Thanks, I liked reading it a lot. It's a good refresher, since I never use haskell but have read about this a couple of times.
The tutorial appears great on mobile, so I was able to refresh my memory very quickly. However, the code boxes had a very small font. I think this could me easily fixed with fome html-fu on the meta tag or the sizing of the box/font.
I also think that it would be great it you could explain at the end what is a counterexample input for the small haskell program. For example, if the user inputs a file that doesn't exist, then the readFile returns Nothing, then putStrLn returns Nothing. There's a sentence in the conclusion that explains that monads are nice to use but not why. I understand that it deals with values inside contexes, but in practice how does it affect development?