Simple utility for COMPLETE pragmas

[treeowl]

I dunno if it belongs here, but COMPLETE pragmas are a bit painful to write by hand for types with many constructors, especially when constructors can be added or removed from time to time. Using th-abstraction, it's easy to fix this. Without th-abstraction, I don't wanna.

import Language.Haskell.TH.Datatype (DatatypeInfo (..), ConstructorInfo(..), reifyDatatype)
import Language.Haskell.TH (Q, Dec, Name, pragCompleteD)
import Data.List ((\\))

-- | Produce a @COMPLETE@ pragma for a type with many constructors,
-- without having to list them all out.
--
-- @completeWithButWithout ''T ['P] ['C1, 'C2]@ produces a @COMPLETE@
-- pragma stating that pattern matching on the type @T@ is complete with
-- with the pattern @P@ and with all the constructors of @T@ other than
-- @C1@ and @C2@.
completeWithButWithout :: Name -> [Name] -> [Name] -> Q [Dec]
completeWithButWithout ty extra_patterns excl_constrs = do
  di <- reifyDatatype ty
  let constrs = map constructorName (datatypeCons di)
  (:[]) <$> pragCompleteD (extra_patterns ++ (constrs \\ excl_constrs))
                          (Just ty)

For truly enormous datatypes, (\\) could be swapped out in favor of something that uses the Ord Name instance.

[mgsloan]

Yeah, seems useful! I'd merge a PR adding such a thing. I suppose putting it in its own module makes sense as TH.Utilities is a grab-bag of utilities useful for writing TH code