DoNotation.rb

class DoThing
  include Dry::Monads::Result::Mixin
  include Dry::Monads::Do.for(:call)

  Context = Struct.new(:user, :params, :accumulator)
  Result  = Struct.new(:output)
  Error   = Struct.new(:step, :errors)

  def call(input)
    context = yield railway(Context.new(input[:user], input[:params], {}))

    context = yield set_one(context)
    context = yield set_two(context)

    result = Result.new(context.accumulator[:some_value])

    Success(result)
  end

  private

  def set_one(context)
    operation_one(context).bind do |v|
      operation_two(v)
    end do |m|
      m.success { |result| Success(result) }
      m.failure { |error|  Success(recover_one(error)) }
    end
  end

  def set_two(context)
    operation_three(context).bind do |v|
      operation_four(v)
    end do |m|
      m.success { |result| Success(result) }
      m.failure { |error|  Success(recover_two(error)) }
    end
  end

  def operation_one(context)
    # ...
  end

  def operation_two(context)
    # ...
  end

  def operation_three(context)
    # ...
  end

  def operation_four(context)
    # ...
  end

  def recover_one(error)
    # Success() with some recovery
  end
end