$ doctest -isrc -Wall -fno-warn-type-defaults src/Course/List.hs
### Failure in src/Course/List.hs:29: expression `instance Arbitrary a => Arbitrary (List a) where arbitrary = P.fmap ((P.foldr (:.) Nil) :: ([a] -> List a)) arbitrary'
expected:
but got:
<interactive>:29:1: warning: [-Worphans]
Orphan instance: instance Arbitrary a => Arbitrary (List a)
To avoid this
move the instance declaration to the module of the class or of the type, or
wrap the type with a newtype and declare the instance on the new type.
Examples: 58 Tried: 4 Errors: 0 Failures: 1