---
- version: 1.6.0
  date: 2022-11-04
  changed:
    - "This version uses dry-core 1.0 (@flash-gordon)"
- version: 1.5.0
  date: 2022-10-16
  changed:
    - "Use zeitwerk for auto-loading dry-monads classes (@flash-gordon)"
    - "`Task#then` is deprecated in favor of `Task#bind` (@flash-gordon)"
    - "Minimal Ruby version is now 2.7 (@flash-gordon)"
    - "Either (old name of Result) was removed (@flash-gordon)"
- version: 1.4.0
  date: 2021-07-20
  fixed:
    - Do notation preserves method visibility (anicholson + flash-gordon)
  changed:
    - |-
      Coercing `nil` values to `None` with `Some#fmap` is officially deprecated. (flash-gordon)
      Switch to `Some#maybe` when you expect `nil`.
      This behavior will be dropped in 2.0 but you can opt out of warnings for the time being
        ```ruby
        Dry::Monads::Maybe.warn_on_implicit_nil_coercion false
        ```
    - Minimal Ruby version is 2.6
  added:
    - "`Unit` destructures to an empty array (flash-gordon)"
    - |-
      When `.value!` called on a `Failure` value the error references to the value (rewritten + flash-gordon)
        ```ruby
        begin
          Failure("oops").value!
        rescue => error
          error.receiver # => Failure("oops")
        end
        ```
    - |-
      `Result#alt_map` for mapping failure values (flash-gordon)
        ```ruby
        Failure("oops").alt_map(&:upcase) # => Failure("OOPS")
        ```
    - |-
      `Try#recover` recovers from errors (flash-gordon)
        ```ruby
        error = Try { Hash.new.fetch(:missing) }
        error.recover(KeyError) { 'default' } # => Try::Value("default")
        ```
    - |-
      `Maybe#filter` runs a predicate against the wrapped value. Returns `None` if the result is false (flash-gordon)
        ```ruby
        Some(3).filter(&:odd?)  # => Some(3)
        Some(3).filter(&:even?) # => None
        # no block given
        Some(3 == 5).filter     # => None
        ```
    - |-
      `RightBiased#|` is an alias for `#or` (flash-gordon)
        ```ruby
        None() | Some(6) | Some(7) # => Some(6)
        Failure() | Success("one") | Success("two") # => Success("one")
        ```

- version: 1.3.5
  date: "2020-01-06"
  added:
    - Smarter keys deconstruction in pattern matching (flash-gordon)
- version: 1.3.4
  date: "2019-12-28"
  fixed:
    - One more delegation warning happenning in do notation (flash-gordon)
- version: 1.3.3
  date: "2019-12-11"
  fixed:
    - Incompatibility with Rails. Internal (!) halt exceptions now use mutable backtraces
      because spring [mutates](https://github.com/rails/spring/blob/ee687859008e947bc905b95121e306e2948d31c9/lib/spring/application.rb#L295-L311)
      (!) them. For the record, this a bug in Rails (johnmaxwell)
- version: 1.3.2
  date: "2019-11-30"
  fixed:
    - Warnings about keywords from Ruby 2.7 (flash-gordon)
  added:
    - |-
      Pattern matching syntax was improved by implementing `#deconstruct_keys`. Now curly braces aren't necessary when the wrapped value is a Hash (flash-gordon)
        ```ruby
        case result
        in Success(code: 200...300) then :ok
        end
        ```
    - "## Internal"
    - Performance of do notation was improved for failing cases (1.2x to 1.3x on synthetic
      benchmarks) (flash-gordon)
- version: 1.3.1
  date: "2019-09-07"
  fixed:
    - "Added missing `None#maybe` :sweat_smile: (flash-gordon)"
- version: 1.3.0
  date: "2019-08-03"
  added:
    - |-
      `Result#either` (waiting-for-dev)
        ```ruby
        Success(1).either(-> x { x + 1 }, -> x { x + 2 }) # => 2
        Failure(1).either(-> x { x + 1 }, -> x { x + 2 }) # => 3
        ```
    - |-
      `Maybe#to_result` (SpyMachine + flash-gordon)
        ```ruby
        Some(3).to_result(:no_value)   # => Success(3)
        None().to_result { :no_value } # => Failure(:no_value)
        None().to_result               # => Failure()
        ```
    - |-
      Do notation can be used with `extend`. This simplifies usage in class methods and in other "complicated" cases (gogiel + flash-gordon)

        ```ruby
        class CreateUser
          extend Dry::Monads::Do::Mixin
          extend Dry::Monads[:result]

          def self.run(params)
            self.call do
              values = bind Validator.validate(params)
              user = bind UserRepository.create(values)

              Success(user)
            end
          end
        end
        ```

        Or you can bind values directly:

        ```ruby
        ma = Dry::Monads.Success(1)
        mb = Dry::Monads.Success(2)

        Dry::Monads::Do.() do
          a = Dry::Monads::Do.bind(ma)
          b = Dry::Monads::Do.bind(mb)

          Dry::Monads.Success(a + b)
        end
        ```
    - |-
      `{Some,Success,Failure}#[]` shortcuts for building arrays wrapped within monadic value (flash-gordon)
        ```ruby
        Success[1, 2] # => Success([1, 2])
        ```
    - |-
      `List.unfold` yields a block returning `Maybe<Any>`. If the block returns `Some(a)` `a` is appended to the output list. Returning `None` halts the unfloding (flash-gordon)

        ```ruby
        List.unfold(0) do |x|
          if x > 5
            None()
          else
            Some[x + 1, 2**x]
          end
        end # => List[1, 2, 3, 4, 5]
        ```
    - |-
      Experimental support for pattern matching! :tada: (flash-gordon)

        ```ruby
        case value
        in Failure(_) then :failure
        in Success(10) then :ten
        in Success(100..500 => code) then code
        in Success() then :empty
        in Success(:code, x) then x
        in Success[:status, x] then x
        in Success({ status: x }) then x
        in Success({ code: 200..300 => x }) then x
        end
        ```

        Read more about pattern matching in Ruby:

        - https://medium.com/@baweaver/ruby-2-7-pattern-matching-destructuring-on-point-90f56aaf7b4e
        - https://bugs.ruby-lang.org/issues/14912

        Keep in mind this feature is experimental and can be changed by 2.7 release. But it rocks already!
- version: 1.2.0
  date: "2019-01-12"
  added:
    - |-
      Most of constructors now have `call` alias so you can compose them with Procs nicely if you've switched to Ruby 2.6 (flash-gordon)
        ```ruby
        pipe = -> x { x.upcase } >> Success
        pipe.('foo') # => Success('FOO')
        ```
    - |-
      `List#collect` gathers `Some` values from the list (flash-gordon)

        ```ruby
        include Dry::Monads::List::Mixin
        include Dry::Monads::Maybe::Mixin
        # ...
        List[10, 5, 0].collect do |divisor|
          if divisor.zero?
            None()
          else
            Some(n / divisor)
          end
        end
        # => List[4, 2]
        ```

        Without block:

        ```ruby
        List[Some(5), None(), Some(3)].collect.map { |x| x * 2 }
        # => [10, 6]
        ```
    - |-
      Right-biased monads got `#flatten` and `#and` (falsh-gordon)

        `#flatten` removes one level of monadic structure, it's useful when you're dealing with things like `Maybe` of `Maybe` of something:

        ```ruby
        include Dry::Monads::Maybe::Mixin

        Some(Some(1)).flatten # => Some(1)
        Some(None()).flatten # => None
        None().flatten # => None
        ```

        In contrast to `Array#flatten`, dry-monads' version removes only 1 level of nesting, that is always acts as `Array#flatten(1)`:

        ```ruby
        Some(Some(Some(1))).flatten # => Some(Some(1))
        ```

        `#and` is handy for combining two monadic values and working with them at once:

        ```ruby
        include Dry::Monads::Maybe::Mixin

        # using block
        Some(5).and(Some(3)) { |x, y| x + y } # => Some(8)
        # without block
        Some(5).and(Some(3)) # => Some([5, 3])
        # other cases
        Some(5).and(None()) # => None()
        None().and(Some(5)) # => None()
        ```
    - |-
      Concise imports with `Dry::Monads.[]`. You're no longer required to require all desired monads and include them one-by-one, the `[]` method handles it for you (flash-gordon)

        ```ruby
        require 'dry/monads'

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

          def initialize(repo, send_email)
            @repo = repo
            @send_email = send_email
          end

          def call(name)
            if @repo.user_exist?(name)
              Failure(:user_exists)
            else
              user = yield @repo.add_user(name)
              yield @send_email.(user)
              Success(user)
            end
          end
        end
        ```
    - "`Task.failed` is a counterpart of `Task.pure`, accepts an exception and returns
      a failed task immediately (flash-gordon)"
- version: 1.1.0
  date: "2018-10-16"
  fixed:
    - Do notation was made to work nicely with inheritance. This shouldn't break any
      existing code but if it does please report (flash-gordon)
  added:
    - |-
      `Success()`, `Failure()`, and `Some()` now have `Unit` as a default argument:

        ```ruby
        include Dry::Monads::Result::Mixin
        include Dry::Monads::Do

        def call
          yield do_1
          yield do_2

          Success() # returns Success(Unit)
        end
        ```
- version: 1.0.1
  date: "2018-08-11"
  fixed:
    - Fixed behavior of `List<Validated>#traverse` in presence of `Valid` values (flash-gordon
      + SunnyMagadan)
  added:
    - |-
      `to_proc` was added to value constructors (flash-gordon)
        ```ruby
        [1, 2, 3].map(&Some) # => [Some(1), Some(2), Some(3)]
        ```
- version: 1.0.0
  date: "2018-06-26"
  added:
    - |-
      `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:

        ```ruby
        class CreateUser
          include Dry::Monads
          include Dry::Monads::Do.for(:call)

          attr_reader :user_repo

          def initialize(:user_repo)
            @user_repo = user_repo
          end

          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])

              Success(user)
            end
          end

          private

          def parse_json(params)
            Try[JSON::ParserError] {
              JSON.parse(params)
            }.to_result
          end

          def validate(json)
            UserSchema.(json).to_monad
          end

          def create_user(user_data)
            Try[Sequel::Error] { user_repo.create(user_data) }.to_result
          end

          def create_profile(user, profile_data)
            Try[Sequel::Error] {
              user_repo.create_profile(user, profile_data)
            }.to_result
          end
        end
        ```

        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](https://github.com/ruby-concurrency/concurrent-ruby/). `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](http://ruby-concurrency.github.io/concurrent-ruby/root/Concurrent.html#executor-class_method) for details. You can provide your own executor if needed (flash-gordon)

        ```ruby
        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
        end
        ```

        `Task` works perfectly with `Do`

        ```ruby
        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))
        end
        ```
    - "`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

        ```ruby
        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+

        ```ruby
          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 `Result`s 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.

        ```ruby
          include Dry::Monads
          include Dry::Monads::Do.for(:call)

          def call(input)
            name, email = yield [
              validate_name(input[:name]),
              validate_email(input[:email])
            ]

            Success(create(name, email))
          end

          # 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

        ```ruby
        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`.

        ```ruby
          # 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)
        ```
    - "## Deprecations"
    - "`Either`, the former name of `Result`, is now deprecated"
    - "## BREAKING CHANGES"
    - "`Either#value` and `Maybe#value` were both droped, use `value_or` or `value!`
      when you :100: 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)"
- version: 0.4.0
  date: "2017-11-11"
  changed:
    - The `Either` monad was renamed to `Result` which sounds less nerdy but better
      reflects the purpose of the type. `Either::Right` became `Result::Success` and
      `Either::Left` became `Result::Failure`. This change is backward-compatible overall
      but you will see the new names when using old `Left` and `Right` methods (citizen428)
    - Consequently, `Try::Success` and `Try::Failure` were renamed to `Try::Value` and
      `Try::Error` (flash-gordon)
  added:
    - "`Try#or`, works as `Result#or` (flash-gordon)"
    - "`Maybe#success?` and `Maybe#failure?` (aliases for `#some?` and `#none?`) (flash-gordon)"
    - "`Either#flip` inverts a `Result` value (flash-gordon)"
    - "`List#map` called without a block returns an `Enumerator` object (flash-gordon)"
    - |-
      Right-biased monads (`Maybe`, `Result`, and `Try`) now implement the `===` operator which is used for equality checks in the `case` statement (flash-gordon)
        ```ruby
          case value
          when Some(1..100)       then :ok
          when Some { |x| x < 0 } then :negative
          when Some(Integer)      then :invalid
          else raise TypeError
          end
        ```
    - "## Deprecated"
    - Direct accessing `value` on right-biased monads has been deprecated, use the `value!`
      method instead. `value!` will raise an exception if it is called on a Failure/None/Error
      instance (flash-gordon)
- version: 0.3.1
  date: "2017-03-18"
  fixed:
    - Fixed unexpected coercing to `Hash` on `.bind` call (flash-gordon)
- version: 0.3.0
  date: "2017-03-16"
  added:
    - Added `Either#either` that accepts two callbacks, runs the first if it is `Right`
      and the second otherwise (nkondratyev)
    - Added `#fmap2` and `#fmap3` for mapping over nested structures like `List Either`
      and `Either Some` (flash-gordon)
    - Added `Try#value_or` (dsounded)
    - Added the `List` monad which acts as an immutable `Array` and plays nice with
      other monads. A common example is a list of `Either`s (flash-gordon)
    - "`#bind` made to work with keyword arguments as extra parameters to the block
      (flash-gordon)"
    - Added `List#traverse` that "flips" the list with an embedded monad (flash-gordon
      + damncabbage)
    - Added `#tee` for all right-biased monads (flash-gordon)
- version: 0.2.1
  date: "2016-11-13"
  added:
    - Added `Either#tee` that is similar to `Object#tap` but executes the block only
      for `Right` instances (saverio-kantox)
  fixed:
    - "`Right(nil).to_maybe` now returns `None` with a warning instead of failing (orisaka)"
    - "`Some#value_or` doesn't require an argument because `None#value_or` doesn't require
      it either if a block was passed (flash-gordon)"
- version: 0.2.0
  date: "2016-09-18"
  added:
    - Added `Maybe#to_json` as an opt-in extension for serialization to JSON (rocknruby)
    - Added `Maybe#value_or` which returns you the underlying value with a fallback
      in a single method call (dsounded)
- version: 0.1.1
  date: "2016-08-25"
  fixed:
    - Added explicit requires of `dry-equalizer`. This allows to safely load only specific
      monads (artofhuman)
- version: 0.1.0
  date: "2016-08-23"
  added:
    - Support for passing extra arguments to the block in `.bind` and `.fmap` (flash-gordon)
  changed:
    - Dropped MRI 2.0 support (flash-gordon)
- version: 0.0.2
  date: "2016-06-29"
  added:
    - Added `Either#to_either` so that you can rely on duck-typing when you work with
      different types of monads (timriley)
    - Added `Maybe#to_maybe` for consistency with `#to_either` (flash-gordon)
- version: 0.0.1
  date: "2016-05-02"
  summary: Initial release containing `Either`, `Maybe`, and `Try` monads.
