Data commit

Task/Combinations/JavaScript/combinations-6.js

@@ -0,0 +1,61 @@
+(() => {
+    'use strict';
+
+    // ------------------ COMBINATIONS -------------------
+
+    // combinations :: Int -> [a] -> [[a]]
+    const combinations = n =>
+        xs => {
+            const comb = n => xs => {
+                return 1 > n ? [
+                    []
+                ] : 0 === xs.length ? (
+                    []
+                ) : (() => {
+                    const
+                        h = xs[0],
+                        tail = xs.slice(1);
+                    return comb(n - 1)(tail)
+                        .map(cons(h))
+                        .concat(comb(n)(tail));
+                })()
+            };
+            return comb(n)(xs);
+        };
+
+    // ---------------------- TEST -----------------------
+    const main = () =>
+        show(
+            combinations(3)(
+                enumFromTo(0)(4)
+            )
+        );
+
+
+    // ---------------- GENERIC FUNCTIONS ----------------
+
+    // cons :: a -> [a] -> [a]
+    const cons = x =>
+        // A list constructed from the item x,
+        // followed by the existing list xs.
+        xs => [x].concat(xs);
+
+
+    // enumFromTo :: Int -> Int -> [Int]
+    const enumFromTo = m =>
+        n => !isNaN(m) ? (
+            Array.from({
+                length: 1 + n - m
+            }, (_, i) => m + i)
+        ) : enumFromTo_(m)(n);
+
+
+    // show :: a -> String
+    const show = (...x) =>
+        JSON.stringify.apply(
+            null, x.length > 1 ? [x[0], null, x[1]] : x
+        );
+
+    // MAIN ---
+    return main();
+})();