Question about OptionalT Applicative instance

[Michael Adlai Arnold]

Hi!

In Course/StateT, we are asked to implement an Applicative instance for OptionalT. I initially wrote the following:

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(OptionalT f) <*> (OptionalT o) = OptionalT $ do
     o' <- o
     f' <- f
     pure (f' <*> o')

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This works for the first few tests, but fails eventually. One of the failed tests:

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testCase "six" $

      let ot = OptionalT (Full (+1) :. Empty :. Nil) <*> OptionalT (Full 1 :. Full 2 :. Nil)

       in runOptionalT ot @?= (Full 2 :. Full 3 :. Empty :. Nil

FAILED: 'StateT.(<*>) for OptionalT.six'

  "Expected [Full 2,Full 3,Empty] but got [Full 2,Empty,Full 3,Empty]"

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At first, I suspected this had to do with the difference between the two "modes" of Applicative on List: zip vs cartesian. But changing the Applicative instance for List in Course/Applicative--which I verified by reloading all loaded modules and running code that exercised it specifically in ghci--had no effect on the test failures. 

Could someone explain why my solution doesn't work? Am I on the right track re: List Applicative?