September 2017 Update

Task/Greatest-element-of-a-list/JavaScript/greatest-element-of-a-list-5.js

@@ -0,0 +1,55 @@
+(() => {
+    'use strict';
+
+    // MAXIMUM BY ... --------------------------------------------------------
+
+    // maximumBy :: (a -> a -> Ordering) -> [a] -> a
+    const maximumBy = (f, xs) =>
+        xs.reduce((a, x) => a === undefined ? x : (
+            f(x, a) > 0 ? x : a
+        ), undefined);
+
+    // comparing :: (a -> b) -> (a -> a -> Ordering)
+    const comparing = f =>
+        (x, y) => {
+            const
+                a = f(x),
+                b = f(y);
+            return a < b ? -1 : a > b ? 1 : 0
+        };
+
+
+    // TEST ------------------------------------------------------------------
+    const words = ["alpha", "beta", "gamma", "delta", "epsilon", "zeta", "eta"];
+    const cities = [{
+        k: 'Bejing',
+        n: 21.5
+    }, {
+        k: 'Delhi',
+        n: 16.7
+    }, {
+        k: 'Karachi',
+        n: 23.5
+    }, {
+        k: 'Lagos',
+        n: 16.0
+    }, {
+        k: 'Shanghai',
+        n: 24.3
+    }];
+
+    // length :: [a] -> Int
+    const length = xs => xs.length;
+
+    // population :: {k: String, n: Float}
+    const population = dct => dct.n;
+
+    // show :: a -> String
+    const show = x => JSON.stringify(x, null, 2);
+
+    // OUTPUT ----------------------------------------------------------------
+    return show({
+        byWordLength: maximumBy(comparing(length), words),
+        byCityPopulation: maximumBy(comparing(population), cities)
+    });
+})();