Factorial function using ST Monad

STRef_example.purs

module Main where

import Prelude

import Effect (Effect)
import Effect.Console (logShow)
import Control.Monad.ST.Internal(ST,for,while,foreach,new,run,write,read,modify)
import Data.Array((..))


facotialUsingFor :: forall r. Int -> ST r Int
facotialUsingFor n = do
    acc <- new 1
    for 2 (n + 1) \i -> 
      void $ modify (\prev -> prev * i) acc

    read acc
    
 
facotialUsingWhile :: forall r. Int -> ST r Int
facotialUsingWhile n = do
    i <- new 1      --- storing the pointer variable 
    acc <- new 1
    while ( (_ <= n) <$> (read i)) do --- checking if i <= n 
        curr_iter <- read i
        _ <- modify (_ * curr_iter) acc
        _ <- write (curr_iter + 1) i   --- increasing the pointer
        pure unit
        
    read acc


factorialUsingForEach :: forall r. Int -> ST r Int
factorialUsingForEach n = do 
    acc <- new 1 
    foreach (2..n) \i -> 
        void $ modify (\prev -> prev * i) acc 
    read acc

computeFactorial :: Int -> Int
computeFactorial = run <<< facotialUsingWhile

main :: Effect Unit
main = do 
   logShow $ run (facotialUsingFor 5)
   logShow $ run (facotialUsingWhile 5)
   logShow $ run (factorialUsingForEach 5)