Question

I have a program that displays a bunch of random triangles using Haskell, OpenGL, and GLUT. However I’d like the random triangles to change every time I click, or something of that nature (not important when they change).

Currently my working code looks like this:

main :: IO ()
main = do
    (pname, _) <- getArgsAndInitialize
    createWindow $ "Haskellisa"
    -- TODO set based on command line args
    windowSize $= (Size 640 480)
    blend $= Enabled
    blendFunc $= (SrcAlpha, OneMinusSrcAlpha)
    displayCallback $= display
    keyboardMouseCallback $= Just (keyboardMouse)
    mainLoop

display :: IO ()
display = do
    clear [ ColorBuffer ]
    gen0 <- getStdGen
    let (tris,gen1) = randomTris 10 gen0
    let (cols,gen2) = randomColor4s 10 gen1
    let triColPairs = zip tris cols
    mapM_ (\(tri, col) -> drawTri tri col) triColPairs
    flush

However display takes the same StdGen every time by calling getStdGen. What I’d like is for the gen2 from the line let (cols,gen2)... to be used as the generator for the next time, but obviously that requires some sort of mutable state or something of that nature.

What’s the best way to do what I’m asking, such that I will get different randomness every time display runs?

Answer 1

Since you’re in IO anyway, you could just store gen2 via setStdGen:

setStdGen gen2

so your next round takes off where your previous stopped.

Another option is to split the StdGen and use one of the results using newStdGen instead of getStdGen.