Portability | non-portable (multi-parameter type classes, undecidable instances) |
---|---|

Stability | experimental |

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

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

MonadTrans MaybeT | |

MonadReader r m => MonadReader r (MaybeT m) | |

MonadState s m => MonadState s (MaybeT m) | |

MonadWriter w m => MonadWriter w (MaybeT m) | |

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) | |

MonadCont m => MonadCont (MaybeT m) |

# Limitations

The instance `MonadPlus`

is not provided, because it has ambiguous
semantics. It could refer to either

instance MonadPlus m => MonadPlus (MaybeT m)

...lifting the semantics of an underlying `MaybeT`

monad, or

instance MonadPlus (MaybeT m)

...with semantics similar to `MonadPlus Maybe`

.

# 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