What would be a typical game skeleton in Haskell

Question

What would be the typical game skeleton for a Haskell game, let's say a simple shoot them up for instance? I am particularly interested on the data structure, and how to manage the update of all the elements in the world, in regards of immutability issue. Just saying that

  new_world=update_world(world)

is a little bit simplistic. For instance, how to deal with multiple interaction that can occurs between element of the world (collisions, reaction to player, etc....) especially when you have a lot of 'independent' elements impacted.

The main concern is about immutability of the world, which makes very difficult to update a "small" part of the world based on another subset of the world.

Answer 1

I love gloss (gloss 2D library. it's very closed to your needs (I think)

A very simple example

import Graphics.Gloss
import Graphics.Gloss.Interface.Pure.Game

-- Your app data
data App = App { mouseX :: Float
               , mouseY :: Float
               }

-- Draw world using your app data
      -- Here thing about *draw* your world (e.g. if radius > MAX then red else blue)
drawWorld (App mousex mousey) = color white $ circle mousex

-- Handle input events and update your app data
      -- Here thing about user interaction (e.g. when press button start jump!)
handleEvent
    (EventMotion (x, y)) -- current viewport mouse coordinates
    (App _ _) = App x y
handleEvent e w = w


-- Without input events you can update your world by time
     -- Here thing about time (e.g. if jumping use `t` to compute new position)
handleTime t w = w

runApp =
    play
        ( InWindow "Test" (300, 300) (0, 0) ) -- full screen or in window
        black                                 -- background color
        20                                    -- frames per second
        ( App 0 0 )                           -- your initial world
        drawWorld                             -- how to draw the world?
        handleEvent                           -- how app data is updated when IO events?
        handleTime                            -- how app data is updated along the time?

-- enjoy!
main = runApp

One simple example modifying some data structure (a list of circle radius) along the three event handlers (draw, input and time)

import Graphics.Gloss
import Graphics.Gloss.Interface.Pure.Game
import System.IO.Unsafe

data App = App { mouseX :: Float
               , mouseY :: Float
               , circleList :: [Float]
               , lastTime :: Float
               , currTime :: Float
               }

drawWorld app =
    color white $ pictures $ map circle $ circleList app

handleEvent
    (EventMotion (x, y)) -- current viewport mouse coordinates
    app = app { mouseX = x, mouseY = y,
                -- drop circles with radius > mouse **MOVED** position
                circleList = filter (<(abs x)) $ circleList app }
handleEvent e app = app

handleTime t app =
    app { currTime = currTime', lastTime = lastTime', circleList = circleList' }
    where currTime' = currTime app + t
          -- create new circle each 1 second
          createNew = currTime' - lastTime app > 1
          lastTime' = if createNew then currTime' else lastTime app
          -- each step, increase all circle radius
          circleList' = (if createNew then [0] else []) ++ map (+0.5) (circleList app)

runApp =
    play
        ( InWindow "Test" (300, 300) (0, 0) )
        black
        20
        ( App 0 0 [] 0 0 )
        drawWorld
        handleEvent
        handleTime

main = runApp

with result