Control.Monad.Maybe

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.)

Documentation

newtype MaybeT m a

A monad transformer which adds Maybe semantics to an existing monad.

Constructors

MaybeT

Fields

runMaybeT :: m (Maybe a)

Instances

MonadTrans MaybeT

Monad m => Monad (MaybeT m)

Functor m => Functor (MaybeT m)

MonadFix m => MonadFix (MaybeT m)

Monad m => MonadPlus (MaybeT m)

MonadIO m => MonadIO (MaybeT m)

MonadWriter m => MonadWriter (MaybeT m)

MonadState m => MonadState (MaybeT m)

MonadReader m => MonadReader (MaybeT m)

MonadCont m => MonadCont (MaybeT m)

Limitations

Example

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