Pattern Synonyms and type families

ashokkimmel:

If you require an explicit annotation on the pattern, …

I like Adam’s type synonym trick. (Should have thought of it myself.) More to the point, I guess ‘promotable’ PattSyns are exactly those for which a type syn can be defined. Then …

• a bit of Template Haskell to make two declarations; or
• a bit of cpp.

In effect by declaring both, you’re saying: I hereby annotate the PattSyn to make it promotable.