Run the test failed with doctest

[Nero Zuo]

Hello,

I encountered a problem when running the test failed with doctest 

$ 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

My computer system is OS X EL Capitan 10.11.6   

GHC version is 8.0.1 

By the way my Ubuntu 16.10 computer works well and its GHC version is 7.10.3

Can anyone help me?   

Thanks

[Zak Patterson]

Hi, 

I just got started here yesterday. The workaround for me right now is to add the option `-fno-warn-orphans` to the doctest command.  I'm using GHC 8.0.2 now. 

Added an issue: https://github.com/data61/fp-course/issues/184

-Zak

[Tony Morris]

Hi Nero,
This is a known problem. Sorry for taking so long to get back to you. We have just now started working toward correcting it. The solution will likely involve removing doctest altogether.