Looping forever with a recursive main. Haskell IO is often explained in terms of main returning an IO value then being executed by the runtime, but it's less clear what goes on when main apparently never terminates. See https://stackoverflow.com/questions/28203214/why-how-does-recursive-io-work

Forever.hs

module Forever where

main :: IO ()
main = do
  line <- getLine
  putStrLn line
  main

-- or if you prefer point-free style:

main2 :: IO ()
main2 = getLine >>= putStrLn >> main2