initial commit

src/List.hs

@@ -0,0 +1,29 @@
+module List (
+  head,
+  ordNub,
+  sortOn,
+) where
+
+import Data.List (sortBy)
+import Data.Maybe (Maybe(..))
+import Data.Ord (Ord, comparing)
+import Data.Foldable (Foldable, foldr)
+import Data.Function ((.))
+import Control.Monad (return)
+import qualified Data.Set as Set
+
+head :: (Foldable f) => f a -> Maybe a
+head = foldr (\x _ -> return x) Nothing
+
+sortOn :: (Ord o) => (a -> o) -> [a] -> [a]
+sortOn = sortBy . comparing
+
+-- O(n * log n)
+ordNub :: (Ord a) => [a] -> [a]
+ordNub l = go Set.empty l
+  where
+    go _ []     = []
+    go s (x:xs) =
+      if x `Set.member` s
+      then go s xs
+      else x : go (Set.insert x s) xs