Add dropEnd and takeEnd (#130)

* added takeEnd and dropEnd

* added tests

* fix the docs

* fix test labels

* fix test message

Author: notgiorgi

src/Data/List.purs

@@ -60,8 +60,10 @@ module Data.List
   , stripPrefix
   , slice
   , take
+  , takeEnd
   , takeWhile
   , drop
+  , dropEnd
   , dropWhile
   , span
   , group
@@ -519,6 +521,13 @@ take = go Nil
   go acc _ Nil = reverse acc
   go acc n (x : xs) = go (x : acc) (n - 1) xs
 
+-- | Take the specified number of elements from the end of a list.
+-- |
+-- | Running time: `O(2n - m)` where `n` is the number of elements in list
+-- | and `m` is number of elements to take.
+takeEnd :: forall a. Int -> List a -> List a
+takeEnd n xs = drop (length xs - n) xs
+
 -- | Take those elements from the front of a list which match a predicate.
 -- |
 -- | Running time (worst case): `O(n)`
@@ -536,6 +545,13 @@ drop 0 xs = xs
 drop _ Nil = Nil
 drop n (x : xs) = drop (n - 1) xs
 
+-- | Drop the specified number of elements from the end of a list.
+-- |
+-- | Running time: `O(2n - m)` where `n` is the number of elements in list
+-- | and `m` is number of elements to drop.
+dropEnd :: forall a. Int -> List a -> List a
+dropEnd n xs = take (length xs - n) xs
+
 -- | Drop those elements from the front of a list which match a predicate.
 -- |
 -- | Running time (worst case): `O(n)`

test/Test/Data/List.purs

@@ -5,7 +5,7 @@ import Data.List.NonEmpty as NEL
 import Control.Monad.Eff (Eff)
 import Control.Monad.Eff.Console (CONSOLE, log)
 import Data.Foldable (foldMap, foldl)
-import Data.List (List(..), (..), stripPrefix, Pattern(..), length, range, foldM, unzip, zip, zipWithA, zipWith, intersectBy, intersect, (\\), deleteBy, delete, unionBy, union, nubBy, nub, groupBy, group', group, partition, span, dropWhile, drop, takeWhile, take, sortBy, sort, catMaybes, mapMaybe, filterM, filter, concat, concatMap, reverse, alterAt, modifyAt, updateAt, deleteAt, insertAt, findLastIndex, findIndex, elemLastIndex, elemIndex, (!!), uncons, unsnoc, init, tail, last, head, insertBy, insert, snoc, null, singleton, fromFoldable, transpose, mapWithIndex, (:))
+import Data.List (List(..), (..), stripPrefix, Pattern(..), length, range, foldM, unzip, zip, zipWithA, zipWith, intersectBy, intersect, (\\), deleteBy, delete, unionBy, union, nubBy, nub, groupBy, group', group, partition, span, dropWhile, drop, dropEnd, takeWhile, take, takeEnd, sortBy, sort, catMaybes, mapMaybe, filterM, filter, concat, concatMap, reverse, alterAt, modifyAt, updateAt, deleteAt, insertAt, findLastIndex, findIndex, elemLastIndex, elemIndex, (!!), uncons, unsnoc, init, tail, last, head, insertBy, insert, snoc, null, singleton, fromFoldable, transpose, mapWithIndex, (:))
 import Data.Maybe (Maybe(..), isNothing, fromJust)
 import Data.Monoid.Additive (Additive(..))
 import Data.NonEmpty ((:|))
@@ -226,16 +226,26 @@ testList = do
   assert $ (take 2 (l [1, 2, 3])) == l [1, 2]
   assert $ (take 1 nil) == nil
 
+  log "takeEnd should keep the specified number of items from the end of an list, discarding the rest"
+  assert $ (takeEnd 1 (l [1, 2, 3])) == l [3]
+  assert $ (takeEnd 2 (l [1, 2, 3])) == l [2, 3]
+  assert $ (takeEnd 1 nil) == nil
+
   log "takeWhile should keep all values that match a predicate from the front of an list"
   assert $ (takeWhile (_ /= 2) (l [1, 2, 3])) == l [1]
   assert $ (takeWhile (_ /= 3) (l [1, 2, 3])) == l [1, 2]
   assert $ (takeWhile (_ /= 1) nil) == nil
 
-  log "drop should remove the specified number of items from the front of an list"
+  log "dropE should remove the specified number of items from the front of an list"
   assert $ (drop 1 (l [1, 2, 3])) == l [2, 3]
   assert $ (drop 2 (l [1, 2, 3])) == l [3]
   assert $ (drop 1 nil) == nil
 
+  log "dropEnd should remove the specified number of items from the end of an list"
+  assert $ (dropEnd 1 (l [1, 2, 3])) == l [1, 2]
+  assert $ (dropEnd 2 (l [1, 2, 3])) == l [1]
+  assert $ (dropEnd 1 nil) == nil
+
   log "dropWhile should remove all values that match a predicate from the front of an list"
   assert $ (dropWhile (_ /= 1) (l [1, 2, 3])) == l [1, 2, 3]
   assert $ (dropWhile (_ /= 2) (l [1, 2, 3])) == l [2, 3]