Ticket #5894 (closed feature request: invalid)
Add generalization of Data.Map.unionWith with (a -> b -> c) as the combining function
The signature of Data.Map.unionWith is:
unionWith :: Ord k => (a -> a -> a) -> Map k a -> Map k a -> Map k a
intersectionWith, on the other hand, has a more general signature:
intersectionWith :: Ord k => (a -> b -> c) -> Map k a -> Map k b -> Map k c
The reason the combining function in unionWith is constrained to (a -> a -> a) is because of the possibility of values in one map not present in the other. unionWith only applies the combining function for values present in both maps; it uses existing values for the rest.
I'd like to be able to write this function:
-- | Combine two maps. Substitute 'Nothing' for any value present in only one -- of the maps. unionMaybes :: Ord k => Map k a -> Map k b -> Map k (Maybe a, Maybe b)
However, it's awkward with unionWith:
unionMaybes left right = Map.unionWith combine (Map.map mkLeft left) (Map.map mkRight right) where mkLeft l = (Just l, Nothing) mkRight r = (Nothing, Just r) combine (Just l, Nothing) (Nothing, Just r) = (Just l, Just r) combine _ _ = error "Shouldn't happen"
It'd be nicer to have a generalization of unionWith that can transform values only present in one of the maps. Here is such a function, implemented in terms of unionWith:
-- | Like 'Map.unionWith', but apply a function even when a key is present in -- only one of the maps. -- -- @'Map.unionWith' = 'combine' 'id' 'id'@ combine :: Ord k => (a -> c) -- ^ For keys only present in the left map -> (b -> c) -- ^ For keys only present in the right map -> (a -> b -> c) -- ^ For keys present in both maps -> Map k a -- ^ Left map -> Map k b -- ^ Right map -> Map k c combine onLeft onRight onBoth left right = Map.map apply $ Map.unionWith combine_ (Map.map LeftOnly left) (Map.map RightOnly right) where combine_ (LeftOnly l) (RightOnly r) = LeftRight l r combine_ _ _ = error "combine: internal error (Data.Map.unionWith is not combining values correctly)" apply (LeftOnly l) = onLeft l apply (RightOnly r) = onRight r apply (LeftRight l r) = onBoth l r data Combine l r = LeftOnly l | RightOnly r | LeftRight l r
This way, unionMaybes can be written as:
unionMaybes :: Ord k => Map k a -> Map k b -> Map k (Maybe a, Maybe b) unionMaybes = combine (\l -> (Just l, Nothing)) (\r -> (Nothing, Just r)) (\l r -> (Just l, Just r))