base-4.9.0.0: Basic libraries

Data.Maybe

Description

The Maybe type, and associated operations.

Synopsis

# Documentation

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

 Nothing Just a

Instances

 Source # Methods(>>=) :: Maybe a -> (a -> Maybe b) -> Maybe b Source #(>>) :: Maybe a -> Maybe b -> Maybe b Source #return :: a -> Maybe a Source # Source # Methodsfmap :: (a -> b) -> Maybe a -> Maybe b Source #(<$) :: a -> Maybe b -> Maybe a Source # Source # Methodsmfix :: (a -> Maybe a) -> Maybe a Source # Source # Methods Source # Methodspure :: a -> Maybe a Source #(<*>) :: Maybe (a -> b) -> Maybe a -> Maybe b Source #(*>) :: Maybe a -> Maybe b -> Maybe b Source #(<*) :: Maybe a -> Maybe b -> Maybe a Source # Source # 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 #sum :: Num a => Maybe a -> a Source #product :: Num a => Maybe a -> a Source # Source # Methodstraverse :: 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 # Source # Associated Typestype Rep1 (Maybe :: * -> *) :: * -> * Source # Methodsfrom1 :: Maybe a -> Rep1 Maybe a Source #to1 :: Rep1 Maybe a -> Maybe a Source # Source # Methodsmplus :: Maybe a -> Maybe a -> Maybe a Source # Source # Methods(<|>) :: Maybe a -> Maybe a -> Maybe a Source #some :: Maybe a -> Maybe [a] Source #many :: Maybe a -> Maybe [a] Source # Source # Methodsmzip :: 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 # Source # MethodsliftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> Maybe a -> ShowS Source #liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [Maybe a] -> ShowS Source # Source # MethodsliftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (Maybe a) Source #liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [Maybe a] Source # Source # MethodsliftCompare :: (a -> b -> Ordering) -> Maybe a -> Maybe b -> Ordering Source # Source # MethodsliftEq :: (a -> b -> Bool) -> Maybe a -> Maybe b -> Bool Source # Eq a => Eq (Maybe a) Source # Methods(==) :: Maybe a -> Maybe a -> Bool #(/=) :: Maybe a -> Maybe a -> Bool # Data a => Data (Maybe a) Source # 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 #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) Source # Methodscompare :: 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 # Read a => Read (Maybe a) Source # Methods Show a => Show (Maybe a) Source # MethodsshowsPrec :: Int -> Maybe a -> ShowS Source #show :: Maybe a -> String Source #showList :: [Maybe a] -> ShowS Source # Generic (Maybe a) Source # Associated Typestype Rep (Maybe a) :: * -> * Source # Methodsfrom :: Maybe a -> Rep (Maybe a) x Source #to :: Rep (Maybe a) x -> Maybe a Source # Semigroup a => Semigroup (Maybe a) Source # Methods(<>) :: Maybe a -> Maybe a -> Maybe a Source #sconcat :: NonEmpty (Maybe a) -> Maybe a Source #stimes :: Integral b => b -> Maybe a -> Maybe a Source # Monoid 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 there is no "Semigroup" typeclass providing just mappend, we use Monoid instead. Methodsmappend :: Maybe a -> Maybe a -> Maybe a Source #mconcat :: [Maybe a] -> Maybe a Source # 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)))) type (==) (Maybe k) a b Source # type (==) (Maybe k) a b 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 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 Basic usage: >>> isJust (Just 3) True  >>> isJust (Just ()) True  >>> isJust Nothing False  Only the outer constructor is taken into consideration: >>> isJust (Just Nothing) True  The isNothing function returns True iff its argument is Nothing. #### Examples 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 :: Maybe a -> a Source # The fromJust function extracts the element out of a Just and throws an error if its argument is Nothing. #### Examples 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. #### 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  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 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.

#### Examples

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

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 $[readMaybe x :: Maybe Int | x <- ["1", "Foo", "3"] ] [1,3]  mapMaybe :: (a -> Maybe b) -> [a] -> [b] Source # The mapMaybe function is a version of map which can throw out elements. In particular, the functional argument returns something of type Maybe b. If this is Nothing, no element is added on to the result list. If it is Just b, then b is included in the result list. #### Examples Using mapMaybe f x is a shortcut for catMaybes$ 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]