|Portability||non-portable (multi-parameter type classes, undecidable instances)|
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
|newtype MaybeT m a|
|A monad transformer which adds Maybe semantics to an existing monad.
|runMaybeT :: (m (Maybe a))|
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.
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.
-- '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> "
s2 <- maybeReadLine "String 2> "
liftIO (putStrLn ("Concatenated: " ++ s1 ++ s2))
-- Loop until failure.
main :: IO ()
main = do
result <- runMaybeT concatTwoInputs
if isNothing result
then putStrLn "Bye!"
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