On Sun, Aug 7, 2016 at 3:31 PM, Henning Thielemann
If I import a package (containers) with functions that are only available on GHC then I need to insert conditionals for whether I am running on GHC or not.
This is true. Unfortunately, the safe, relatively pleasant, relatively comprehensible, and relatively general API this function offers simply can't be supported without GADTs. It would be possible to retain safety and generality in a potentially-more-portable fashion using rank 2 types, but the API is unlikely to be pleasant or easy to understand. The alternatives are to use mergeWithKey, which is unpleasant, difficult to understand, and unsafe, or to export four functions instead of one. I don't particularly care for either of those options. If you are more concerned about portability than safety or ease of understanding, you can always use mergeWithKey.