IO does not do side-effects. Start here.

IO.hs

{-# LANGUAGE NoImplicitPrelude #-}

import Control.Monad.Free
import Prelude(Functor(..), Monad(..), (.), Char)

data IO'Op a =
  PutChar Char a
  | GetChar (Char -> a)

instance Functor IO'Op where
  fmap f (PutChar c a) =
    PutChar c (f a)
  fmap f (GetChar k) =
    GetChar (f . k)

type IO' a =
  Free IO'Op a

putChar ::
  Char
  -> IO' ()
putChar c =
  Free (PutChar c (return ()))

getChar ::
  IO' Char
getChar =
  Free (GetChar return)

program =
  do
    c1 <- getChar
    c2 <- getChar
    putChar c1
    putChar c2