| 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 |
Data.Maybe
Description
The Maybe type, and associated operations.
Synopsis
- data Maybe a
- maybe :: b -> (a -> b) -> Maybe a -> b
- isJust :: Maybe a -> Bool
- isNothing :: Maybe a -> Bool
- fromJust :: HasCallStack => Maybe a -> a
- fromMaybe :: a -> Maybe a -> a
- listToMaybe :: [a] -> Maybe a
- maybeToList :: Maybe a -> [a]
- catMaybes :: [Maybe a] -> [a]
- mapMaybe :: (a -> Maybe b) -> [a] -> [b]
Documentation
The Maybe type encapsulates an optional value. A value of type
Maybe aa (represented as Just aNothing ). 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.
Instances
Instances details
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 #
Instance details
Defined in Data.Functor.Classes
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 #
Instance details
Defined in Data.Traversable
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 #
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.Maybe
Instance details
Defined in GHC.Generics
Instance details
Defined in GHC.Generics
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 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""
fromJust :: HasCallStack => Maybe a -> a Source #
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 aa 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 Maybe s 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 bNothing , no element
is added on to the result list. If it is Just bb is
included in the result list.
Examples
Expand
Using mapMaybe f xcatMaybes $ map f x
>>>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]