{-# LANGUAGE TypeFamilies #-} {- | Copyright : 2007 Eric Kidd License : BSD3 Stability : experimental Portability : non-portable (type families) The 'MaybeT' monad. See <http://www.haskell.org/haskellwiki/New_monads/MaybeT> for more widely-used version. Our 'Functor' instance and our implementation of '>>=' are borrowed from there. [Computation type:] Computations which may fail or return nothing. [Binding strategy:] Failure returns the value 'Nothing', bypassing any bound functions which follow. Success returns a value wrapped in 'Just'. [Useful for:] Building computations from steps which may fail. No error information is returned. (If error information is required, see 'Control.Monad.Error'.) -} module Control.Monad.Maybe ( MaybeT(..) -- * Example -- $MaybeExample ) where import Control.Monad.Trans() import Control.Monad.Trans.Maybe import Control.Monad.Fix instance (MonadFix m) => MonadFix (MaybeT m) where -- Ref: http://www.haskell.org/pipermail/libraries/2011-April/016202.html mfix f = MaybeT (mfix (runMaybeT . f . unJust)) where unJust (Just x) = x -- if f is strict then mfix produces bottom, so f must produce a -- constructor without inspecting its argument. if that's Nothing, f -- cannot inspect its argument at all; if it's Just, then the call to -- unJust is safe. unJust Nothing = error "MaybeT mfix: Nothing! (this should be impossible)" {- $MaybeExample Here is an example that shows how to use 'MaybeT' to propagate an end-of-file condition in the IO monad. In the example below, both @maybeReadLine@ and @failIfQuit@ may cause a failure, which will propagate out to @main@ without further intervention. >import System.Console.Readline >import Data.Maybe >import Control.Monad >import Control.Monad.Trans >import Control.Monad.Maybe > >-- 'MaybeIO' is the type of computations which do IO, and which may fail. >type MaybeIO = MaybeT IO > >-- 'readline' already has type 'String -> IO (Maybe String)'; we just need >-- to wrap it. >maybeReadLine :: String -> MaybeIO String >maybeReadLine prompt = MaybeT (readline prompt) > >-- Fail if 'str' equals "quit". >failIfQuit :: (Monad m) => String -> m () >failIfQuit str = when (str == "quit") (fail "Quitting") > >-- This task may fail in several places. Try typing Control-D or "quit" at >-- any prompt. >concatTwoInputs :: MaybeIO () >concatTwoInputs = do > s1 <- maybeReadLine "String 1> " > failIfQuit s1 > s2 <- maybeReadLine "String 2> " > failIfQuit s2 > liftIO (putStrLn ("Concatenated: " ++ s1 ++ s2)) > >-- Loop until failure. >main :: IO () >main = do > result <- runMaybeT concatTwoInputs > if isNothing result > then putStrLn "Bye!" > else main -}