base-4.6.0.1: Basic libraries

Portability | portable
Stability | stable
Maintainer | libraries@haskell.org
Safe Haskell | Trustworthy

Data.Maybe

Description

The Maybe type, and associated operations.

Synopsis

- data Maybe a
  - = Nothing
  - | Just a
- maybe :: b -> (a -> b) -> Maybe a -> b
- isJust :: Maybe a -> Bool
- isNothing :: Maybe a -> Bool
- fromJust :: 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

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 |
Functor Maybe |
Typeable1 Maybe |
MonadFix Maybe |
MonadPlus Maybe |
Applicative Maybe |
Foldable Maybe |
Traversable Maybe |
Generic1 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 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.

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.

isJust :: Maybe a -> Bool

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

isNothing :: Maybe a -> Bool

The `isNothing` function returns `True` iff its argument is `Nothing`.

fromJust :: Maybe a -> a

The `fromJust` function extracts the element out of a `Just` and throws an error if its argument is `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`.

listToMaybe :: [a] -> Maybe a

The `listToMaybe` function returns `Nothing` on an empty list or ``Just` a` where `a` is the first element of the list.

maybeToList :: Maybe a -> [a]

The `maybeToList` function returns an empty list when given `Nothing` or a singleton list when not given `Nothing`.

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

The `catMaybes` function takes a list of `Maybe`s and returns a list of all the `Just` values.

mapMaybe :: (a -> Maybe b) -> [a] -> [b]

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 just ``Just` b`, then `b` is included in the result list.