The unit test definition seems to be out of date with the version of HSpec used:

/tmp/haskell11634-20-12vowd8/SimpleInteractiveInterpreter/Test.hs:40:20:
    Ambiguous occurrence `pendingWith'
    It could refer to either `SimpleInteractiveInterpreter.Test.pendingWith',
                             defined at /tmp/haskell11634-20-12vowd8/SimpleInteractiveInterpreter/Test.hs:71:1
                          or `Test.Hspec.pendingWith',
                             imported from `Test.Hspec' at /tmp/haskell11634-20-12vowd8/SimpleInteractiveInterpreter/Test.hs:4:1-17
                             (and originally defined in `hspec-1.12.1:Test.Hspec.Core.Type')

The unit test definition seems to be out of date with the version of HSpec used:

/tmp/haskell11634-20-12vowd8/SimpleInteractiveInterpreter/Test.hs:40:20:
    Ambiguous occurrence `pendingWith'
    It could refer to either `SimpleInteractiveInterpreter.Test.pendingWith',
                             defined at /tmp/haskell11634-20-12vowd8/SimpleInteractiveInterpreter/Test.hs:71:1
                          or `Test.Hspec.pendingWith',
                             imported from `Test.Hspec' at /tmp/haskell11634-20-12vowd8/SimpleInteractiveInterpreter/Test.hs:4:1-17
                             (and originally defined in `hspec-1.12.1:Test.Hspec.Core.Type')

I think it's way too easy. Just playing type tetris / cat-programming ("if it fits, it sits") can be used to solve it in a couple minutes without actually having to think about it.

However, one could add a remark about closed type families and point to external resources, as long as the GHC version stays 7.6.x.

Well, Codewars still uses GHC 7.6.x. That settles it. (Probably 7.6.3, since that one is in Ubuntu 14.04 LTS and similar LTS distributions)

Closed type families were introduced in 7.8.1.

I think when I made the kata that they were still using GHC 7.6 which doesn't have support for closed type families.

Which GHC version introduced closed type families?

The fact that it doesn't use (and doesn't even seem to allow!) closed type families is potentially teaching an obsolete pattern.