Mutable variables are not used often in Haskell, but there is time that you need to 
use mutable variables in your code. There are many ways to create mutable variables.

Here is how to use mutable variables in Haskell
The idea is very simple:
1. Create a mutable variable
2. Modify or write the variable 
3. Read the variable whatever the new value is inside.

var <- newIORef 2
x <- readIORef var
print x 
writeIORef var 100
x1 <- readIORef var
print x1
-------------------------------------------------------------------
import Data.IORef
ref <- newIORef 0
replicateM_ 4 $ modifyIORef ref (+5)
readIORef ref >>= print  -- output 20

ref <- newIORef 1 
replicateM_ 4 $ modifyIORef ref (+5)
readIORef ref >>= print  -- output 21

-- modify (Set Int)
set <- newIORef Set.empty 
modifyIORef set (Set.insert 2) 
set <- readIORef set 

total <- newIORef 0
let loop i = 
        if i > 100
            then do 
                 readIORef total
            else do
                 modifyIORef total (+1)
                 pp $ "i=" <<< i
                 loop (i + 1)

loop 1