Lazy evaluation.

Lazy evaluation example with ES6 generators.

readme.md

@@ -121,6 +121,22 @@ referential transparent.
 
 ---
 
+## Lazy evalution
+> aka call-by-need is an evaluation machanism which delays the evaluation of an expression until its value is needed.
+
+```js
+let rand = function*() {
+    while(1<2) {
+        yield Math.random();
+    }
+}
+```
+```
+let randIter = random();
+randIter.next(); // Each exectuion gives a random value, expression is evluated on need.
+```
+---
+
 ## Monoid
 
 ---