September 2017 Update

Task/Power-set/Haskell/power-set-3.hs

@@ -1,2 +1,2 @@
 powerSet :: [a] -> [[a]]
-powerset = foldr (\x acc -> acc ++ map (x:) acc) [[]]
+powerSet = foldr (\x acc -> acc ++ map (x:) acc) [[]]

Task/Power-set/Haskell/power-set-4.hs

@@ -1,15 +1,2 @@
-import qualified Data.Set as Set
-type Set=Set.Set
-unionAll :: (Ord a) => Set (Set a) -> Set a
-unionAll = Set.fold Set.union Set.empty
-
---slift is the analogue of liftA2 for sets.
-slift :: (Ord a, Ord b, Ord c) => (a->b->c) -> Set a -> Set b -> Set c
-slift f s0 s1 = unionAll (Set.map (\e->Set.map (f e) s1) s0)
-
---a -> {{},{a}}
-makeSet :: (Ord a) => a -> Set (Set a)
-makeSet = (Set.insert Set.empty) . Set.singleton.Set.singleton
-
-powerSet :: (Ord a) => Set a -> Set (Set a)
-powerSet = (Set.fold (slift Set.union) (Set.singleton Set.empty)) . Set.map makeSet
+powerSet :: [a] -> [[a]]
+powerSet = foldr ((mappend <*>) . fmap . (:)) (pure [])

Task/Power-set/Haskell/power-set-5.hs

@@ -1,2 +1,15 @@
-Prelude Data.Set> powerSet fromList [1,2,3]
-fromList [fromList [], fromList [1], fromList [1,2], fromList [1,2,3], fromList [1,3], fromList [2], fromList [2,3], fromList [3]]
+import qualified Data.Set as Set
+type Set=Set.Set
+unionAll :: (Ord a) => Set (Set a) -> Set a
+unionAll = Set.fold Set.union Set.empty
+
+--slift is the analogue of liftA2 for sets.
+slift :: (Ord a, Ord b, Ord c) => (a->b->c) -> Set a -> Set b -> Set c
+slift f s0 s1 = unionAll (Set.map (\e->Set.map (f e) s1) s0)
+
+--a -> {{},{a}}
+makeSet :: (Ord a) => a -> Set (Set a)
+makeSet = (Set.insert Set.empty) . Set.singleton.Set.singleton
+
+powerSet :: (Ord a) => Set a -> Set (Set a)
+powerSet = (Set.fold (slift Set.union) (Set.singleton Set.empty)) . Set.map makeSet

Task/Power-set/Haskell/power-set-6.hs

@@ -0,0 +1,2 @@
+Prelude Data.Set> powerSet fromList [1,2,3]
+fromList [fromList [], fromList [1], fromList [1,2], fromList [1,2,3], fromList [1,3], fromList [2], fromList [2,3], fromList [3]]