Why is Bag's Data instance "broken"?

[Philip Holzenspies]

Dear GHCers,

I'm performing traversals over GHC-API results (HsSyn et al). For this purpose, I'm using SYB generics.

I found that I couldn't use "ext1Q" for a function with type "Data x => Bag x -> String", i.e. that this function was never applied. The source of Bag's instance of the Data class seems to explain why:


instance Data a => Data (Bag a) where
  gfoldl k z b = z listToBag `k` bagToList b -- traverse abstract type abstractly
  toConstr _   = abstractConstr $ "Bag("++show (typeOf (undefined::a))++")"
  gunfold _ _  = error "gunfold"
  dataTypeOf _ = mkNoRepType "Bag"


Is there a rationale to not allow gunfolds and to keep toConstr abstract? More to the point for my needs, is there a reason to not allow dataCast1 casting of Bags?

Regards,
Philip