Example.hs

{-# LANGUAGE DeriveGeneric              #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE MultiWayIf                 #-}
{-# LANGUAGE OverloadedLabels           #-}

module Example where

import GHC.Generics             ( Generic )
import Control.Lens             ( (+=)
                                , (*=)
                                , use
                                , view
                                )
import Control.Monad.State.Lazy ( State
                                , execState
                                , put
                                , replicateM
                                , when
                                )
import Data.Generics.Labels     ()

newtype S = S { n :: Int }
  deriving (Eq, Generic, Ord, Show)

foo :: Bool -> State S ()
foo flag = do
  #n += 2
  when flag $ #n *= 10
  x <- use #n
  put . S $ if | even x         -> x + 1
               | x > 100        -> x `rem` 100
               | x < -100       -> x `rem` 100
               | x `rem` 5 == 0 -> x + 11
               | otherwise      -> x - 25

main :: IO ()
main = print . n . flip execState initialVal . replicateM r $ foo flag
  where
    flag       = True
    initialVal = S 0
    r          = 3