On Sunday 26 December 2010 20:00:02, michael rice wrote:
I lifted the code below from here:
http://en.wikibooks.org/wiki/Haskell/Monad_transformers
Since the wiki page doesn't say what needs to be imported, I'm guessing.
Not sure what is happening. Maybe someone can tell me.
It loops trying to show `askPassword'. You have declared an instance Show (MaybeT m a) without defining any methods, so when you type ghci> askPassword at the prompt, it tries to print the action, putStrLn (show askPassword) show has a default definition show x = showsPrec 0 x "" so it looks for showsPrec, which has a default definition showsPrec _ x s = show x ++ s ~> loop, that overflows the stack. Try ghci> runMaybeT askPassword
Michael
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import Control.Monad import Control.Monad.Trans.Class import Data.Char
newtype (Monad m) => MaybeT m a = MaybeT { runMaybeT :: m (Maybe a) }
instance Monad m => Monad (MaybeT m) where return = MaybeT . return . Just x >>= f = MaybeT $ do maybe_value <- runMaybeT x case maybe_value of Nothing -> return Nothing Just value -> runMaybeT $ f value
instance Monad m => MonadPlus (MaybeT m) where mzero = MaybeT $ return Nothing mplus x y = MaybeT $ do maybe_value <- runMaybeT x case maybe_value of Nothing -> runMaybeT y Just value -> runMaybeT x instance MonadTrans MaybeT where lift = MaybeT . (liftM Just)
instance Show (MaybeT m a)
getValidPassword :: MaybeT IO String getValidPassword = do s <- lift getLine guard (isValid s) return s
isValid :: String -> Bool isValid s = (length s > 8) && ((filter isAlphaNum s) == s) && any isDigit s && any isAlpha s askPassword :: MaybeT IO () askPassword = do lift $ putStrLn "Insert your new password:" value <- getValidPassword lift $ putStrLn "Storing in database..."
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[michael@localhost ~]$ ghci GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude> :l test5 [1 of 1] Compiling Main ( test5.hs, interpreted ) Ok, modules loaded: Main. *Main> askPassword Loading package transformers-0.2.2.0 ... linking ... done. *** Exception: stack overflow *Main>