-- Unsafe.hs
import Data.IORef (IORef, newIORef, readIORef, writeIORef)
import System.IO.Unsafe (unsafePerformIO)
var :: a -> IORef a
var = unsafePerformIO . newIORef
(<==) :: IORef a -> a -> ()
r <== a = unsafePerformIO $ writeIORef r a
star :: IORef a -> a
star = unsafePerformIO . readIORef
Haskell, a purely functional programming language, supports global mutable variables. Load the above script with ghci to see how it works.
*Main> let helloWorld = var "Hello, World!"
*Main> star helloWorld
"Hello, World!"
*Main> helloWorld <== "Surprise!"
()
*Main> star helloWorld
"Surprise!"
Notice, in line 1, let is used instead of <-. That's because var is not a monadic action. unsafePerformIO has removed the IO context and made it a "pure" function. unsafePerformIO is also used to achieve false purity in the definitions of the assignment operator <== and the dereference function star. These impurely pure functions can be buried anywhere in a codebase, turning the application into a game of minesweeper.
The unsafePerformIO documentation provides one of the more humorous lines technical writing has to offer:
This is the "back door" into the IO monad