Twelve Monads of Christmas

An overview

So we reach the end of our twelve(ish) monads, or maybe if you are visiting from the future, the beginning.

Some Basics

On Day 1, I emphasised how IO actions are just values and that >>= is always in charge. On Day 2, I showed the fmap and join version of monads, with [] as an example where you can see intermediate steps of a program materialised as values. On Day 3, I used Maybe to introduce a bit of category theory and how monad morphisms let you interpret Maybe actions in [] or in IO. Day 4 talked about monads that add extra parameters or return values, such as Reader.

Specific Monads

Day 5 puzzled over why it is difficult to implement Set as a monad, even though it seems quite like Maybe or []. Day 6 introduces co-routines via Conduit. Day 7 gives an example where it is useful to not have the full power of a monad, Haxl. Day 8 discusses the difficulties in making a list transformer. Skipping ahead a bit, Day 11 brings in some concepts from dependently typed programming to use compile-time proofs of facts only known at runtime.

Building monads

Day 9 introduces free and freer monads and Day 10 extends that give composable effects and interpreters. And finally, Day 12 introduces continuations which you probably shouldn’t use.

There’s a video

I gave an overview at London Haskell, and a video is available on YouTube.

Happy Christmas!