MaybeTについて学んだ

MaybeT.hs

module Main where

import System.Environment
import Data.Maybe
import Control.Monad.Trans.Maybe
import Control.Monad.Trans.Class


-- sample >>= fromMaybe (putStrLn "not found")
sample1= do
  maybeHoge <- lookupEnv "Hoge"
  return $ do
    hoge <- maybeHoge
    return $ putStrLn hoge

sample1_5 = do
  maybeHoge <- lookupEnv "Hoge"
  case maybeHoge of
    Just hoge -> putStrLn hoge >> return (Just ())
    Nothing -> return Nothing

-- sample2
sample2 = do
  maybeHoge <- lookupEnv "Hoge" :: IO (Maybe String)
  let hoge = fromMaybe "not found" maybeHoge
  putStrLn hoge

-- sample3
sample3 = do
  hoge <- MaybeT $ lookupEnv "Hoge"
  lift $ putStrLn hoge


sample4 = do
  maybe <- runMaybeT sample3
  case maybe of
    Nothing -> putStrLn "not found"
    _ -> return ()

sample5 = do
  maybe <- sample1_5
  case maybe of
    Nothing -> putStrLn "not found"
    _ -> return ()

main = runMaybeT sample3
➜  IoMaybeA