@flash-gordon flash-gordon released this Jun 26, 2018 · 18 commits to master since this release

Assets 2

v1.0.0 2018-06-26


  • do-like notation (the idea comes from Haskell of course). This is the biggest and most important addition to the release which greatly increases the ergonomics of using monads in Ruby. Basically, almost everything it does is passing a block to a given method. You call yield on monads to extract the values. If any operation fails i.e. no value can be extracted, the whole computation is halted and the failing step becomes a result. With Do you don't need to chain monadic values with fmap/bind and block, everything can be done on a single level of indentation. Here is a more or less real-life example:

    class CreateUser
      include Dry::Monads
      include Dry::Monads::Do.for(:call)
      attr_reader :user_repo
      def initialize(:user_repo)
        @user_repo = user_repo
      def call(params)
        json = yield parse_json(params)
        hash = yield validate(json)
        user_repo.transaction do
          user = yield create_user(hash[:user])
          yield create_profile(user, hash[:profile])
      def parse_json(params)
        Try[JSON::ParserError] {
      def validate(json)
      def create_user(user_data)
        Try[Sequel::Error] { user_repo.create(user_data) }.to_result
      def create_profile(user, profile_data)
        Try[Sequel::Error] {
          user_repo.create_profile(user, profile_data)

    In the code above any yield can potentially fail and return the failure reason as a result. In other words, yield None acts as return None. Internally, Do uses exceptions, not return, this is somewhat slower but allows to detect failed operations in DB-transactions and roll back the changes which far more useful than an unjustifiable speed boost (flash-gordon)

  • The Task monad based on Promise from the concurrent-ruby gem. Task represents an asynchronous computation which can be (doesn't have to!) run on a separated thread. Promise already offers a good API and implemented in a safe manner so dry-monads just adds a monad-compatible interface for it. Out of the box, concurrent-ruby has three types of executors for running blocks: :io, :fast, :immediate, check out the docs for details. You can provide your own executor if needed (flash-gordon)

    include Dry::Monads::Task::Mixin
    def call
      name = Task { get_name_via_http }    # runs a request in the background
      email = Task { get_email_via_http }  # runs another one request in the background
      # to_result forces both computations/requests to complete by pausing current thread
      # returns `Result::Success/Result::Failure`
      name.bind { |n| email.fmap { |e| create(e, n) } }.to_result

    Task works perfectly with Do

    include Dry::Monads::Do.for(:call)
    def call
      name, email = yield Task { get_name_via_http }, Task { get_email_via_http }
      Success(create(e, n))
  • Lazy is a copy of Task that isn't run until you ask for the value for the first time. It is guaranteed the evaluation is run at most once as opposed to lazy assignment ||= which isn't synchronized. Lazy is run on the same thread asking for the value (flash-gordon)

  • Automatic type inference with .typed for lists was deprecated. Instead, typed list builders were added

    list = List::Task[Task { get_name }, Task { get_email }]
    list.traverse # => Task(List['John', 'john@doe.org'])

    The code above runs two tasks in parallel and automatically combines their results with traverse (flash-gordon)

  • Try got a new call syntax supported in Ruby 2.5+

      Try[ArgumentError, TypeError] { unsafe_operation }

    Prior to 2.5, it wasn't possible to pass a block to [].

  • The Validated “monad” that represents a result of a validation. Suppose, you want to collect all the errors and return them at once. You can't have it with Result because when you traverse a List of Results it returns the first value and this is the correct behavior from the theoretical point of view. Validated, in fact, doesn't have a monad instance but provides a useful variant of applicative which concatenates the errors.

      include Dry::Monads
      include Dry::Monads::Do.for(:call)
      def call(input)
        name, email = yield [
        Success(create(name, email))
      # can return
      # * Success(User(...))
      # * Invalid(List[:invalid_name])
      # * Invalid(List[:invalid_email])
      # * Invalid(List[:invalid_name, :invalid_email])

    In the example above an array of Validated values is implicitly coerced to List::Validated. It's supported because it's useful but don't forget it's all about types so don't mix different types of monads in a single array, the consequences are unclear. You always can be explicit with List::Validated[validate_name(...), ...], choose what you like (flash-gordon).

  • Failure, None, and Invalid values now store the line where they were created. One of the biggest downsides of dealing with monadic code is lack of backtraces. If you have a long list of computations and one of them fails how do you know where did it actually happen? Say, you've got None and this tells you nothing about what variable was assigned to None. It makes sense to use Result instead of Maybe and use distinct errors everywhere but it doesn't always look good and forces you to think more. TLDR; call .trace to get the line where a fail-case was constructed

    Failure(:invalid_name).trace # => app/operations/create_user.rb:43
  • Dry::Monads::Unit which can be used as a replacement for Success(nil) and in similar situations when you have side effects yet doesn't return anything meaningful as a result. There's also the .discard method for mapping any successful result (i.e. Success(?), Some(?), Value(?), etc) to Unit.

      # we're making an HTTP request but "forget" any successful result,
      # we only care if the task was complete without an error
      Task { do_http_request }.discard
      # ... wait for the task to finish ...
      # => Task(valut=Unit)


  • Either, the former name of Result, is now deprecated


  • Either#value and Maybe#value were both droped, use value_or or value! when you 💯 sure it's safe
  • require 'dry/monads' doesn't load all monads anymore, use require 'dry/monads/all' instead or cherry pick them with require 'dry/monads/maybe' etc (timriley)

Compare v0.4.0...v1.0.0