Add difference for Map

src/Data/Map.purs

@@ -5,7 +5,7 @@ module Data.Map
 
 import Prelude
 
-import Data.Map.Internal (Map, alter, checkValid, delete, empty, filter, filterKeys, filterWithKey, findMax, findMin, foldSubmap, fromFoldable, fromFoldableWith, insert, isEmpty, isSubmap, lookup, lookupGE, lookupGT, lookupLE, lookupLT, member, pop, showTree, singleton, size, submap, toUnfoldable, toUnfoldableUnordered, union, unionWith, unions, update, values)
+import Data.Map.Internal (Map, alter, checkValid, delete, empty, filter, filterKeys, filterWithKey, findMax, findMin, foldSubmap, fromFoldable, fromFoldableWith, insert, isEmpty, isSubmap, lookup, lookupGE, lookupGT, lookupLE, lookupLT, member, pop, showTree, singleton, size, submap, toUnfoldable, toUnfoldableUnordered, union, unionWith, unions, difference, update, values)
 import Data.Set (Set)
 import Unsafe.Coerce (unsafeCoerce)
 

src/Data/Map/Internal.purs

@@ -32,6 +32,7 @@ module Data.Map.Internal
   , union
   , unionWith
   , unions
+  , difference
   , isSubmap
   , size
   , filterWithKey
@@ -612,6 +613,11 @@ union = unionWith const
 unions :: forall k v f. Ord k => Foldable f => f (Map k v) -> Map k v
 unions = foldl union empty
 
+-- | Difference of two maps. Return elements of the first map where
+-- | the keys do not exist in the second map.
+difference :: forall k v w. Ord k => Map k v -> Map k w -> Map k v
+difference m1 m2 = foldl (flip delete) m1 (keys m2)
+
 -- | Test whether one map contains all of the keys and values contained in another map
 isSubmap :: forall k v. Ord k => Eq v => Map k v -> Map k v -> Boolean
 isSubmap m1 m2 = LL.all f $ (toUnfoldable m1 :: LL.List (Tuple k v))

test/Test/Data/Map.purs

@@ -4,7 +4,7 @@ import Prelude
 
 import Control.Alt ((<|>))
 import Data.Array as A
-import Data.Foldable (foldl, for_, all)
+import Data.Foldable (foldl, for_, all, and)
 import Data.FoldableWithIndex (foldrWithIndex)
 import Data.Function (on)
 import Data.FunctorWithIndex (mapWithIndex)
@@ -215,6 +215,12 @@ mapTests = do
           Just v              -> Just v == v2
           Nothing             -> not (in1 || in2)
 
+  log "difference"
+  quickCheck $ \(TestMap m1) (TestMap m2) ->
+    let d = M.difference (m1 :: M.Map SmallKey Int) (m2 :: M.Map SmallKey String)
+    in and (map (\k -> M.member k m1) (A.fromFoldable $ M.keys d)) &&
+       and (map (\k -> not $ M.member k d) (A.fromFoldable $ M.keys m2))
+
   log "size"
   quickCheck $ \xs ->
     let xs' = nubBy ((==) `on` fst) xs