IO (Either e a) を ExceptT を使ってうまいことする

Example-ExceptT.hs

module Main where

import Control.Monad.Except

type IOE a = IO (Either String a)

req1 :: IOE Int
req1 = return . Right $ 1

req2 :: Int -> IOE Int
req2 n = return . Right $ n + 2
-- req2 n = return . Left $ "error"

req3 :: Int -> IOE Int
req3 n = return . Right $ n + 3

main :: IO ()
main = do
  a <- runExceptT $ do
    n1 <- ExceptT $ req1
    n2 <- ExceptT $ req2 n1
    n3 <- ExceptT $ req3 n2
    return n3

  b <- do
    en1 <- req1
    en2 <- case en1 of
      Left e   -> return $ Left e
      Right n1 -> req2 n1
    en3 <- case en2 of
      Left e   -> return $ Left e
      Right n2 -> req3 n2
    return en3

  putStrLn $ show a
  putStrLn $ show b