# Data.Maybe

- Package
- purescript-maybe
- Repository
- purescript/purescript-maybe

### #Maybe Source

`data Maybe a`

The `Maybe`

type is used to represent optional values and can be seen as
something like a type-safe `null`

, where `Nothing`

is `null`

and `Just x`

is the non-null value `x`

.

#### Constructors

#### Instances

`Functor Maybe`

`Apply Maybe`

The

`Apply`

instance allows functions contained within a`Just`

to transform a value contained within a`Just`

using the`apply`

operator:`Just f <*> Just x == Just (f x)`

`Nothing`

values are left untouched:`Just f <*> Nothing == Nothing Nothing <*> Just x == Nothing`

Combining

`Functor`

's`<$>`

with`Apply`

's`<*>`

can be used transform a pure function to take`Maybe`

-typed arguments so`f :: a -> b -> c`

becomes`f :: Maybe a -> Maybe b -> Maybe c`

:`f <$> Just x <*> Just y == Just (f x y)`

The

`Nothing`

-preserving behaviour of both operators means the result of an expression like the above but where any one of the values is`Nothing`

means the whole result becomes`Nothing`

also:`f <$> Nothing <*> Just y == Nothing f <$> Just x <*> Nothing == Nothing f <$> Nothing <*> Nothing == Nothing`

`Applicative Maybe`

The

`Applicative`

instance enables lifting of values into`Maybe`

with the`pure`

function:`pure x :: Maybe _ == Just x`

Combining

`Functor`

's`<$>`

with`Apply`

's`<*>`

and`Applicative`

's`pure`

can be used to pass a mixture of`Maybe`

and non-`Maybe`

typed values to a function that does not usually expect them, by using`pure`

for any value that is not already`Maybe`

typed:`f <$> Just x <*> pure y == Just (f x y)`

Even though

`pure = Just`

it is recommended to use`pure`

in situations like this as it allows the choice of`Applicative`

to be changed later without having to go through and replace`Just`

with a new constructor.`Alt Maybe`

The

`Alt`

instance allows for a choice to be made between two`Maybe`

values with the`<|>`

operator, where the first`Just`

encountered is taken.`Just x <|> Just y == Just x Nothing <|> Just y == Just y Nothing <|> Nothing == Nothing`

`Plus Maybe`

The

`Plus`

instance provides a default`Maybe`

value:`empty :: Maybe _ == Nothing`

`Alternative Maybe`

The

`Alternative`

instance guarantees that there are both`Applicative`

and`Plus`

instances for`Maybe`

.`Bind Maybe`

The

`Bind`

instance allows sequencing of`Maybe`

values and functions that return a`Maybe`

by using the`>>=`

operator:`Just x >>= f = f x Nothing >>= f = Nothing`

`Monad Maybe`

The

`Monad`

instance guarantees that there are both`Applicative`

and`Bind`

instances for`Maybe`

. This also enables the`do`

syntactic sugar:`do x' <- x y' <- y pure (f x' y')`

Which is equivalent to:

`x >>= (\x' -> y >>= (\y' -> pure (f x' y')))`

Which is equivalent to:

`case x of Nothing -> Nothing Just x' -> case y of Nothing -> Nothing Just y' -> Just (f x' y')`

`MonadZero Maybe`

`Extend Maybe`

The

`Extend`

instance allows sequencing of`Maybe`

values and functions that accept a`Maybe a`

and return a non-`Maybe`

result using the`<<=`

operator.`f <<= Nothing = Nothing f <<= x = Just (f x)`

`Invariant Maybe`

`(Semigroup a) => Semigroup (Maybe a)`

The

`Semigroup`

instance enables use of the operator`<>`

on`Maybe`

values whenever there is a`Semigroup`

instance for the type the`Maybe`

contains. The exact behaviour of`<>`

depends on the "inner"`Semigroup`

instance, but generally captures the notion of appending or combining things.`Just x <> Just y = Just (x <> y) Just x <> Nothing = Just x Nothing <> Just y = Just y Nothing <> Nothing = Nothing`

`(Semigroup a) => Monoid (Maybe a)`

`(Eq a) => Eq (Maybe a)`

The

`Eq`

instance allows`Maybe`

values to be checked for equality with`==`

and inequality with`/=`

whenever there is an`Eq`

instance for the type the`Maybe`

contains.`Eq1 Maybe`

`(Ord a) => Ord (Maybe a)`

The

`Ord`

instance allows`Maybe`

values to be compared with`compare`

,`>`

,`>=`

,`<`

and`<=`

whenever there is an`Ord`

instance for the type the`Maybe`

contains.`Nothing`

is considered to be less than any`Just`

value.`Ord1 Maybe`

`(Bounded a) => Bounded (Maybe a)`

`(Show a) => Show (Maybe a)`

The

`Show`

instance allows`Maybe`

values to be rendered as a string with`show`

whenever there is an`Show`

instance for the type the`Maybe`

contains.`Generic (Maybe a) _`

### #maybe Source

`maybe :: forall a b. b -> (a -> b) -> Maybe a -> b`

Takes a default value, a function, and a `Maybe`

value. If the `Maybe`

value is `Nothing`

the default value is returned, otherwise the function
is applied to the value inside the `Just`

and the result is returned.

```
maybe x f Nothing == x
maybe x f (Just y) == f y
```

### #maybe' Source

`maybe' :: forall a b. (Unit -> b) -> (a -> b) -> Maybe a -> b`

Similar to `maybe`

but for use in cases where the default value may be
expensive to compute. As PureScript is not lazy, the standard `maybe`

has
to evaluate the default value before returning the result, whereas here
the value is only computed when the `Maybe`

is known to be `Nothing`

.

```
maybe' (\_ -> x) f Nothing == x
maybe' (\_ -> x) f (Just y) == f y
```

### #fromMaybe' Source

`fromMaybe' :: forall a. (Unit -> a) -> Maybe a -> a`

Similar to `fromMaybe`

but for use in cases where the default value may be
expensive to compute. As PureScript is not lazy, the standard `fromMaybe`

has to evaluate the default value before returning the result, whereas here
the value is only computed when the `Maybe`

is known to be `Nothing`

.

```
fromMaybe' (\_ -> x) Nothing == x
fromMaybe' (\_ -> x) (Just y) == y
```

### #optional Source

`optional :: forall f a. Alt f => Applicative f => f a -> f (Maybe a)`

One or none.

```
optional empty = pure Nothing
```

The behaviour of `optional (pure x)`

depends on whether the `Alt`

instance
satisfy the left catch law (`pure a <|> b = pure a`

).

`Either e`

does:

```
optional (Right x) = Right (Just x)
```

But `Array`

does not:

```
optional [x] = [Just x, Nothing]
```

The

`Functor`

instance allows functions to transform the contents of a`Just`

with the`<$>`

operator:`Nothing`

values are left untouched: