Another update from ingydotnet^djgoku

Task/Roman-numerals-Decode/JavaScript/roman-numerals-decode-2.js

@@ -0,0 +1,48 @@
+(function (lstTest) {
+
+    var mapping = [["M", 1000], ["CM", 900], ["D", 500], ["CD", 400], ["C", 100], [
+        "XC", 90], ["L", 50], ["XL", 40], ["X", 10], ["IX", 9], ["V", 5], ["IV",
+        4], ["I", 1]];
+
+    // s -> n
+    function romanValue(s) {
+        // recursion over list of characters
+        // [c] -> n
+        function toArabic(lst) {
+            return lst.length ? function (xs) {
+                var lstParse = chain(mapping, function (lstPair) {
+                    return isPrefixOf(
+                        lstPair[0], xs
+                    ) ? [lstPair[1], drop(lstPair[0].length, xs)] : []
+                });
+                return lstParse[0] + toArabic(lstParse[1]);
+            }(lst) : 0
+        }
+        return toArabic(s.split(''));
+    }
+
+    // Monadic bind (chain) for lists
+    function chain(xs, f) {
+        return [].concat.apply([], xs.map(f));
+    }
+
+    // [a] -> [a] -> Bool
+    function isPrefixOf(lstFirst, lstSecond) {
+        return lstFirst.length ? (
+            lstSecond.length ?
+            lstFirst[0] === lstSecond[0] && isPrefixOf(
+                lstFirst.slice(1), lstSecond.slice(1)
+            ) : false
+        ) : true;
+    }
+
+    // Int -> [a] -> [a]
+    function drop(n, lst) {
+        return n <= 0 ? lst : (
+            lst.length ? drop(n - 1, lst.slice(1)) : []
+        );
+    }
+
+    return lstTest.map(romanValue);
+
+})(['MCMXC', 'MDCLXVI', 'MMVIII']);