Plurrrr

If you’ve been around Haskell circles for a bit, you’ve probably seen the term “free monads”. This article aims to introduce free monads and explain why they are useful.

To whet your appetite a little, free monads are basically a way to easily get a generic pure Monad instance for any Functor. This can be rather useful in many cases when you’re dealing with tree-like structures, but to name a few:

• To build an AST for an eDSL using do-notation.
• To have different semantics for the same monad in different contexts, e.g., define an interpreter and a pretty-printer for an eDSL, or have a mock interpreter in addition to a real one.
• To build a decision-tree type structure harnessing the do-notation for non-determinism (like with lists, but for trees).

I originally wrote this at work, after my team spent far too many days yelling at the computer because of Mojibake. Thanks to my employer for allowing me to publish it, and the several colleagues who provided helpful feedback. Any errors are, naturally, not their fault.

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