A simple While-loop in Haskell using the State Monad

while.hs

-- Inspired by https://stackoverflow.com/questions/17719620/while-loop-in-haskell-with-a-condition

import Control.Monad.State

type IsZero = Bool

{- 
 This is in State monad. I.e. it is a function, that when given an initial state (Int,IsZero) will produce a tuple of ((Int,IsZero),())
 We can see this as an action on our state space with no output value.
-}
decrementAndCheck :: State (Int, IsZero) ()
decrementAndCheck = do
    (m,_) <- get
    let n = m-1
    put (n, n==0)
    return ()

{- 
  This function applies some stateful processing decrementAndCheck
  After each such processing, it makes a termination check on the state
  In this specific example, it simply pulls the second component of the state
  When that second component is True, we are done
  Since we are in the state monad, the return type () really means ((Int,IsZero),())
  and we can interrogate the final state if we like
-}
stateProcessor :: State (Int, IsZero) ()
stateProcessor = do
    let loop True = return ()
        loop False = do
            decrementAndCheck
            (_,isDone) <- get
            loop isDone
    
    loop False

main = print $ runState stateProcessor (12,False)