Question

How could I go about getting put to work on the StateT monad while inside forM_?

loop :: Integer -> StateT World IO ()
loop passes = do
    (scene, b1) <- get
    forM_ [1..passes] $ \pass -> do
        let b2 = foo b1 pass
        -- other stuff --
        put (scene, b2) -- this no longer puts into the StateT monad 

Or is there a better approach all-together?

Answer 1

I think it works fine, for example:

--I don't know what World is so I can't use your example

loop :: StateT Integer IO ()
loop = do
    forM_ [1..10] put
    x <- get
    liftIO $ print x

running evalStateT loop 0 prints 10 like I’d expect. mapM_ under the hood does this:

1. Actually do the map, which for the above example returns a list [StateT Integer IO ()]

2. Do a right fold like: foldr (>>) (return ()) listOfMappedValues

It’s not clear what exactly your expecting but this would let you put things into the State from a mapM_