Add union, unionBy

Author: garyb

docs/Data.Array.md

@@ -443,6 +443,25 @@ nubBy :: forall a. (a -> a -> Boolean) -> Array a -> Array a
 Remove the duplicates from an array, where element equality is determined
 by the specified equivalence relation, creating a new array.
 
+#### `union`
+
+``` purescript
+union :: forall a. (Eq a) => Array a -> Array a -> Array a
+```
+
+Calculate the union of two lists.
+
+Running time: `O(n^2)`
+
+#### `unionBy`
+
+``` purescript
+unionBy :: forall a. (a -> a -> Boolean) -> Array a -> Array a -> Array a
+```
+
+Calculate the union of two arrays, using the specified function to
+determine equality of elements.
+
 #### `delete`
 
 ``` purescript

src/Data/Array.purs

@@ -61,8 +61,8 @@ module Data.Array
 
   , nub
   , nubBy
-  -- , union
-  -- , unionBy
+  , union
+  , unionBy
   , delete
   , deleteBy
 
@@ -84,6 +84,7 @@ import Control.Alternative (Alternative)
 import Control.Lazy (Lazy, defer)
 import Control.MonadPlus (MonadPlus)
 import Control.Plus (Plus)
+import Data.Foldable (foldl)
 import Data.Functor.Invariant (Invariant, imapF)
 import Data.Maybe (Maybe(..), maybe, isJust)
 import Data.Monoid (Monoid, mempty)
@@ -466,6 +467,17 @@ nubBy _ [] = []
 nubBy eq xs = case uncons xs of
                 Just o -> o.head : nubBy eq (filter (\y -> not (o.head `eq` y)) o.tail)
 
+-- | Calculate the union of two lists.
+-- |
+-- | Running time: `O(n^2)`
+union :: forall a. (Eq a) => Array a -> Array a -> Array a
+union = unionBy (==)
+
+-- | Calculate the union of two arrays, using the specified function to
+-- | determine equality of elements.
+unionBy :: forall a. (a -> a -> Boolean) -> Array a -> Array a -> Array a
+unionBy eq xs ys = xs ++ foldl (flip (deleteBy eq)) (nubBy eq ys) xs
+
 -- | Delete the first element of an array which is equal to the specified value,
 -- | creating a new array.
 delete :: forall a. (Eq a) => a -> Array a -> Array a

test/Test/Data/Array.purs

@@ -236,6 +236,13 @@ testArray = do
   let nubPred = \x y -> if odd x then false else x == y
   assert $ nubBy nubPred [1, 2, 2, 3, 3, 4, 4, 1] == [1, 2, 3, 3, 4, 1]
 
+  log "union should produce the union of two arrays"
+  assert $ union [1, 2, 3] [2, 3, 4] == [1, 2, 3, 4]
+  assert $ union [1, 1, 2, 3] [2, 3, 4] == [1, 1, 2, 3, 4]
+
+  log "unionBy should produce the union of two arrays using the specified equality relation"
+  assert $ unionBy (\_ y -> y < 5) [1, 2, 3] [2, 3, 4, 5, 6] == [1, 2, 3, 5, 6]
+
   log "delete should remove the first matching item from an array"
   assert $ delete 1 [1, 2, 1] == [2, 1]
   assert $ delete 2 [1, 2, 1] == [1, 1]