haskell - understanding MonadTransformer examples -

i going through tutorial @ https://en.wikibooks.org/wiki/haskell/monad_transformers

i wrote following piece of codes. 1 without , other monadtransformer instance :

-- simple password functions.  getpassphrase1 :: io (maybe string) getpassphrase1 =   password <- getline   if isvalid password   return $ password   else return nothing   askpassphrase1 :: io () askpassphrase1 =   putstrln "enter password < 8 , alpha, number , punctuation:"   p <- getpassphrase1   case p of     nothing ->          -- q1. ### how implement `monadtrans` ?       putstrln "invalid password. enter again:"       askpassphrase1     password ->       putstrln $ "your password " ++ password  -- validation test want be. isvalid :: string -> bool isvalid s = length s >= 8             && isalpha s             && isnumber s             && ispunctuation s 

another using monadt wrote myself.

getpassphrase2 :: maybet io string getpassphrase2 =   password <- lift getline   guard $ isvalid password   return password  askpassphrase2 :: maybet io () askpassphrase2 =   lift $ putstrln "enter password < 8 , alpha, number , punctuation:"   p <- getpassphrase2   -- q1. how print "invalid password" ?   lift $ putstrln $ "your password " ++ p  -- validation test want be. isvalid :: string -> bool isvalid s = length s >= 8             && isalpha s             && isnumber s             && ispunctuation s  main :: io () main =   <- runmaybet askpassphrase2   return () 

both works.

but unable understand how add wrong password support in monadtrans example. ?

also, main method ok.. or can written in better way ?

guard not want in maybet approach. check invalid password and able provide own processing in case use original version of getpassphase , lift the maybet io monad:

getpassphease2 = result <- lift $ getpassphrase1                     case result of                         nothing -> lift $ putstrln "bad password"                         pw -> lift $ putstrln "your password is: " ++ pw 

explanation...

the maybet io monad giving io-actions capability fail and having failure automatically handled monad. if step fails, control goes way runmaybet , runmaybet returns nothing. it's throwing exception.

the point of using maybet no have explicitly check see if step has failed - checking performed maybet monad after each step called. means can write code assuming each preceding step has succeeded - i.e. if on "happy path". means can't different if previous step failed.

one possibility using maybet version of getpassphrase this:

main = result <- runmaybet askpassphrase2           case result of             _  -> return ()             nothing  -> putstrln "some failure happened... perhaps wrong password?" 

the problem if runmaybet returns nothing mean step in askpassphrase failed, not guard.

another way use maybet version of getpassphrase have askpassphrase run runmaybet:

askpassphrase2 = result <- lift $ runmaybet getpassphrase2                     case result of                       nothing -> lift $ putstrln "bad password"                       pw -> lift $ putstrln $ "your password " ++ pw 

here we're using runmaybet try-catch block.