lazy IO と imperative programming とが相性がわるい例

lazyIO.hs

module Main where

import System.IO.Unsafe

lazy :: IO a -> IO a
lazy = unsafeInterleaveIO

main :: IO ()
main = do
  { input1 <- getLine
  ; input2 <- getLine
  ; putStrLn ("out: " ++ input2)
  ; putStrLn "1.------"
  ; input3 <- lazy getLine
  ; input4 <- lazy getLine
  ; putStrLn ("out: " ++ input4)
  ; putStrLn "2.------"
  }
{- ^
>>> :main
foo
bar
out: bar
1.------
baz
out: baz
2.------
-}