Initial data commit

Task/Symmetric-difference/JavaScript/symmetric-difference-3.js

@@ -0,0 +1,64 @@
+(() => {
+    'use strict';
+
+    const symmetricDifference = (xs, ys) =>
+        union(difference(xs, ys), difference(ys, xs));
+
+
+    // GENERIC FUNCTIONS ------------------------------------------------------
+
+    // First instance of x (if any) removed from xs
+    // delete_ :: Eq a => a -> [a] -> [a]
+    const delete_ = (x, xs) => {
+        const i = xs.indexOf(x);
+        return i !== -1 ? (xs.slice(0, i)
+            .concat(xs.slice(i, -1))) : xs;
+    };
+
+    //  (\\)  :: (Eq a) => [a] -> [a] -> [a]
+    const difference = (xs, ys) =>
+        ys.reduce((a, x) => filter(z => z !== x, a), xs);
+
+    // filter :: (a -> Bool) -> [a] -> [a]
+    const filter = (f, xs) => xs.filter(f);
+
+    // flip :: (a -> b -> c) -> b -> a -> c
+    const flip = f => (a, b) => f.apply(null, [b, a]);
+
+    // foldl :: (b -> a -> b) -> b -> [a] -> b
+    const foldl = (f, a, xs) => xs.reduce(f, a);
+
+    // nub :: [a] -> [a]
+    const nub = xs => {
+        const mht = unconsMay(xs);
+        return mht.nothing ? xs : (
+            ([h, t]) => [h].concat(nub(t.filter(s => s !== h)))
+        )(mht.just);
+    };
+
+    // show :: a -> String
+    const show = x => JSON.stringify(x, null, 2);
+
+    // unconsMay :: [a] -> Maybe (a, [a])
+    const unconsMay = xs => xs.length > 0 ? {
+        just: [xs[0], xs.slice(1)],
+        nothing: false
+    } : {
+        nothing: true
+    };
+
+    // union :: [a] -> [a] -> [a]
+    const union = (xs, ys) => {
+        const sx = nub(xs);
+        return sx.concat(foldl(flip(delete_), nub(ys), sx));
+    };
+
+    // TEST -------------------------------------------------------------------
+    const
+        a = ["John", "Serena", "Bob", "Mary", "Serena"],
+        b = ["Jim", "Mary", "John", "Jim", "Bob"];
+
+    return show(
+        symmetricDifference(a, b)
+    );
+})();