Safe Haskell | None |
---|---|

Language | Haskell2010 |

Maybe type

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, where all errors are represented by `Nothing`

. A richer error monad can be built using the `Either`

type.

Monad Maybe | Since: base-2.1 |

Functor Maybe | Since: base-2.1 |

MonadFix Maybe | Since: base-2.1 |

Defined in Control.Monad.Fix | |

MonadFail Maybe | Since: base-4.9.0.0 |

Defined in Control.Monad.Fail | |

Applicative Maybe | Since: base-2.1 |

Foldable Maybe | Since: base-2.1 |

Defined in Data.Foldable ## Methodsfold :: Monoid m => Maybe m -> m Source foldMap :: Monoid m => (a -> m) -> Maybe a -> m Source foldr :: (a -> b -> b) -> b -> Maybe a -> b Source foldr' :: (a -> b -> b) -> b -> Maybe a -> b Source foldl :: (b -> a -> b) -> b -> Maybe a -> b Source foldl' :: (b -> a -> b) -> b -> Maybe a -> b Source foldr1 :: (a -> a -> a) -> Maybe a -> a Source foldl1 :: (a -> a -> a) -> Maybe a -> a Source toList :: Maybe a -> [a] Source null :: Maybe a -> Bool Source length :: Maybe a -> Int Source elem :: Eq a => a -> Maybe a -> Bool Source maximum :: Ord a => Maybe a -> a Source minimum :: Ord a => Maybe a -> a Source | |

Traversable Maybe | Since: base-2.1 |

MonadPlus Maybe | Since: base-2.1 |

Alternative Maybe | Since: base-2.1 |

MonadZip Maybe | Since: base-4.8.0.0 |

Show1 Maybe | Since: base-4.9.0.0 |

Read1 Maybe | Since: base-4.9.0.0 |

Defined in Data.Functor.Classes ## MethodsliftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (Maybe a) Source liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [Maybe a] Source liftReadPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec (Maybe a) Source liftReadListPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec [Maybe a] Source | |

Ord1 Maybe | Since: base-4.9.0.0 |

Defined in Data.Functor.Classes | |

Eq1 Maybe | Since: base-4.9.0.0 |

Eq a => Eq (Maybe a) | Since: base-2.1 |

Data a => Data (Maybe a) | Since: base-4.0.0.0 |

Defined in Data.Data ## Methodsgfoldl :: (forall d b. Data d => c (d -> b) -> d -> c b) -> (forall g. g -> c g) -> Maybe a -> c (Maybe a) Source gunfold :: (forall b r. Data b => c (b -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c (Maybe a) Source toConstr :: Maybe a -> Constr Source dataTypeOf :: Maybe a -> DataType Source dataCast1 :: Typeable t => (forall d. Data d => c (t d)) -> Maybe (c (Maybe a)) Source dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c (Maybe a)) Source gmapT :: (forall b. Data b => b -> b) -> Maybe a -> Maybe a Source gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> Maybe a -> r Source gmapQr :: (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> Maybe a -> r Source gmapQ :: (forall d. Data d => d -> u) -> Maybe a -> [u] Source gmapQi :: Int -> (forall d. Data d => d -> u) -> Maybe a -> u Source gmapM :: Monad m => (forall d. Data d => d -> m d) -> Maybe a -> m (Maybe a) Source gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> Maybe a -> m (Maybe a) Source gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> Maybe a -> m (Maybe a) Source | |

Ord a => Ord (Maybe a) | Since: base-2.1 |

Read a => Read (Maybe a) | Since: base-2.1 |

Show a => Show (Maybe a) | Since: base-2.1 |

Generic (Maybe a) | |

Semigroup a => Semigroup (Maybe a) | Since: base-4.9.0.0 |

Semigroup a => Monoid (Maybe a) |
Lift a semigroup into Since 4.11.0: constraint on inner Since: base-2.1 |

Generic1 Maybe | |

type Rep (Maybe a) | Since: base-4.6.0.0 |

type Rep1 Maybe | Since: base-4.6.0.0 |

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Licensed under a BSD-style license (see top of the page).

https://downloads.haskell.org/~ghc/8.6.1/docs/html/libraries/base-4.12.0.0/GHC-Maybe.html