Functional Ruby gem

November 28, 2015

Ruby is a great example of multi-paradigm programming language: it allows you to write code in object-oriented, imperative or functional styles. Ruby have much in common with functional programming languages: high-order functions, closures, anonymous functions, continuations, statements all values. If you want to use more functional programming patterns and tools, you might want to take a look on Functional Ruby gem.


  • Thread-safe, immutable data structures
  • Protocol specifications
  • Functions overloading
  • Either, Option classes
  • Immutable variation of Ruby’s OpenStruct class
  • Memoization
  • Lazy execution
  • Tuples
  • Pattern matching


Install this gem with or without bundler:

gem install functional-ruby
gem 'functional-ruby'

And then require it in your project:

require 'functional'

Immutable data structures

Address =, :country, :street, :house) do
  mandatory :country, :city
  default :city, 'Moscow'
  default :country, 'Russia'
end # <record Address :city=>"Moscow", :country=>"Russia", :street=>nil, :house=>nil>

Immutable OpenStruct

Immutable, thread-safe, write-once and read-only object variation of OpenStruct:

name = firstname: 'Hodor', lastname: 'Hodor'
name.get :firstname # Hodor
name.lastname # Hodor
name.firstname? # true


Tuple is a data structure that is similar to array, but is immutable and has a fixed length.

tuple = %w(one two three) 0 # one
tuple.last 0 # three
tuple.fetch 4, 'four' # four
tuple.tail.to_a # ['two', 'three']
tuple.repeat(2).to_a.join ', ' # one, two, three, one, two, three


Protocols are specifications to provide polymorphism and method-dispatch mechanism with strong typing, inspired by Clojure protocols:

Functional::SpecifyProtocol(:Address) do
  attr_accessor :city
  attr_accessor :country
  attr_accessor :street
  attr_accessor :house

Pattern matching

# Pattern matching with type and protocol checking
class AddressChecker
  include Functional::PatternMatching
  include Functional::Protocol
  include Functional::TypeCheck

  def msg
    'You are live in Moscow, Russia'

  defn(:msg, _) do |addr|
    "You are live in #{addr}"

  defn(:msg, _) { |addr|
    "You are live in #{}, #{addr.street}, #{}, #{}"
  }.when { |addr| Satisfy?(addr, :Address) }

  defn(:msg, :name, _) do |addr|
    "Somebody live in #{addr}"

  defn(:msg, _) { |zip|
    "Your zip is #{zip}"
  }.when { |addr| Type?(addr, Fixnum) }


If you like functional programming, and want to use it’s patterns and tools with Ruby, then you can use Functional Ruby gem to write code in more functional style. You can find more information in API documentation.

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