MonadTransformers.hs

-- https://en.wikibooks.org/wiki/Haskell/Monad_transformers

import Data.Char
import Control.Monad
import Control.Monad.Trans
import Control.Monad.Trans.Maybe

-- getPassphrase :: IO (Maybe String)
-- getPassphrase =
--   getLine >>=
--   \s -> if isValid s
--         then return $ Just s
--         else return Nothing

isValid :: String -> Bool
isValid s =
  length s >= 8 &&
  any isAlpha s &&
  any isNumber s &&
  any isPunctuation s

-- askPassphrase :: IO ()
-- askPassphrase =
--   putStrLn "Insert your new passphrase:" >>
--   getPassphrase >>=
--   \maybe_value -> case maybe_value of
--     Just value -> putStrLn "Strong in database..."
--     Nothing    -> putStrLn "Passphrase invalid."

newtype MaybeT' m a = MaybeT' { runMaybeT' :: m (Maybe a) }
-- MaybeT' $ Just (Just Nothing)
-- MaybeT' $ \c -> (Just Nothing)
-- MaybeT' $ \c -> (Just (c : "hoge"))
-- (runMaybeT' $ MaybeT' $ \c -> (Just (c : "hoge"))) '5'

-- instance MonadTrans MaybeT where
--   lift = MaybeT . (liftM Just)

getPassphrase :: MaybeT IO String
getPassphrase =
  lift getLine >>=
  \s -> guard (isValid s) >>
  return s

askPassphrase :: MaybeT IO ()
askPassphrase =
  (lift $ putStrLn "Insert your new passphrase:") >>
  (msum $ repeat getPassphrase) >>=
  \value -> lift $ putStrLn "Storing in database..."