Is it correct to use the Reader monad in this turn-based game?

Question

I need to learn to use the Reader monad. (And also the Writer and State).

The game-contests I do always consists in global parameters that are valid throughout the Game. And a bunch of parameters that are valid only during one Turn.

I ended all the latest contests with lots and lots of functions that almost all had Game and Turn as input parameters.

After having read about the reader monad, I am thinking about having:

data Game = Game AllNeeded StrategicPositions AndGameRelatedStuffs
data Turn = Turn PlayersPositions AndTurnDependentStuffs 

and for each loop when I receive turn dependent data, I plan to have a variable Reader Game Turn:

turnReader = Reader currentGame currentTurn
let actionToPerform = play turnReader

Do you think it's the good example of application for the Reader monad ? Do you think I can/should go blindfold and learn to use the Reader monad in this context ? Or is there a more appropriate monad for this, like State ?

Answer 1

The Reader monad is used when you have an immutable state that you want to read in your computation. Is basically like having an extra argument implicitly passed to all the monadic actions.

It seems like in your game you actually want to change something (i.e. turn, or state of the game). In this case a State monad may be easier to work with.

In order to use Reader you have to group all actions that depend on the state, execute them in the monad then outside the monad change the state and run a new monadic computation with the modified state etc. This sounds cumbersome.

Note that using Reader is actually similar to using State. Simply: the put operation (and derivatives) are disallowed. So you could start writing using Reader and if you encounter a place were you'd like to modify the state you know that it would probably be better to use State. Or viceversa: start by using State and if you end up never modifying the state replace it with Reader.

Answer 2

It seems reasonable to have something like StateT Turn (ReaderT Game IO), assuming Turn value changes every turn and Game doesn't. You'll need to read up on monad transformers for this (I personally like "Monad Transformers Step by Step" (PDF)).