test.hs

instance Monad f => Applicative (OptionalT f) where
  pure ::
    a
    -> OptionalT f a
  pure =
    OptionalT . pure . pure
  (<*>) ::
    OptionalT f (a -> b)
     -> OptionalT f a
     -> OptionalT f b
  (OptionalT f) <*> (OptionalT a) =
    OptionalT (lift2 (<*>) f a)

-- (<$>) for OptionalT: OK
-- (<*>) for OptionalT
--   one:               OK
--   two:               OK
--   three:             OK
--   four:              OK
--   five:              FAIL
--     test/Course/StateTTest.hs:133:
--     expected: [Empty]
--      but got: [Empty,Empty]
--   six:               FAIL
--     test/Course/StateTTest.hs:136:
--     expected: [Full 2,Full 3,Empty]
--      but got: [Full 2,Full 3,Empty,Empty]
--   seven:             OK