Many people bemoan the sharp divide between experts and beginning Haskell programmers. One thing I've noticed is that "advanced" Haskell topics all have one thing in common: there exists only one good tutorial on that topic and only the experts have found it. This post is a collection of links to what I consider to be the definitive tutorials on these topics.
Monads are technically not an advanced Haskell topic, but I include this topic in the list because the vast majority of monad tutorials are terrible and/or misleading. In my opinion, the best monad tutorial (Titled: "You could have invented monads") was written 8 years ago by Dan Piponi and nobody has ever improved on it since.
One thing that perpetually boggles me is that the blogosphere has almost no posts explaining how to use monad transformers. In fact, the only good explanation I know of is a very simple paper by Martin Grabmuller (Titled: "Monad Transformers - Step by Step"). Most beginners will skip over an academic paper like this when searching for tutorials since most people don't associate the word "academic" with beginner-friendly. However, don't let that fool you: this paper is very well written and easy to read.
Outsiders comment that monads only exist to paper over Haskell's weaknesses until they discover monadic parsers. The truly mind-blowing moment is when you learn how simple they are to implement when you read Hutton and Meijer's functional pearl on monadic parsing (Titled: "Monadic Parsing in Haskell"). They walk through how to implement a backtracking parser, how to implement the
Monad type class for this parser, and how to define several parsing primitives and chain them together into more sophisticated functionality. After you read this functional pearl you will never doubt the significance of monads ever again.
You also want to understand parser combinators for another reason: Haskell parser combinator libraries are light-years ahead of Haskell regular expression libraries. This confuses many people who come to Haskell from a language that uses regular expressions heavily (such as Python or Perl) and they wonder why regular expression support is so bad in Haskell. The reason is that nearly all Haskell programmers use parser combinator libraries such as parsec or attoparsec.
This is the only case where I will plug a post of my own. I wrote a tutorial on free monads (Titled: "Why free monads matter") because back when I was trying to understand free monads I could not find any beginner-friendly explanation. Everything on this subject was written for a target audience of mathematicians and would motivate their use cases using mathematical examples.
I wanted to explain the motivation behind free monads in terms that a programmer would understand and my post explains the topic in terms of creating a syntax tree and interpreting that syntax tree. Understanding free monads will greatly improve your understanding for how to build monadic domain-specific languages within Haskell. Also, many people comment that understanding free monads greatly improves their mental model for Haskell
IO as well.
Haskell coroutine libraries (like
machines) have what appear to be impenetrable implementations ... until you read Mario Blazevic's article on coroutines in issue 19 of The Monad Reader (Titled: Coroutine Pipelines").
In that article he introduces the
Coroutine type (which is now more commonly known as the free monad transformer) which unifies every coroutine implementation into a single pattern. Once you understand his article, you will understand the internals of coroutine libraries and you will be able to easily write a streaming library of your own.
Lens tutorials are the new monad tutorials, and the one mistake that all lens tutorials make is that none of them clearly explain how lenses work. The only post that ever got this right was the original post by Twan van Laarhoven that first introduced the modern formulation of lenses (Titled: "CPS Functional references"). The problem is that post is incredibly hard to find even through a dedicated Google search for lens tutorials because the title is obscure and the post only has a single mention of the word "lens". I actually had trouble finding this post even by browsing his blog archive because I didn't recognize the title.
Honorable mention: The
lens-family-core library is the only lens library that actually hews closely to Twan's original presentation, so I highly recommend that you study its source code to further improve your understanding.
I still can't find a good blog post on Church encoding, so I will not link to anything. However, once you learn how to Church encode a data type you will suddenly see this pattern everywhere.
Equally important, many high-performance Haskell libraries (like
attoparsec) use Church encoding for efficiency reasons and understanding Church encoding will allow you to understand their internal implementations.
I'm really disappointed that so many critical Haskell topics only have a single source of truth that is difficult to find. I hypothesize that the problem is that Haskell is rooted in computer science academia, which discourages duplicate publications on the same topic. If you want to prove me wrong, then please take the time to increase the number of high quality Haskell tutorials on these and other key topics.
Another troubling trend is that tendency for people to bandwagon on particular tutorial topics that are sexy (first monads, now lenses). I wish people would spend more time diversifying coverage on more mundane topics, like how to apply well-established libraries to architect various types of applications.