/monads

Simple Ruby implementations of some common monads.

Primary LanguageRubyMIT LicenseMIT

Monads

Build Status

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