Issue when running test: Failure in src/Course/List.hs:28: expression `instance Arbitrary a => Arbi

[cd]

Running doctests without any changes to Course.List with ghc 7.10.1 on OSX, I get:

### Failure in src/Course/List.hs:28: expression `instance Arbitrary a => Arbitrary (List a) where arbitrary = P.fmap (P.foldr (:.) Nil) arbitrary'

expected:

 but got:

          <interactive>:26:70:

              Could not deduce (P.Foldable t0) arising from a use of ‘P.foldr’

              from the context (Arbitrary a)

                bound by the instance declaration at <interactive>:26:10-42

              The type variable ‘t0’ is ambiguous

              Note: there are several potential instances:

                instance P.Foldable (A.Const m) -- Defined in ‘Control.Applicative’

                instance P.Foldable (Either a) -- Defined in ‘Data.Foldable’

                instance P.Foldable Data.Proxy.Proxy -- Defined in ‘Data.Foldable’

                ...plus five others

              In the first argument of ‘P.fmap’, namely ‘(P.foldr (:.) Nil)’

              In the expression: P.fmap (P.foldr (:.) Nil) arbitrary

              In an equation for ‘arbitrary’:

                  arbitrary = P.fmap (P.foldr (:.) Nil) arbitrary

         

          <interactive>:26:88:

              Could not deduce (Arbitrary (t0 a))

                arising from a use of ‘arbitrary’

              from the context (Arbitrary a)

                bound by the instance declaration at <interactive>:26:10-42

              The type variable ‘t0’ is ambiguous

              Relevant bindings include

                arbitrary :: Gen (List a) (bound at <interactive>:26:50)

              Note: there are several potential instances:

                instance [overlap ok] [safe] (P.RealFloat a, Arbitrary a) =>

                                             Arbitrary (Data.Complex.Complex a)

                  -- Defined in ‘Test.QuickCheck.Arbitrary’

                instance [overlap ok] [safe] (Arbitrary a, Arbitrary b) =>

                                             Arbitrary (Either a b)

                  -- Defined in ‘Test.QuickCheck.Arbitrary’

                instance [overlap ok] [safe] Data.Fixed.HasResolution a =>

                                             Arbitrary (Data.Fixed.Fixed a)

                  -- Defined in ‘Test.QuickCheck.Arbitrary’

                ...plus 9 others

              In the second argument of ‘P.fmap’, namely ‘arbitrary’

              In the expression: P.fmap (P.foldr (:.) Nil) arbitrary

              In an equation for ‘arbitrary’:

                  arbitrary = P.fmap (P.foldr (:.) Nil) arbitrary