Preprocessing library course-0.1.4...
[26 of 27] Compiling Course.StateT    ( src/Course/StateT.hs, dist/build/Course/StateT.o )

src/Course/StateT.hs:232:18-23:
    Couldn't match expected type ‘a’ with actual type ‘f0 a’
      ‘a’ is a rigid type variable bound by
          the type signature for pure :: a -> OptionalT f a
          at src/Course/StateT.hs:232:3
    Relevant bindings include
      x :: a (bound at src/Course/StateT.hs:232:8)
      pure :: a -> OptionalT f a (bound at src/Course/StateT.hs:232:3)
    In the first argument of ‘pure’, namely ‘(pure x)’
    In the expression: pure (pure x)