Hi Edward,
I would like to propose improving the Data instances for a number of currently completely opaque data types in the containers package, by using virtual constructors.
The instance for Data.Map already uses fromList for gfoldl, it just stops there.
Extending it to be able to gunfold and mention the name of that constructor would enable generic traversal libraries like uniplate, etc. to work over the contents of the Map, rather than bailing out in fear or crashing at the sight of a mkNoRepType.
An example of the changes for Data.Map are highlighted below.
instance (Data k, Data a, Ord k) => Data (Map k a) where gfoldl f z m = z fromList `f` toList m toConstr _ = fromListConstr gunfold k z c = case constrIndex c of 1 -> k (z fromList) _ -> error "gunfold" dataTypeOf _ = mapDataType dataCast2 f = gcast2 f
fromListConstr :: Constr fromListConstr = mkConstr mapDataType "fromList" [] Prefix
mapDataType :: DataType mapDataType = mkDataType "Data.Map.Map" [fromListConstr]
I've used this approach for years on my own libraries to great effect.
+1 here. I am not very familiar with the Data instances -- is it true that the parameter of the `fromList` in the Data instance will often be sorted (i.e., result of `toList` or `filter . toList`)? If so, we could use fromMaybeAscList which would look like fromMaybeAscList list | isDistinctAsc list = fromDistinctAscList list | otherwise = fromList list There is a big gain in using a linear-time fromDistinctAscList over O(N log N) fromList, but there is a linear-time check and the list must be kept around until isDistinctAsc finishes. Cheers, Milan