GHC Trac user nimnul reports in
https://ghc.haskell.org/trac/ghc/ticket/10540:

  Hoopl exports

    showGraph :: Showing n -> Graph n e x -> String

  But it doesn't export Showing. This doesn't make sense, as it's a
  user-supplied function. So to know what I should pass to showGraph I can only
  look at the source. :i Showing doesn't work.

  Below is a patch to export the innocent type synonym: