Further reading

Here is a list of information you can refer to:

1. Basic Category Theory for Computer Scientists (Foundations of Computing), Benjamin C. Pierce: http://www.amazon.com/Category-Computer-Scientists-FoundationsComputing-ebook/dp/B00MG7E5WE/ref=sr_1_7?ie=UTF8&qid=1423484917&sr=8-7&keywords=category+theory
2. Category Theory (Oxford Logic Guides), Steve Awodey: http://www.amazon.com/Category-Theory-Oxford-Logic-Guides/dp/0199237182/ref=sr_1_2?ie=UTF8&qid=1423484917&sr=8-2&keywords=category+theory
3. Null References: The Billion Dollar Mistake, Tony Hoare: http://www.infoq.com/presentations/Null-References-The-Billion-Dollar-Mistake-Tony-Hoare
4. Monads in Small Bites, Leonardo Borges: http://www.leonardoborges.com/writings/2012/11/30/monads-in-small-bites-part-i-functors