Getting started

Installation

Add this line to your Gemfile

gem 'dry-monads'

Then run

$ bundle

Usage

Every monad has corresponding value constructors. For example, the Maybe monad has two of them: Some(...) and None(). It also has the Maybe(...) method. All three methods start with a capital letter similarly to built-in Ruby methods like Kernel#Array(...) and Kernel#Hash(...). Value constructors are not available globally, you need to add them with a mixin.

To add the Maybe constructors add Dry::Monads[:maybe] to your class:

require 'dry/monads'

class CreateUser
  # this line loads the Maybe monad and adds
  # Some(...), None(), and Maybe(...) to CreateUser
  include Dry::Monads[:maybe]

  def call(params)
    # ...
    if valid?(params)
      None()
    else
      Some(create_user(params))
    end
  end
end

Example in the docs may use extend Dry::Monads[...] for brevity but you normally want to use include in production code.

Including multiple monads

require 'dry/monads'

class CreateUser
  # Adds Maybe and Result. The order doesn't matter
  include Dry::Monads[:maybe, :result]
end

Using with do notation

A very common case is using the Result monad with do notation:

require 'dry/monads'

class ResultCalculator
  include Dry::Monads[:result, :do]

  def calculate(input)
    value = Integer(input)

    value = yield add_3(value)
    value = yield mult_2(value)

    Success(value)
  end

  def add_3(value)
    if value > 1
      Success(value + 3)
    else
      Failure("value was less than 1")
    end
  end

  def mult_2(value)
    if value % 2 == 0
      Success(value * 2)
    else
      Failure("value was not even")
    end
  end
end


c = ResultCalculator.new
c.calculate(3) # => Success(12)
c.calculate(0) # => Failure("value was less than 1")
c.calculate(2) # => Failure("value was not even")

Constructing array values

Some constructors have shortcuts for wrapping arrays:

require 'dry/monads'

class CreateUser
  include Dry::Monads[:result]

  def call(params)
    # ...
    # Same as Failure([:user_exists, params: params])
    Failure[:user_exists, params: params]
  end
end

Interaction between monads and constructors availability

Some values can be converted to others or they can have methods that use other monads. By default, dry-monads doesn’t load all monads so you may have troubles like this:

extend Dry::Monads[:result]

Success(:foo).to_maybe # RuntimeError: Load Maybe first with require 'dry/monads/maybe'

To work around you may either load dry/monads/maybe add maybe to the mixin:

extend Dry::Monads[:result, :maybe]

Success(:foo).to_maybe # => Some(:foo)

For the same reason Dry::Monads.Some(...), Dry::Monads.Success(...), and some other constructors are not available until you explicitly load the monads with require 'dry/monads/%{monad_name}'.

Loading everything

Just require 'dry/monads/all'