module Concurrent

  RSpec.describe Maybe do

    context 'construction' do

      it 'hides Maybe.new' do
        expect {
          Maybe.new
        }.to raise_error(NoMethodError)
      end

      context 'Maybe.from' do

        it 'raises an exception when no block is given' do
          expect {
            Maybe.from
          }.to raise_error(ArgumentError)
        end

        it 'passes all arguments to the block' do
          expected = [1, 2, 3]
          actual = nil
          Maybe.from(*expected) do |*args|
            actual = args
          end
          expect(actual).to eq expected
        end

        it 'creates a Just Maybe on success' do
          maybe = Maybe.from{ 42 }
          expect(maybe).to be_just
        end

        it 'sets the value to the block result on success' do
          maybe = Maybe.from{ 42 }
          expect(maybe.just).to eq 42
        end

        it 'creates a Nothing Maybe on exception' do
          maybe = Maybe.from{ raise StandardError.new }
          expect(maybe).to be_nothing
        end

        it 'sets the reason to the error object on exception' do
          ex = StandardError.new
          maybe = Maybe.from{ raise ex }
          expect(maybe.nothing).to eq ex
        end
      end

      context 'Maybe.just' do

        let!(:value) { 42 }
        subject { Maybe.just(value) }

        it 'creates a new  Just Maybe' do
          expect(subject).to be_a Maybe
          expect(subject).to be_just
        end
      end

      context 'Maybe.nothing' do

        let!(:error) { StandardError.new }
        subject { Maybe.nothing(error) }

        it 'creates a new Nothing Maybe' do
          expect(subject).to be_a Maybe
          expect(subject).to be_nothing
        end

        it 'uses the given Error object' do
          error = NoMethodError.new
          maybe = Maybe.nothing(error)
          expect(maybe.nothing).to be error
        end

        it 'creates a new error object with the given string' do
          message = 'What do you get when you multiply six by nine?'
          maybe = Maybe.nothing(message)
          expect(maybe.nothing).to be_a StandardError
          expect(maybe.nothing.message).to eq message
        end

        it 'creates a new error object when given nothing' do
          maybe = Maybe.nothing
          expect(maybe.nothing).to be_a StandardError
          expect(maybe.nothing.message).to be_empty
        end
      end
    end

    context 'when just' do

      let!(:value) { 42 }
      subject { Maybe.just(value) }

      specify '#just? returns true' do
        expect(subject).to be_just
      end

      specify '#fulfilled? returns true' do
        expect(subject).to be_fulfilled
      end

      specify '#nothing? returns false' do
        expect(subject).to_not be_nothing
      end

      specify '#rejected? returns false' do
        expect(subject).to_not be_rejected
      end

      specify '#just returns the value' do
        expect(subject.just).to eq value
      end

      specify '#value returns the value' do
        expect(subject.value).to eq value
      end

      specify '#nothing returns NONE' do
        expect(subject.nothing).to be Maybe::NONE
      end

      specify '#reason returns NONE' do
        expect(subject.reason).to be Maybe::NONE
      end
    end

    context 'when nothing' do

      let!(:error) { StandardError.new }
      subject { Maybe.nothing(error) }

      specify '#just? returns false' do
        expect(subject).to_not be_just
      end

      specify '#fulfilled? returns false' do
        expect(subject).to_not be_fulfilled
      end

      specify '#nothing? returns true' do
        expect(subject).to be_nothing
      end

      specify '#rejected? returns true' do
        expect(subject).to be_rejected
      end

      specify '#just returns NONE' do
        expect(subject.just).to eq Maybe::NONE
      end

      specify '#value returns NONE' do
        expect(subject.value).to eq Maybe::NONE
      end

      specify '#nothing returns the raised error' do
        expect(subject.nothing).to be error
      end

      specify '#reason returns the raised error' do
        expect(subject.reason).to be error
      end
    end

    context 'comparison' do

      let!(:something_big)   { Maybe.just(42) }
      let!(:something_small) { Maybe.just(1) }
      let!(:nothing_big)     { Maybe.nothing(StandardError.new) }
      let!(:nothing_small)   { Maybe.nothing(ArgumentError.new) }

      specify 'something is not equal to nothing' do
        expect(something_big).to_not eq nothing_big
      end

      specify 'nothing is equal to nothing' do
        expect(nothing_big).to eq nothing_small
      end

      specify 'something is equal to the same value' do
        first = Maybe.just(42)
        second = Maybe.just(42)
        expect(first).to eq second
      end

      specify 'something is not equal to a different value' do
        expect(something_big).to_not eq something_small
      end

      specify 'something is greater than a smaller value' do
        expect(something_big).to be > something_small
        expect(something_big).to be >= something_small
      end

      specify 'something is less than a bigger value' do
        expect(something_small).to be < something_big
        expect(something_small).to be <= something_big
      end

      specify 'nothing is not less than nothing' do
        expect(nothing_small).to_not be < nothing_big
      end

      specify 'nothing is not greater than nothing' do
        expect(nothing_big).to_not be > nothing_small
      end
    end

    context '#or' do

      it 'returns the value when something' do
        maybe = Maybe.just(42)
        actual = maybe.or(100)
        expect(actual).to eq 42
      end

      it 'returns the other when nothing' do
        maybe = Maybe.nothing
        actual = maybe.or(100)
        expect(actual).to eq 100
      end
    end
  end
end
