dropEnd and takeEnd functions (#114)

* add takeEnd and dropEnd functions with docs

* add tests

* change order of functions

Author: notgiorgi

src/Data/Array.purs

@@ -79,8 +79,10 @@ module Data.Array
   , sortWith
   , slice
   , take
+  , takeEnd
   , takeWhile
   , drop
+  , dropEnd
   , dropWhile
   , span
   , group
@@ -495,6 +497,11 @@ foreign import slice :: forall a. Int -> Int -> Array a -> Array a
 -- | array.
 foreign import take :: forall a. Int -> Array a -> Array a
 
+-- | Keep only a number of elements from the end of an array, creating a new
+-- | array.
+takeEnd :: forall a. Int -> Array a -> Array a
+takeEnd n xs = drop (length xs - n) xs
+
 -- | Calculate the longest initial subarray for which all element satisfy the
 -- | specified predicate, creating a new array.
 takeWhile :: forall a. (a -> Boolean) -> Array a -> Array a
@@ -503,6 +510,10 @@ takeWhile p xs = (span p xs).init
 -- | Drop a number of elements from the start of an array, creating a new array.
 foreign import drop :: forall a. Int -> Array a -> Array a
 
+-- | Drop a number of elements from the start of an array, creating a new array.
+dropEnd :: forall a. Int -> Array a -> Array a
+dropEnd n xs = take (length xs - n) xs
+
 -- | Remove the longest initial subarray for which all element satisfy the
 -- | specified predicate, creating a new array.
 dropWhile :: forall a. (a -> Boolean) -> Array a -> Array a

test/Test/Data/Array.purs

@@ -251,6 +251,11 @@ testArray = do
   assert $ (A.takeWhile (_ /= 3) [1, 2, 3]) == [1, 2]
   assert $ (A.takeWhile (_ /= 1) nil) == nil
 
+  log "take should keep the specified number of items from the end of an array, discarding the rest"
+  assert $ (A.takeEnd 1 [1, 2, 3]) == [3]
+  assert $ (A.takeEnd 2 [1, 2, 3]) == [2, 3]
+  assert $ (A.takeEnd 1 nil) == nil
+
   log "drop should remove the specified number of items from the front of an array"
   assert $ (A.drop 1 [1, 2, 3]) == [2, 3]
   assert $ (A.drop 2 [1, 2, 3]) == [3]
@@ -261,6 +266,11 @@ testArray = do
   assert $ (A.dropWhile (_ /= 2) [1, 2, 3]) == [2, 3]
   assert $ (A.dropWhile (_ /= 1) nil) == nil
 
+  log "drop should remove the specified number of items from the end of an array"
+  assert $ (A.dropEnd 1 [1, 2, 3]) == [1, 2]
+  assert $ (A.dropEnd 2 [1, 2, 3]) == [1]
+  assert $ (A.dropEnd 1 nil) == nil
+
   log "take and drop should treat negative arguments as zero"
   assert $ (A.take (-2) [1, 2, 3]) == nil
   assert $ (A.drop (-2) [1, 2, 3]) == [1, 2, 3]