tomstuart/monads

---

tomstuart/monads.json

{
  "createdAt": "2014-08-02T13:52:07Z",
  "defaultBranch": "master",
  "description": "Simple Ruby implementations of some common monads.",
  "fullName": "tomstuart/monads",
  "homepage": "https://tomstu.art/refactoring-ruby-with-monads",
  "language": "Ruby",
  "name": "monads",
  "pushedAt": "2019-09-08T09:18:50Z",
  "stargazersCount": 616,
  "topics": [],
  "updatedAt": "2025-11-14T07:04:27Z",
  "url": "https://github.com/tomstuart/monads"
}

Monads

This library provides simple Ruby implementations of some common monads.

The most important method of each implementation is #and_then (a.k.a. bind or >>=), which is used to connect together a sequence of operations involving the value(s) inside the monad. Each monad also has a .from_value class method (a.k.a. return or unit) for constructing an instance of that monad from an arbitrary value.

Optional

(a.k.a. the maybe monad)

An Optional object contains a value that might be nil.

>> require 'monads/optional'
=> true

>> include Monads
=> Object

>> optional_string = Optional.new('hello world')
=> #<struct Monads::Optional value="hello world">

>> optional_result = optional_string.and_then { |string| Optional.new(string.upcase) }
=> #<struct Monads::Optional value="HELLO WORLD">

>> optional_result.value
=> "HELLO WORLD"

>> optional_string = Optional.new(nil)
=> #<struct Monads::Optional value=nil>

>> optional_result = optional_string.and_then { |string| Optional.new(string.upcase) }
=> #<struct Monads::Optional value=nil>

>> optional_result.value
=> nil

Many

(a.k.a. the list monad)

A Many object contains multiple values.

>> require 'monads/many'
=> true

>> include Monads
=> Object

>> many_strings = Many.new(['hello world', 'goodbye world'])
=> #<struct Monads::Many values=["hello world", "goodbye world"]>

>> many_results = many_strings.and_then { |string| Many.new(string.split(/ /)) }
=> #<struct Monads::Many values=["hello", "world", "goodbye", "world"]>

>> many_results.values
=> ["hello", "world", "goodbye", "world"]

Eventually

(a.k.a. the continuation monad)

An Eventually object contains a value that will eventually be available, perhaps as the result of an asynchronous process (e.g. a network request).

>> require 'monads/eventually'
=> true

>> include Monads
=> Object

>> eventually_string = Eventually.new do |success|
     Thread.new do
       sleep 5
       success.call('hello world')
     end
   end
=> #<struct Monads::Eventually block=#<Proc>>

>> eventually_result = eventually_string.and_then do |string|
     Eventually.new do |success|
       Thread.new do
         sleep 5
         success.call(string.upcase)
       end
     end
   end
=> #<struct Monads::Eventually block=#<Proc>>

>> eventually_result.run { |string| puts string }
=> #<Thread run>
HELLO WORLD