How to execute monadic function combining state management and random number generation

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1

I have written a Haskell program that uses both state and randomness. Here’s my code:

import Control.Monad.State
import Control.Monad.Identity  
import Control.Monad.Random.Class

-- dice roll function
coinFlip :: MonadRandom m => m Bool
coinFlip = (< 0.5) <$> getRandomR (0,1 :: Double)

-- main computation
process :: (MonadState Bool m, MonadRandom m) => m Bool
process = coinFlip >>= \result -> put result >> return result

I need help figuring out the proper way to execute the process function. The goal is to make sure it returns True approximately 50% of the time when run multiple times. What’s the correct approach to run this monadic computation that requires both state handling and random generation capabilities?

2

The main thing is picking the right execution strategy for your monad stack. I’ve worked with similar setups and found that putting RandT as the outer transformer usually works better than nesting it inside StateT. Try this:

import System.Random

runProcess :: IO Bool
runProcess = do
    gen <- getStdGen
    return $ evalState (evalRandT process gen) False

This puts the random generator in the outer layer, which makes testing more predictable. Order matters because different stacking changes how the computations interact. When I debugged similar code, flipping the transformer order fixed weird behavior where random values weren’t distributing properly across multiple runs. Use evalState instead of runState since you only want the result, not the final state.

3

You need a concrete monad transformer stack to run your computation. Since your process function needs both MonadState Bool and MonadRandom instances, go with StateT Bool (RandT StdGen Identity) or something similar.

Here’s what worked for me:

import System.Random

execute :: IO Bool
execute = do
    gen <- newStdGen
    let result = evalRandT (evalStateT process False) gen
    return result

Getting the transformer order right is key. I’ve found evalStateT on the outside with evalRandT inside gives you the cleanest execution. Don’t forget your imports - you’ll need Control.Monad.Random and either Control.Monad.State.Lazy or the strict variants depending on what you’re doing.

4

try runRandT with evalState - it’s way easier to combine state and randomness. somethng like runRandT process False should work. just double-check ur types and transformers first!