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I owe so much to this blog! It was such an inspiring read when I started out programming in Haskell back in 2007.

Today I am a professor in computer science and still draw on it for examples in my advanced functional programming course. Just last week we did the loeb function, as an example of interesting use of Functor.

Loeb function: http://blog.sigfpe.com/2006/11/from-l-theorem-to-spreadsheet...

I keep bouncing off this stuff due to the lack of concrete examples where it would be useful. Maybe some documentation organized like the Design Patterns book would be helpful?

It's serendipitous that I'm seeing this blog post on the front page today, because I'm currently writing an article discussing the free monad.

In addition to the free monad presented in this post, there is a variant, called the "freer" monad, based on the "bind" operation instead of the "join" operation:

    data Freer f a where
      Pure :: a -> Freer f a
      Bind :: f a -> (a -> Freer f b) -> Freer f b

I believe this definition originates from the following paper by Oleg Kiselyov and Hiromi Ishii: https://okmij.org/ftp/Haskell/extensible/more.pdf

When thinking of monads as giving the semantics of some computational strategy, it's easier to define them in terms of "bind" instead of "join." This way of defining monads is sometimes called a "Kleisli triple" because it is better suggestive of "Kleisli arrows," or functions of the signature `a -> m b`. The "bind" operation defines how to compose a monadic computation with its continuation, and from this perspective, the "freer" monad resembles an abstract syntax tree.

Originally, Eugenio Moggi proposed monads as a technique for specifying the denotational semantics of programming languages. All Java programs "really" happen in the IO + Either monads, because all Java programs may perform IO and throw exceptions. To my understanding, free monads are the monad that OCaml 5 runs in, because they give the semantics for effect handlers (or resumable exceptions).

Free Monads are everywhere. Learning Haskell at this time was such an amazing experience. Haskell has incredible library stability, the kmettoverse feels the same, my is still good enough for most situations, there are new streaming libraries but accomplish the same things as conduit and pipes. LLMs are as decent as you would expect on Haskell, and have helped me debug some situations where I would be fighting GHC usually with some flags turned out. AI has actually has been helpful in learning since in Haskell once you figure something out you solve it for a whole class of problems, the issues is sometimes figuring that one thing out it's so abstract you feel like you are hitting a cliff. Excited to be writing Haskell still in 2026, I hope it continues to avoid success at all cost.