Unit-testing IO in Haskell

demo.hs

module Main where

import Data.IORef
import Data.Time.Clock.POSIX

realIoUnderTest :: IORef Int -> IORef [[Char]] -> IORef Int -> IORef String -> IO ()
realIoUnderTest a b c d = do
  b' <- readIORef b 
  putStrLn $ Prelude.unlines b'
  
  a' <- readIORef a
  c' <- readIORef c
  writeIORef d $ "This took " ++ show (c' - a') ++ " units of time"

ioUnderTest = do
  a <- newIORef 0
  writeIORef a 3

  b <- newIORef []
  writeIORef b ["a", "b", "c"]

  c <- newIORef 0
  writeIORef c 3

  putStrLn "done stuff."

  d <- newIORef ""
  realIoUnderTest a b c d

  d' <- readIORef d
  if (d' == "This took 4 units of time")
    then putStrLn "success!"
    else putStrLn "fail!"
  
  
ioUnderTest' = do
  t0 <- getPOSIXTime
  a <- newIORef $ round t0

  line1 <- readLn
  line2 <- readLn
  b <- newIORef [line1, line2]
  
  t1 <- getPOSIXTime
  c <- newIORef $ round t1

  d <- newIORef ""
  realIoUnderTest a b c d

  d' <- readIORef d
  putStrLn d'

test = ioUnderTest

main = ioUnderTest'