How do I write a game loop in Haskell?

Question

I want to code a game in Haskell where every iteration of the loop computes the state of the world. I thought I should create a function:

gameLoop :: World -> World
-- ...

and have main :: IO () call it:

main = do
    gameLoop -- ...

But the problem is that I'm missing some fundamental understanding of how to wrap the gameLoop function so that it returns main's parameter value.

How would one go about creating a game loop in Haskell?

Answer 1

You'll probably want something like this

import Control.Monad.Loops

main = iterateM_ 
       (\w -> displayWorld w >> return (gameLoop w))
       initWorld
-- iterateM_ ((>>) <$> displayWorld <*> return . gameLoop) initWorld

Or if you don't want to use the whole monad-loops package (even though it rocks)

main = loop initWorld
  where loop w = displayWorld w >> loop (gameLoop w)

Basically you're just drawing the world, then looping again to with the next state.

More likely you want something like this though

 -- False when the user wants to exit the game
 keepGoing :: World -> Bool

 main = iterateUntilM_ keepGoing displayLoop initWorld
   where displayLoop w = displayWorld w >> return (gameLoop w)

Since otherwise you can't stop :)