union maps, reject and collect duplicate keys

Example.hs

module Example where

import Data.Map
import Data.Validation
import Data.List.NonEmpty (NonEmpty((:|)))

-- | Merge maps rejecting duplicate keys, which are collected in the 'Failure' value
--
-- >>> awsm (fromList [('a', 1)]) (fromList [('b', 2)])
-- Success (fromList [('a',1),('b',2)])
-- >>> awsm (fromList [('a', 1)]) (fromList [('b', 2), ('a', 3)])
-- Failure ('a' :| "")
-- >>> awsm (fromList [('a', 1), ('b', 4)]) (fromList [('b', 2), ('a', 3)])
-- Failure ('a' :| "b")
--
awsm :: Ord k => Map k v -> Map k v -> Validation (NonEmpty k) (Map k v)
awsm m1 m2 = sequenceA $ unionWithKey (\k _ _ -> Failure (k:|[])) (Success <$> m1) (Success <$> m2)