Maybe -package -cgi

data Maybe a
base Prelude, base Data.Maybe
The Maybe type encapsulates an optional value. A value of type Maybe a either contains a value of type a (represented as Just a), or it is empty (represented as Nothing). Using Maybe is a good way to deal with errors or exceptional cases without resorting to drastic measures such as error. The Maybe type is also a monad. It is a simple kind of error monad, error monad can be built using the Data.Either.Either type.
module Data.Maybe
base Data.Maybe
The Maybe type, and associated operations.
module Control.Monad.Trans.Maybe
transformers Control.Monad.Trans.Maybe
The MaybeT monad transformer adds the ability to fail to a monad. A sequence of actions succeeds, producing a value, only if all the actions in the sequence are successful. If one fails, the rest of the sequence is skipped and the composite action fails. For a variant allowing a range of error values, see Control.Monad.Trans.Error.
MaybeT :: m (Maybe a) -> MaybeT m a
transformers Control.Monad.Trans.Maybe
newtype MaybeT m a
transformers Control.Monad.Trans.Maybe
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.
maybe :: b -> (a -> b) -> Maybe a -> b
base Prelude, base Data.Maybe
The maybe function takes a default value, a function, and a Maybe value. If the Maybe value is Nothing, the function returns the default value. Otherwise, it applies the function to the value inside the Just and returns the result.
maybeNew :: (a -> IO (Ptr a)) -> (Maybe a -> IO (Ptr a))
base Foreign.Marshal.Utils
Allocate storage and marshal a storable value wrapped into a Maybe * the nullPtr is used to represent Nothing
maybePeek :: (Ptr a -> IO b) -> Ptr a -> IO (Maybe b)
base Foreign.Marshal.Utils
Convert a peek combinator into a one returning Nothing if applied to a nullPtr
maybeToList :: Maybe a -> [a]
base Data.Maybe
The maybeToList function returns an empty list when given Nothing or a singleton list when not given Nothing.
maybeWith :: (a -> (Ptr b -> IO c) -> IO c) -> (Maybe a -> (Ptr b -> IO c) -> IO c)
base Foreign.Marshal.Utils
Converts a withXXX combinator into one marshalling a value wrapped into a Maybe, using nullPtr to represent Nothing.
catMaybes :: [Maybe a] -> [a]
base Data.Maybe
The catMaybes function takes a list of Maybes and returns a list of all the Just values.
fromMaybe :: a -> Maybe a -> a
base Data.Maybe
The fromMaybe function takes a default value and and Maybe value. If the Maybe is Nothing, it returns the default values; otherwise, it returns the value contained in the Maybe.
listToMaybe :: [a] -> Maybe a
base Data.Maybe
The listToMaybe function returns Nothing on an empty list or Just a of the list.
mapMaybe :: (a -> Maybe b) -> [a] -> [b]
base Data.Maybe
The mapMaybe function is a version of map which can throw out elements. In particular, the functional argument returns something of type Maybe b. If this is Nothing, no element is added on to the result list. If it just Just b, then b is included in the result list.
mapMaybe :: (a -> Maybe b) -> IntMap a -> IntMap b
containers Data.IntMap.Strict, containers Data.IntMap.Lazy
O(n). Map values and collect the Just results. > let f x = if x == "a" then Just "new a" else Nothing > mapMaybe f (fromList [(5,"a"), (3,"b")]) == singleton 5 "new a"
mapMaybe :: (a -> Maybe b) -> Map k a -> Map k b
containers Data.Map.Lazy, containers Data.Map.Strict
O(n). Map values and collect the Just results. > let f x = if x == "a" then Just "new a" else Nothing > mapMaybe f (fromList [(5,"a"), (3,"b")]) == singleton 5 "new a"
mapMaybeT :: (m (Maybe a) -> n (Maybe b)) -> MaybeT m a -> MaybeT n b
transformers Control.Monad.Trans.Maybe
Transform the computation inside a MaybeT. *  (mapMaybeT f m) = f (runMaybeT >  
mapMaybeWithKey :: (Key -> a -> Maybe b) -> IntMap a -> IntMap b
containers Data.IntMap.Strict, containers Data.IntMap.Lazy
O(n). Map keys/values and collect the Just results. > let f k _ = if k < 5 then Just ("key (:) " ++ (show k)) else Nothing > mapMaybeWithKey f (fromList [(5,"a"), (3,"b")]) == singleton 3 "key (:) 3"
mapMaybeWithKey :: (k -> a -> Maybe b) -> Map k a -> Map k b
containers Data.Map.Lazy, containers Data.Map.Strict
O(n). Map keys/values and collect the Just results. > let f k _ = if k < 5 then Just ("key (:) " ++ (show k)) else Nothing > mapMaybeWithKey f (fromList [(5,"a"), (3,"b")]) == singleton 3 "key (:) 3"
optionMaybe :: Stream s m t => ParsecT s u m a -> ParsecT s u m (Maybe a)
parsec Text.Parsec.Combinator, parsec Text.ParserCombinators.Parsec.Combinator
optionMaybe p tries to apply parser p. If p fails without consuming input, it return Nothing, otherwise it returns Just the value returned by p.

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