Copyright | (c) The University of Glasgow 2001 |
---|---|

License | BSD-style (see the file libraries/base/LICENSE) |

Maintainer | [email protected] |

Stability | stable |

Portability | portable |

Safe Haskell | Trustworthy |

Language | Haskell2010 |

The Maybe type, and associated operations.

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 |

maybe :: b -> (a -> b) -> Maybe a -> b Source

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.

Basic usage:

>>>maybe False odd (Just 3)True

>>>maybe False odd NothingFalse

Read an integer from a string using `readMaybe`

. If we succeed, return twice the integer; that is, apply `(*2)`

to it. If instead we fail to parse an integer, return `0`

by default:

>>>import Text.Read ( readMaybe )>>>maybe 0 (*2) (readMaybe "5")10 >>>maybe 0 (*2) (readMaybe "")0

Apply `show`

to a `Maybe Int`

. If we have `Just n`

, we want to show the underlying `Int`

`n`

. But if we have `Nothing`

, we return the empty string instead of (for example) "Nothing":

>>>maybe "" show (Just 5)"5" >>>maybe "" show Nothing""

isJust :: Maybe a -> Bool Source

The `isJust`

function returns `True`

iff its argument is of the form `Just _`

.

Basic usage:

>>>isJust (Just 3)True

>>>isJust (Just ())True

>>>isJust NothingFalse

Only the outer constructor is taken into consideration:

>>>isJust (Just Nothing)True

isNothing :: Maybe a -> Bool Source

The `isNothing`

function returns `True`

iff its argument is `Nothing`

.

Basic usage:

>>>isNothing (Just 3)False

>>>isNothing (Just ())False

>>>isNothing NothingTrue

Only the outer constructor is taken into consideration:

>>>isNothing (Just Nothing)False

fromJust :: Maybe a -> a Source

The `fromJust`

function extracts the element out of a `Just`

and throws an error if its argument is `Nothing`

.

Basic usage:

>>>fromJust (Just 1)1

>>>2 * (fromJust (Just 10))20

>>>2 * (fromJust Nothing)*** Exception: Maybe.fromJust: Nothing

fromMaybe :: a -> Maybe a -> a Source

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`

.

Basic usage:

>>>fromMaybe "" (Just "Hello, World!")"Hello, World!"

>>>fromMaybe "" Nothing""

Read an integer from a string using `readMaybe`

. If we fail to parse an integer, we want to return `0`

by default:

>>>import Text.Read ( readMaybe )>>>fromMaybe 0 (readMaybe "5")5 >>>fromMaybe 0 (readMaybe "")0

listToMaybe :: [a] -> Maybe a Source

The `listToMaybe`

function returns `Nothing`

on an empty list or `Just a`

where `a`

is the first element of the list.

Basic usage:

>>>listToMaybe []Nothing

>>>listToMaybe [9]Just 9

>>>listToMaybe [1,2,3]Just 1

Composing `maybeToList`

with `listToMaybe`

should be the identity on singleton/empty lists:

>>>maybeToList $ listToMaybe [5][5] >>>maybeToList $ listToMaybe [][]

But not on lists with more than one element:

>>>maybeToList $ listToMaybe [1,2,3][1]

maybeToList :: Maybe a -> [a] Source

The `maybeToList`

function returns an empty list when given `Nothing`

or a singleton list when not given `Nothing`

.

Basic usage:

>>>maybeToList (Just 7)[7]

>>>maybeToList Nothing[]

One can use `maybeToList`

to avoid pattern matching when combined with a function that (safely) works on lists:

>>>import Text.Read ( readMaybe )>>>sum $ maybeToList (readMaybe "3")3 >>>sum $ maybeToList (readMaybe "")0

catMaybes :: [Maybe a] -> [a] Source

The `catMaybes`

function takes a list of `Maybe`

s and returns a list of all the `Just`

values.

Basic usage:

>>>catMaybes [Just 1, Nothing, Just 3][1,3]

When constructing a list of `Maybe`

values, `catMaybes`

can be used to return all of the "success" results (if the list is the result of a `map`

, then `mapMaybe`

would be more appropriate):

>>>import Text.Read ( readMaybe )>>>[readMaybe x :: Maybe Int | x <- ["1", "Foo", "3"] ][Just 1,Nothing,Just 3] >>>catMaybes $ [readMaybe x :: Maybe Int | x <- ["1", "Foo", "3"] ][1,3]

mapMaybe :: (a -> Maybe b) -> [a] -> [b] Source

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 is `Just b`

, then `b`

is included in the result list.

Using `mapMaybe f x`

is a shortcut for `catMaybes $ map f x`

in most cases:

>>>import Text.Read ( readMaybe )>>>let readMaybeInt = readMaybe :: String -> Maybe Int>>>mapMaybe readMaybeInt ["1", "Foo", "3"][1,3] >>>catMaybes $ map readMaybeInt ["1", "Foo", "3"][1,3]

If we map the `Just`

constructor, the entire list should be returned:

>>>mapMaybe Just [1,2,3][1,2,3]

© The University of Glasgow and others

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/Data-Maybe.html