haskell / 9 / libraries / base-4.17.0.0 / data-maybe.html

Data.Maybe

Copyright (c) The University of Glasgow 2001
License BSD-style (see the file libraries/base/LICENSE)
Maintainer libraries@haskell.org
Stability stable
Portability portable
Safe Haskell Trustworthy
Language Haskell2010

Description

The Maybe type, and associated operations.

data Maybe a Source

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.

Constructors

Instances
Instances details
MonadFail Maybe Source

Since: base-4.9.0.0

Instance details

Defined in Control.Monad.Fail

Methods

fail :: String -> Maybe a Source

MonadFix Maybe Source

Since: base-2.1

Instance details

Defined in Control.Monad.Fix

Methods

mfix :: (a -> Maybe a) -> Maybe a Source

MonadZip Maybe Source

Since: base-4.8.0.0

Instance details

Defined in Control.Monad.Zip

Methods

mzip :: Maybe a -> Maybe b -> Maybe (a, b) Source

mzipWith :: (a -> b -> c) -> Maybe a -> Maybe b -> Maybe c Source

munzip :: Maybe (a, b) -> (Maybe a, Maybe b) Source

Foldable Maybe Source

Since: base-2.1

Instance details

Defined in Data.Foldable

Methods

fold :: Monoid m => Maybe m -> m Source

foldMap :: Monoid m => (a -> m) -> Maybe a -> 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

sum :: Num a => Maybe a -> a Source

product :: Num a => Maybe a -> a Source

Eq1 Maybe Source

Since: base-4.9.0.0

Instance details

Defined in Data.Functor.Classes

Methods

liftEq :: (a -> b -> Bool) -> Maybe a -> Maybe b -> Bool Source

Ord1 Maybe Source

Since: base-4.9.0.0

Instance details

Defined in Data.Functor.Classes

Methods

liftCompare :: (a -> b -> Ordering) -> Maybe a -> Maybe b -> Ordering Source

Read1 Maybe Source

Since: base-4.9.0.0

Instance details

Defined in Data.Functor.Classes

Methods

liftReadsPrec :: (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

Show1 Maybe Source

Since: base-4.9.0.0

Instance details

Defined in Data.Functor.Classes

Methods

liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> Maybe a -> ShowS Source

liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [Maybe a] -> ShowS Source

Traversable Maybe Source

Since: base-2.1

Instance details

Defined in Data.Traversable

Methods

traverse :: Applicative f => (a -> f b) -> Maybe a -> f (Maybe b) Source

sequenceA :: Applicative f => Maybe (f a) -> f (Maybe a) Source

mapM :: Monad m => (a -> m b) -> Maybe a -> m (Maybe b) Source

sequence :: Monad m => Maybe (m a) -> m (Maybe a) Source

Alternative Maybe Source

Since: base-2.1

Instance details

Defined in GHC.Base

Methods

empty :: Maybe a Source

(<|>) :: Maybe a -> Maybe a -> Maybe a Source

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

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

Applicative Maybe Source

Since: base-2.1

Instance details

Defined in GHC.Base

Methods

pure :: a -> Maybe a Source

(<*>) :: Maybe (a -> b) -> Maybe a -> Maybe b Source

liftA2 :: (a -> b -> c) -> Maybe a -> Maybe b -> Maybe c Source

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

(<*) :: Maybe a -> Maybe b -> Maybe a Source

Functor Maybe Source

Since: base-2.1

Instance details

Defined in GHC.Base

Methods

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

(<$) :: a -> Maybe b -> Maybe a Source

Monad Maybe Source

Since: base-2.1

Instance details

Defined in GHC.Base

Methods

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

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

return :: a -> Maybe a Source

MonadPlus Maybe Source

Since: base-2.1

Instance details

Defined in GHC.Base

Methods

mzero :: Maybe a Source

mplus :: Maybe a -> Maybe a -> Maybe a Source

Generic1 Maybe Source
Instance details

Defined in GHC.Generics

Associated Types

type Rep1 Maybe :: k -> Type Source

Methods

from1 :: forall (a :: k). Maybe a -> Rep1 Maybe a Source

to1 :: forall (a :: k). Rep1 Maybe a -> Maybe a Source

Data a => Data (Maybe a) Source

Since: base-4.0.0.0

Instance details

Defined in Data.Data

Methods

gfoldl :: (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 :: forall r r'. (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

Semigroup a => Monoid (Maybe a) Source

Lift a semigroup into Maybe forming a Monoid according to http://en.wikipedia.org/wiki/Monoid: "Any semigroup S may be turned into a monoid simply by adjoining an element e not in S and defining e*e = e and e*s = s = s*e for all s ∈ S."

Since 4.11.0: constraint on inner a value generalised from Monoid to Semigroup.

Since: base-2.1

Instance details

Defined in GHC.Base

Methods

mempty :: Maybe a Source

mappend :: Maybe a -> Maybe a -> Maybe a Source

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

Semigroup a => Semigroup (Maybe a) Source

Since: base-4.9.0.0

Instance details

Defined in GHC.Base

Methods

(<>) :: Maybe a -> Maybe a -> Maybe a Source

sconcat :: NonEmpty (Maybe a) -> Maybe a Source

stimes :: Integral b => b -> Maybe a -> Maybe a Source

Generic (Maybe a) Source
Instance details

Defined in GHC.Generics

Associated Types

type Rep (Maybe a) :: Type -> Type Source

Methods

from :: Maybe a -> Rep (Maybe a) x Source

to :: Rep (Maybe a) x -> Maybe a Source

Read a => Read (Maybe a) Source

Since: base-2.1

Instance details

Defined in GHC.Read

Show a => Show (Maybe a) Source

Since: base-2.1

Instance details

Defined in GHC.Show

Methods

showsPrec :: Int -> Maybe a -> ShowS Source

show :: Maybe a -> String Source

showList :: [Maybe a] -> ShowS Source

Eq a => Eq (Maybe a) Source

Since: base-2.1

Instance details

Defined in GHC.Maybe

Methods

(==) :: Maybe a -> Maybe a -> Bool Source

(/=) :: Maybe a -> Maybe a -> Bool Source

Ord a => Ord (Maybe a) Source

Since: base-2.1

Instance details

Defined in GHC.Maybe

Methods

compare :: Maybe a -> Maybe a -> Ordering Source

(<) :: Maybe a -> Maybe a -> Bool Source

(<=) :: Maybe a -> Maybe a -> Bool Source

(>) :: Maybe a -> Maybe a -> Bool Source

(>=) :: Maybe a -> Maybe a -> Bool Source

max :: Maybe a -> Maybe a -> Maybe a Source

min :: Maybe a -> Maybe a -> Maybe a Source

type Rep1 Maybe Source

Since: base-4.6.0.0

Instance details

Defined in GHC.Generics

type Rep1 Maybe = D1 ('MetaData "Maybe" "GHC.Maybe" "base" 'False) (C1 ('MetaCons "Nothing" 'PrefixI 'False) (U1 :: Type -> Type) :+: C1 ('MetaCons "Just" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) Par1))
type Rep (Maybe a) Source

Since: base-4.6.0.0

Instance details

Defined in GHC.Generics

type Rep (Maybe a) = D1 ('MetaData "Maybe" "GHC.Maybe" "base" 'False) (C1 ('MetaCons "Nothing" 'PrefixI 'False) (U1 :: Type -> Type) :+: C1 ('MetaCons "Just" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 a)))

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.

Examples
Expand

Basic usage:

>>> maybe False odd (Just 3)
True
>>> maybe False odd Nothing
False

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 _.

Examples
Expand

Basic usage:

>>> isJust (Just 3)
True
>>> isJust (Just ())
True
>>> isJust Nothing
False

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.

Examples
Expand

Basic usage:

>>> isNothing (Just 3)
False
>>> isNothing (Just ())
False
>>> isNothing Nothing
True

Only the outer constructor is taken into consideration:

>>> isNothing (Just Nothing)
False

fromJust :: HasCallStack => Maybe a -> a Source

The fromJust function extracts the element out of a Just and throws an error if its argument is Nothing.

Examples
Expand

Basic usage:

>>> fromJust (Just 1)
1
>>> 2 * (fromJust (Just 10))
20
>>> 2 * (fromJust Nothing)
*** Exception: Maybe.fromJust: Nothing
...

WARNING: This function is partial. You can use case-matching instead.

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

The fromMaybe function takes a default value and a Maybe value. If the Maybe is Nothing, it returns the default value; otherwise, it returns the value contained in the Maybe.

Examples
Expand

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.

Examples
Expand

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 given Just.

Examples
Expand

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 Maybes and returns a list of all the Just values.

Examples
Expand

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.

Examples
Expand

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/9.4.2/docs/libraries/base-4.17.0.0/Data-Maybe.html