Array semigroup optimisation

Author: garyb

src/Data/Semigroup.js

@@ -10,6 +10,8 @@ exports.concatString = function (s1) {
 
 exports.concatArray = function (xs) {
   return function (ys) {
+    if (xs.length === 0) return ys;
+    if (ys.length === 0) return xs;
     return xs.concat(ys);
   };
 };