Why is Bag's Data instance "broken"?
pkfh at st-andrews.ac.uk
Wed Aug 29 13:01:57 CEST 2012
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?
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