September 2017 Update

Task/Permutations/JavaScript/permutations-6.js

@@ -1,19 +1,38 @@
-(function (lst) {
+(() => {
     'use strict';
 
-    const permutations = (xs) => xs.length ? (
-            flatMap((x) => flatMap((xs) => [[x].concat(xs)],
-                permutations(del(x, xs))), xs)
-        ) : [[]],
+    // permutations :: [a] -> [[a]]
+    const permutations = xs =>
+        xs.length ? concatMap(x => concatMap(ys => [
+                [x].concat(ys)
+            ],
+            permutations(delete_(x, xs))), xs) : [
+            []
+        ];
 
-        flatMap = (f, xs) => [].concat.apply([], xs.map(f)),
+    // GENERIC FUNCTIONS
 
-        del = (x, xs) => xs.length ? x === xs[0] ? (
-            xs.slice(1)
-        ) : [xs[0]].concat(del(x, xs.slice(1))
+    // concatMap :: (a -> [b]) -> [a] -> [b]
+    const concatMap = (f, xs) => [].concat.apply([], xs.map(f));
+    //
+    // // delete :: Eq a => a -> [a] -> [a]
+    // const delete_ = (x, xs) =>
+    //     deleteBy((a, b) => a === b, x, xs);
+
+    // delete_ :: Eq a => a -> [a] -> [a]
+    const delete_ = (x, xs) =>
+        xs.length > 0 ? (
+            (x === xs[0]) ? (
+                xs.slice(1)
+            ) : [xs[0]].concat(delete_(x, xs.slice(1)))
         ) : [];
 
+    // range :: Int -> Int -> [Int]
+    const range = (m, n) =>
+        Array.from({
+            length: Math.floor(n - m) + 1
+        }, (_, i) => m + i);
 
-    return permutations(lst);
-
-})(["Aardvarks", "eat", "ants"]);
+    // TEST
+    return permutations(['Aardvarks', 'eat', 'ants']);
+})();