby!" })   # Output: Received a string: Hello, Ruby!
process_data({ type: "array", value: [1, 2, 3] })         # Output: Received an array: [1, 2, 3]
process_data({ type: "hash", value: { key: "value" } })   # Output: Received a hash: {:key=>"value"}
process_data({ type: "unknown", value: "unknown data" })  # Output: Received something else.

In this example, we show how to perform a search based on a Hash pattern. We highlight one of the powerful features of Pattern Matching: variable binding. We manage to assign a value from the unstructured hash to a variable, which allows us to work with that value later in our code.

Deconstruct and Deconstruct_keys

There are two special methods in pattern matching: deconstruct, called when evaluating on an Array, and deconstruct_keys, called when evaluating on a Hash. Let’s see an example:

class Coordinate
  attr_accessor :x, :y

  def initialize(x, y)
    @x = x
    @y = y

  def deconstruct
    [@x, @y]

  def deconstruct_key
    {x: @x, y: @y}

In the Coordinate class, we define a deconstruct and deconstruct_key method that return an Array and a Hash respectively.

So, when an instance of the Coordinate class is evaluated on an array, what happens is that the deconstruct method is called on the instance to be evaluated:

c = Coordinates.new(32,50)

case c
in [a,b]
  p a #=> 32
  p b #=> 50

And when the same instance is evaluated on a Hash, then the deconstruct_key method is called:

case c
in {x:, y:}
  p x #=> 32
  p y #=> 50

If you are interested in the topic, I invite you to look for more information in the documentation. There are other interesting elements of pattern matching, such as the use of the pin operator (^) and Guard clauses (if and unless).

So much for the short introduction to the topic. If you didn’t know this syntax, I hope you leave with a new tool to further develop your Ruby projects.

Happy Coding!

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