MaybeT-monads-tf- MaybeT monad transformer compatible with monads-tf (deprecated)

Portabilitynon-portable (type families)




The MaybeT monad. See 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.)



newtype MaybeT m a

The parameterizable maybe monad, obtained by composing an arbitrary monad with the Maybe monad.

Computations are actions that may produce a value or fail.

The return function yields a successful computation, while >>= sequences two subcomputations, failing on the first error.




runMaybeT :: m (Maybe a)



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