Twelve Monads of Christmas

An overview

A continuation is “the rest of the program” represented as a value that you can do value things to: stick it in a variable, store it in a…

I saw mention of this at Haskell eXchange 2016. I haven’t done anything with it, but it intrigued me so I decided to include a post in this…

Free monads let you take a data type that represents instructions, or (approximately the same) effects that you want to happen, and then…

On day 5, I wrote about how to make a set monad — first build a sequence of instructions that describe how to build a Set, and then…

The list monad lets you perform non-deterministic searches for a solution to a problem. At each step of your program, you can provide a…

In the Reader monad, all you can do is query the environment by asking for the whole environment. If you only want a little bit of that…

Lazy evaluation lets us compose two different computations without thinking too much about who is “in control”: we can generate primes…

Sets are a bit like lists, and so you can probably imagine that there should be a Monad instance for them. The first part of this post will…

Reader, Writer and State usually come presented together, because they’re very similar: these monads wire together functions by adding in…

Maybe is like [] but “smaller” — Maybe t can store zero or one elements of type t, but not more.

Here’s a list comprehension: