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.
(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
(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"]
(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