2016 Update

Task/Permutations/JavaScript/permutations-4.js

@@ -0,0 +1,39 @@
+(function () {
+
+    // permutations :: [a] -> [[a]]
+    function permutations(xs) {
+        return xs.length ? (concatMap(
+            function (x) {
+                return concatMap(
+                    function (ys) {
+                        return ([[x].concat(ys)]);
+                    }, permutations(delete1(x, xs)))
+            }, xs)) : [[]]
+    }
+
+
+    // GENERIC LIBRARY FUNCTIONS
+
+    // concatMap :: (a -> [b]) -> [a] -> [b]
+    function concatMap(f, xs) {
+        return [].concat.apply([], xs.map(f));
+    }
+
+    // delete first instance of a in [a]
+    // delete1 :: a -> [a] -> [a]
+    function delete1(x, xs) {
+        return deleteBy(function (a, b) {
+            return a === b;
+        }, x, xs);
+    }
+
+    // deleteBy :: (a -> a -> Bool) -> a -> [a] -> [a]
+    function deleteBy(fnEq, x, xs) {
+        return xs.length ? fnEq(x, xs[0]) ? xs.slice(1) : [xs[0]]
+            .concat(deleteBy(fnEq, x, xs.slice(1))) : [];
+    }
+
+
+    return permutations(['Aardvarks', 'eat', 'ants'])
+
+})();