IO Either

Main.hs

import Control.Monad.Except (ExceptT, runExceptT)
import Data.Functor.Identity (Identity)

start :: ExceptT String IO String
start =
    return "start"


next1 :: String -> String
next1 first = first ++ " next1"

next2 :: Monad m => String -> ExceptT String m String
next2 first =
    return $ first ++ " next2"

next3 :: String -> ExceptT String IO String
next3 first =
    return $ first ++ " next3"


main' :: ExceptT String IO String
main' =
    do
        a <- start
        let b = next1 a
        c <- next2 b
        d <- next3 c
        return d

main :: IO ()
main =
    do
        result <- runExceptT main'
        putStrLn $ show result