Add Map.catMaybes function (#25)

* Add catMaybes function

* Add catMaybes to Set

Author: drewolson

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, fromFoldableWithIndex, insert, insertWith, isEmpty, isSubmap, lookup, lookupGE, lookupGT, lookupLE, lookupLT, member, pop, showTree, singleton, size, submap, toUnfoldable, toUnfoldableUnordered, union, unionWith, unions, intersection, intersectionWith, difference, update, values, mapMaybeWithKey, mapMaybe)
+import Data.Map.Internal (Map, alter, catMaybes, checkValid, delete, empty, filter, filterKeys, filterWithKey, findMax, findMin, foldSubmap, fromFoldable, fromFoldableWith, fromFoldableWithIndex, insert, insertWith, isEmpty, isSubmap, lookup, lookupGE, lookupGT, lookupLE, lookupLT, member, pop, showTree, singleton, size, submap, toUnfoldable, toUnfoldableUnordered, union, unionWith, unions, intersection, intersectionWith, difference, update, values, mapMaybeWithKey, mapMaybe)
 import Data.Set (Set)
 import Unsafe.Coerce (unsafeCoerce)
 

src/Data/Map/Internal.purs

@@ -44,6 +44,7 @@ module Data.Map.Internal
   , filter
   , mapMaybeWithKey
   , mapMaybe
+  , catMaybes
   ) where
 
 import Prelude
@@ -689,3 +690,8 @@ mapMaybeWithKey f = foldrWithIndex (\k a acc → maybe acc (\b -> insert k b acc
 -- | function returns `Nothing`.
 mapMaybe :: forall k a b. Ord k => (a -> Maybe b) -> Map k a -> Map k b
 mapMaybe = mapMaybeWithKey <<< const
+
+-- | Filter a map of optional values, keeping only the key/value pairs which
+-- | contain a value, creating a new map.
+catMaybes :: forall k v. Ord k => Map k (Maybe v) -> Map k v
+catMaybes = mapMaybe identity

src/Data/Set.purs

@@ -26,6 +26,7 @@ module Data.Set
   , intersection
   , filter
   , mapMaybe
+  , catMaybes
   ) where
 
 import Prelude hiding (map)
@@ -190,3 +191,7 @@ filter f (Set s) = Set (M.filterWithKey (\k _ -> f k) s)
 -- | function returns `Nothing`.
 mapMaybe :: forall a b. Ord b => (a -> Maybe b) -> Set a -> Set b
 mapMaybe f = foldr (\a acc -> maybe acc (\b -> insert b acc) (f a)) empty
+
+-- | Filter a set of optional values, discarding values that contain `Nothing`
+catMaybes :: forall a. Ord a => Set (Maybe a) -> Set a
+catMaybes = mapMaybe identity

test/Test/Data/Map.purs

@@ -18,7 +18,7 @@ import Data.Tuple (Tuple(..), fst, uncurry)
 import Effect (Effect)
 import Effect.Console (log)
 import Partial.Unsafe (unsafePartial)
-import Test.QuickCheck ((<?>), (===), quickCheck, quickCheck')
+import Test.QuickCheck ((<?>), (<=?), (===), quickCheck, quickCheck')
 import Test.QuickCheck.Arbitrary (class Arbitrary, arbitrary)
 import Test.QuickCheck.Gen (elements, oneOf)
 
@@ -370,3 +370,15 @@ mapTests = do
   quickCheck \(TestMap m :: TestMap Int Int) ->
     let outList = foldrWithIndex (\i a b -> (Tuple i a) : b) Nil m
     in outList == sort outList
+
+  log "catMaybes creates a new map of size less than or equal to the original"
+  quickCheck \(TestMap m :: TestMap Int (Maybe Int)) -> do
+    let result = M.catMaybes m
+    M.size result <=? M.size m
+
+  log "catMaybes drops key/value pairs with Nothing values"
+  quickCheck \(TestMap m :: TestMap Int Int) -> do
+    let maybeMap = M.alter (const $ Just Nothing) 1 $ map Just m
+    let result = M.catMaybes maybeMap
+    let expected = M.delete 1 m
+    result === expected

test/Test/Data/Set.purs

@@ -2,6 +2,7 @@ module Test.Data.Set where
 
 import Prelude
 
+import Data.Maybe (Maybe(..))
 import Data.Set (Set)
 import Data.Set as S
 import Effect (Effect)
@@ -25,3 +26,8 @@ setTests = do
          s2 = S.fromFoldable [2,4,6,8,10]
          s3 = S.fromFoldable [2,4]
      assert $ S.intersection s1 s2 == s3
+
+  log "catMaybes - drops Nothing values"
+  do let s1 = S.fromFoldable [Just 1,Just 2,Just 3,Nothing]
+         s2 = S.fromFoldable [1,2,3]
+     assert $ S.catMaybes s1 == s2