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 |

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

either contains a value of type `Maybe`

a`a`

(represented as

),
or it is empty (represented as `Just`

a`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.

#### Instances

MonadFail Maybe Source # |
| ||||

MonadFix Maybe Source # |
| ||||

MonadZip Maybe Source # |
| ||||

Foldable Maybe Source # |
| ||||

Defined in Data.Foldable 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 # | |||||

Eq1 Maybe Source # |
| ||||

Ord1 Maybe Source # |
| ||||

Defined in Data.Functor.Classes | |||||

Read1 Maybe Source # |
| ||||

Defined in Data.Functor.Classes 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 # |
| ||||

Traversable Maybe Source # |
| ||||

Alternative Maybe Source # | Picks the leftmost
| ||||

Applicative Maybe Source # |
| ||||

Functor Maybe Source # |
| ||||

Monad Maybe Source # |
| ||||

MonadPlus Maybe Source # | Picks the leftmost
| ||||

Generic1 Maybe Source # | |||||

Defined in GHC.Generics | |||||

Data a => Data (Maybe a) Source # |
| ||||

Defined in Data.Data 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
| ||||

Semigroup a => Semigroup (Maybe a) Source # |
| ||||

Generic (Maybe a) Source # | |||||

Defined in GHC.Generics
| |||||

Read a => Read (Maybe a) Source # |
| ||||

Show a => Show (Maybe a) Source # |
| ||||

Eq a => Eq (Maybe a) Source # |
| ||||

Ord a => Ord (Maybe a) Source # |
| ||||

type Rep1 Maybe Source # |
| ||||

type Rep (Maybe a) Source # |
| ||||

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

Basic usage:

`>>>`

True`maybe False odd (Just 3)`

`>>>`

False`maybe False odd Nothing`

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 )`

`>>>`

10`maybe 0 (*2) (readMaybe "5")`

`>>>`

0`maybe 0 (*2) (readMaybe "")`

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":

`>>>`

"5"`maybe "" show (Just 5)`

`>>>`

""`maybe "" show Nothing`

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

Basic usage:

`>>>`

1`fromJust (Just 1)`

`>>>`

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

`>>>`

*** Exception: Maybe.fromJust: Nothing ...`2 * (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

Basic usage:

`>>>`

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

`>>>`

5`fromMaybe 0 (readMaybe "5")`

`>>>`

0`fromMaybe 0 (readMaybe "")`

listToMaybe :: [a] -> Maybe a Source #

The `listToMaybe`

function returns `Nothing`

on an empty list
or

where `Just`

a`a`

is the first element of the list.

#### Examples

Basic usage:

`>>>`

Nothing`listToMaybe []`

`>>>`

Just 9`listToMaybe [9]`

`>>>`

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

Composing `maybeToList`

with `listToMaybe`

should be the identity
on singleton/empty lists:

`>>>`

[5]`maybeToList $ listToMaybe [5]`

`>>>`

[]`maybeToList $ listToMaybe []`

But not on lists with more than one element:

`>>>`

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

maybeToList :: Maybe a -> [a] Source #

The `maybeToList`

function returns an empty list when given
`Nothing`

or a singleton list when given `Just`

.

#### Examples

Basic usage:

`>>>`

[7]`maybeToList (Just 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 )`

`>>>`

3`sum $ maybeToList (readMaybe "3")`

`>>>`

0`sum $ maybeToList (readMaybe "")`

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

The `catMaybes`

function takes a list of `Maybe`

s and returns
a list of all the `Just`

values.

#### Examples

Basic usage:

`>>>`

[1,3]`catMaybes [Just 1, Nothing, Just 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 )`

`>>>`

[Just 1,Nothing,Just 3]`[readMaybe x :: Maybe Int | x <- ["1", "Foo", "3"] ]`

`>>>`

[1,3]`catMaybes $ [readMaybe x :: Maybe Int | x <- ["1", "Foo", "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

. If this is `Maybe`

b`Nothing`

, no element
is added on to the result list. If it is

, then `Just`

b`b`

is
included in the result list.

#### Examples

Using

is a shortcut for `mapMaybe`

f x

in most cases:`catMaybes`

$ `map`

f x

`>>>`

`import Text.Read ( readMaybe )`

`>>>`

`let readMaybeInt = readMaybe :: String -> Maybe Int`

`>>>`

[1,3]`mapMaybe readMaybeInt ["1", "Foo", "3"]`

`>>>`

[1,3]`catMaybes $ map readMaybeInt ["1", "Foo", "3"]`

If we map the `Just`

constructor, the entire list should be returned:

`>>>`

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