The Monad Challenges

Welcome

The guys who wrote the matasano crypto challenges realized that you cannot get a working knowledge of crypto without investing blood, sweat, and tears into playing with and attempting to break crypto. They even say, “But more people ‘know how’ to break it than can actually break it.” Their crypto challenges provide a structured way for people to make the requisite blood, sweat, and tears investment to get to a place of understanding.

We believe the same is true when learning monads. Monads cannot be taught. They must be discovered. The problem with monad tutorials is that they are predicated on someone teaching/explaining monads. But you must explore them yourself. You will never really understand monads until you do this. The problem is that beginners do not usually know how to explore. We put together this set of challenges to show you how to do that.

Please do NOT publish the answers to these challenges. Reading answers that you did not create yourself defeats the whole purpose.

Do I need to know category theory?

No.

Do I need to know Haskell?

Yes, we assume that you have some beginner “pre-monad” level knowledge of the following Haskell concepts:

• Basic syntax
• Type signatures
• Higher order functions
• Type aliases
• newtypes
• Polymorphism
• Type classes

We are not trying to teach you Haskell here. We are trying to guide you down the path to a fully internalized working knowledge of monads. If you need to learn Haskell, go work through some beginning Haskell materials and come back when you think you need to learn monads.

Organizing Your Code

It will probably help if you organize your code into one file per set of challenges. Some of the problem sets build on previous ones, so it will be convenient to be able to easily import your code for a whole set or find it for reference.

Who did this?

• Doug Beardsley (with feedback and suggestions from Brent Yorgey)

Code Template

For all of these problem sets you should use the following language pragmas and imports:

{-# LANGUAGE MonadComprehensions #-}
{-# LANGUAGE RebindableSyntax  #-}

module Set1 where

import MCPrelude

MCPrelude should be the only module you import. Do not import any other modules. We are giving you a special prelude because we want you to solve these challenges without the use of monad libraries that have already done the work for you. Also, don’t look at the code in Control.Monad, the transformers package, or the mtl package. You’re doing this to learn. To do that you need to struggle with the concepts. Don’t cheat yourself out of the benefit you can get by working through things yourself.

Getting Started

First clone the monad-challenges github repository and build the associated library.

git clone https://github.com/mightybyte/monad-challenges.git
cd monad-challenges
cabal install

This makes the MCPrelude module available on your system. Now head on over to Set 1 and start coding!