early return

main.hs

module Main where

import Control.Monad.Trans.Class
import Control.Monad.Trans.Cont

foo :: ContT r IO String
foo = callCC $ \k -> do
    b <- lift $ readLn
    if b then k "early return" else return ()
    lift $ putStrLn "Hello world!"
    return "return"

main :: IO ()
main = do
    res <- evalContT foo
    putStrLn res
    return ()

test

ghci> main
True
early return
ghci> main
False
Hello world!
return