Precursor.Data.Maybe

Documentation

data Maybe a :: * -> *

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

Nothing

Just a

Instances

Monad Maybe

Methods

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

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

return :: a -> Maybe a

fail :: String -> Maybe a

Functor Maybe

Methods

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

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

MonadFix Maybe

Methods

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

Applicative Maybe

Methods

pure :: a -> Maybe a

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

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

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

Foldable Maybe

Methods

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

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

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

foldr' :: (a -> b -> b) -> b -> Maybe a -> b

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

foldl' :: (b -> a -> b) -> b -> Maybe a -> b

foldr1 :: (a -> a -> a) -> Maybe a -> a

foldl1 :: (a -> a -> a) -> Maybe a -> a

toList :: Maybe a -> [a]

null :: Maybe a -> Bool

length :: Maybe a -> Int

elem :: Eq a => a -> Maybe a -> Bool

maximum :: Ord a => Maybe a -> a

minimum :: Ord a => Maybe a -> a

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

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

Traversable Maybe

Methods

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

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

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

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

Generic1 Maybe

Associated Types

type Rep1 (Maybe :: * -> *) :: * -> *

Methods

from1 :: Maybe a -> Rep1 Maybe a

to1 :: Rep1 Maybe a -> Maybe a

Eq1 Maybe

Methods

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

Ord1 Maybe

Methods

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

Read1 Maybe

Methods

liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (Maybe a)

liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [Maybe a]

Show1 Maybe

Methods

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

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

Alternative Maybe

Methods

empty :: Maybe a

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

some :: Maybe a -> Maybe [a]

many :: Maybe a -> Maybe [a]

MonadPlus Maybe

Methods

mzero :: Maybe a

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

Eq a => Eq (Maybe a)

Methods

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

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

Ord a => Ord (Maybe a)

Methods

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

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

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

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

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

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

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

Generic (Maybe a)

Associated Types

type Rep (Maybe a) :: * -> *

Methods

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

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

Semigroup a => Semigroup (Maybe a)

Methods

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

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

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

Monoid a => Monoid (Maybe a)

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 there is no "Semigroup" typeclass providing just mappend, we use Monoid instead.

Methods

mempty :: Maybe a

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

mconcat :: [Maybe a] -> Maybe a

Lift a => Lift (Maybe a)

Methods

lift :: Maybe a -> Q Exp

SingI (Maybe a) (Nothing a)

Methods

sing :: Sing (Nothing a) a

SingKind a (KProxy a) => SingKind (Maybe a) (KProxy (Maybe a))

Associated Types

type DemoteRep (KProxy (Maybe a)) (kparam :: KProxy (KProxy (Maybe a))) :: *

Methods

fromSing :: Sing (KProxy (Maybe a)) a -> DemoteRep (KProxy (Maybe a)) kparam

SingI a a1 => SingI (Maybe a) (Just a a1)

Methods

sing :: Sing (Just a a1) a

type Rep1 Maybe

type Rep1 Maybe = D1 (MetaData "Maybe" "GHC.Base" "base" False) ((:+:) (C1 (MetaCons "Nothing" PrefixI False) U1) (C1 (MetaCons "Just" PrefixI False) (S1 (MetaSel (Nothing Symbol) NoSourceUnpackedness NoSourceStrictness DecidedLazy) Par1)))

type Rep (Maybe a)

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

data Sing (Maybe a)

data Sing (Maybe a) where

• SNothing :: Sing (Maybe a) (Nothing a)
• SJust :: Sing (Maybe a) (Just a a1)

type (==) (Maybe k) a b

type (==) (Maybe k) a b = EqMaybe k a b

type DemoteRep (Maybe a) (KProxy (Maybe a))

type DemoteRep (Maybe a) (KProxy (Maybe a)) = Maybe (DemoteRep a (KProxy a))

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

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

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
""

fromMaybe :: a -> Maybe a -> a

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.

Examples

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