September 2017 Update

Task/Roman-numerals-Decode/JavaScript/roman-numerals-decode-8.js

@@ -0,0 +1,52 @@
+(() => {
+
+    // Folding from right to left,
+    // lower leftward characters are subtracted,
+    // others are added.
+
+    // fromRoman :: String -> Int
+    const fromRoman = s =>
+        snd(foldr(
+            ([r, n], l) => [l, l >= r ? n + l : n - l], [0, 0],
+            map(charVal, stringChars(s))
+        ));
+
+    // charVal :: Char -> Maybe Int
+    const charVal = k => {
+        const v = {
+            I: 1,
+            V: 5,
+            X: 10,
+            L: 50,
+            C: 100,
+            D: 500,
+            M: 1000
+        }[k];
+        return v !== undefined ? v : 0;
+    };
+
+    // GENERIC FUNCTIONS ------------------------------------------------------
+
+    // foldr (a -> b -> b) -> b -> [a] -> b
+    const foldr = (f, a, xs) => xs.reduceRight(f, a);
+
+    // map :: (a -> b) -> [a] -> [b]
+    const map = (f, xs) => xs.map(f);
+
+    // snd :: (a, b) -> b
+    const snd = tpl => Array.isArray(tpl) ? tpl[1] : undefined;
+
+    // stringChars :: String -> [Char]
+    const stringChars = s => s.split('');
+
+    // show :: a -> String
+    const show = (...x) =>
+        JSON.stringify.apply(
+            null, x.length > 1 ? [x[1], null, x[0]] : x
+        );
+
+    // TEST -------------------------------------------------------------------
+    return show(
+        map(fromRoman, ["MDCLXVI", "MCMXC", "MMVIII", "MMXVI", "MMXVII"])
+    );
+})();