Haskell Gloss Not Animating

Question

I have a program which simulates the interaction of many agents in a community. I'm animating the interaction using the Gloss library , the pictures of the agents render correctly, just not the animation. I animate it by generating a simulation, which is a list of list of interactions, which I then take the one corresponding to the second of the animation, where I then render that interaction. The code for simulating works fine when outputting it to terminal. The code:

render ::  Int -> History -> Picture -- assume window to be square
render size (History int agents) =  Pictures $ map (drawAgent (step`div`2) colors step) agents
    where step = size*6 `div` (length agents)
          --agents = nub $ concat $ map (\(Interaction a1 a2 _ ) -> [a1,a2]) int
          nubNames = nub $ map (getName . name) agents --ignore the first two letters of name
          colors = Map.fromList $ zipWith (\name color-> (name, color)) nubNames (cycle colorlist)
          colorlist = [red,green,blue,yellow,cyan,magenta,rose,violet,azure,aquamarine,chartreuse,orange]

drawAgent :: Int -> Map.Map String Color -> Int -> Agent -> Picture
drawAgent size colors step agent =
    color aColor (Polygon [(posA,posB),(posA,negB),(negA,negB),(negA,posB)])
    where aColor = fromMaybe black $ Map.lookup (getName $ name agent ) colors
          a = (fst $ position agent) * step
          b = (snd $ position agent) * step
          posA = fromIntegral $ a+size
          negA = fromIntegral $ a-size
          posB = fromIntegral $ b+size
          negB = fromIntegral $ b-size

simulate :: Int -> [Agent] -> [History]
simulate  len  agents = trace ("simulation"
                        (playRound agents len) : 
                         simulate len (reproduce (playRound agents len))

main =  do
    a <- getStdGen
          G.animate (G.InWindow "My Window" (400, 400) (0,0)) G.white 
          (\time ->(render 400) $ ((simulate 5 (agent a))!!(floor time)))

    where agent a = generate a 9
          sim a = simulate 40 (agent a)

When I execute this, it will say the simulation is running, but only render the first interaction.

  $ ghc -O2 -threaded main.hs && ./main
  [6 of 8] Compiling Prisoners        ( Prisoners.hs, Prisoners.o )
  Linking main ...
  simulation
  simulation
  simulation

It will continue like this until I stop it, rendering the same picture each time. What am I doing wrong?

Answer 1

(SO Comment box won't let me paste code)

Your doesn't compile for me. What version of Gloss are you using, the API has changed from v1.0 to v1.7.x. What version of GHC? What OS?

Does this simple example work for you?

{- left click to create a circle;  escape to quit  -}
import Graphics.Gloss
import Graphics.Gloss.Interface.Pure.Game

initial _ = [(0.0,0.0) :: Point]

event (EventMotion (x,y)) world = world --(x,y)
event (EventKey (MouseButton LeftButton) Up mods (x,y)) world = (x,y):world
event _                   world = world

step time world = world 

draw pts = Pictures $ map f pts
  where f (x,y) = translate x y (circle 10)

m = play (InWindow "Hi" (600,600) (200,200)) white 1 (initial 0) draw event step