base- Basic libraries

Copyright(c) The University of Glasgow 2001
LicenseBSD-style (see the file libraries/base/LICENSE)
Safe HaskellTrustworthy



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.


Just a 


Monad Maybe 
Functor Maybe 
MonadFix Maybe 
Applicative Maybe 
Foldable Maybe 
Traversable Maybe 
Generic1 Maybe 
MonadPlus Maybe 
Alternative Maybe 
Eq a => Eq (Maybe a) 
Data a => Data (Maybe a) 
Ord a => Ord (Maybe a) 
Read a => Read (Maybe a) 
Show a => Show (Maybe a) 
Generic (Maybe a) 
Monoid a => Monoid (Maybe a)

Lift a semigroup into Maybe forming a Monoid according to "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.

type Rep1 Maybe 
type Rep (Maybe a) 
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.


Basic usage:

>>> maybe False odd (Just 3)
>>> 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 )
>>> maybe 0 (*2) (readMaybe "5")
>>> 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":

>>> maybe "" show (Just 5)
>>> maybe "" show Nothing

isJust :: Maybe a -> Bool Source

The isJust function returns True iff its argument is of the form Just _.


Basic usage:

>>> isJust (Just 3)
>>> isJust (Just ())
>>> isJust Nothing

Only the outer constructor is taken into consideration:

>>> isJust (Just Nothing)

isNothing :: Maybe a -> Bool Source

The isNothing function returns True iff its argument is Nothing.


Basic usage:

>>> isNothing (Just 3)
>>> isNothing (Just ())
>>> isNothing Nothing

Only the outer constructor is taken into consideration:

>>> isNothing (Just Nothing)

fromJust :: Maybe a -> a Source

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


Basic usage:

>>> fromJust (Just 1)
>>> 2 * (fromJust (Just 10))
>>> 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.


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")
>>> fromMaybe 0 (readMaybe "")

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.


Basic usage:

>>> listToMaybe []
>>> 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]
>>> maybeToList $ listToMaybe []

But not on lists with more than one element:

>>> 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 not given Nothing.


Basic usage:

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

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

The catMaybes function takes a list of Maybes and returns a list of all the Just values.


Basic usage:

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


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

If we map the Just constructor, the entire list should be returned:

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