Add helpers for partial application

lib/utils/helpers.js

@@ -1,6 +1,5 @@
 // A collection of useful helper functions, used internally but not exported
 // with the rest of Cylon. 
-
 "use strict";
 
 var __slice = Array.prototype.slice;
@@ -151,6 +150,24 @@ function arity(fn, n) {
   };
 }
 
+function partial(fn) {
+  var args = __slice.call(arguments, 1);
+
+  return function() {
+    return fn.apply(null, args.concat(__slice.call(arguments)));
+  };
+}
+
+function partialRight(fn) {
+  var args = __slice.call(arguments, 1);
+
+  return function() {
+    return fn.apply(null, __slice.call(arguments).concat(args));
+  };
+}
+
 extend(H, {
-  arity: arity
+  arity: arity,
+  partial: partial,
+  partialRight: partialRight
 });

spec/lib/utils/helpers.spec.js

@@ -220,4 +220,22 @@ describe("Helpers", function() {
       expect(fn).to.be.calledWith("one");
     });
   });
+
+  describe("#partial", function() {
+    it("partially applies a function's arguments", function() {
+      var fn = spy();
+      var one = _.partial(fn, "one", "two");
+      one("three");
+      expect(fn).to.be.calledWith("one", "two", "three");
+    });
+  });
+
+  describe("#partialRight", function() {
+    it("partially applies arguments to the end of a fn call", function() {
+      var fn = spy();
+      var one = _.partialRight(fn, "two", "three");
+      one("one");
+      expect(fn).to.be.calledWith("one", "two", "three");
+    });
+  });
 });