# Control.Plus

- Package
- purescript-control
- Repository
- purescript/purescript-control

### #Plus Source

`class (Alt f) <= Plus f where`

The `Plus`

type class extends the `Alt`

type class with a value that
should be the left and right identity for `(<|>)`

.

It is similar to `Monoid`

, except that it applies to types of
kind `* -> *`

, like `Array`

or `List`

, rather than concrete types like
`String`

or `Number`

.

`Plus`

instances should satisfy the following laws:

- Left identity:
`empty <|> x == x`

- Right identity:
`x <|> empty == x`

- Annihilation:
`f <$> empty == empty`

#### Members

`empty :: forall a. f a`

#### Instances

## Re-exports from **Control.**Alt

### #Alt Source

`class (Functor f) <= Alt f where`

The `Alt`

type class identifies an associative operation on a type
constructor. It is similar to `Semigroup`

, except that it applies to
types of kind `* -> *`

, like `Array`

or `List`

, rather than concrete types
`String`

or `Number`

.

`Alt`

instances are required to satisfy the following laws:

- Associativity:
`(x <|> y) <|> z == x <|> (y <|> z)`

- Distributivity:
`f <$> (x <|> y) == (f <$> x) <|> (f <$> y)`

For example, the `Array`

(`[]`

) type is an instance of `Alt`

, where
`(<|>)`

is defined to be concatenation.

A common use case is to select the first "valid" item, or, if all items are "invalid", the last "invalid" item.

For example:

```
import Control.Alt ((<|>))
import Data.Maybe (Maybe(..)
import Data.Either (Either(..))
Nothing <|> Just 1 <|> Just 2 == Just 1
Left "err" <|> Right 1 <|> Right 2 == Right 1
Left "err 1" <|> Left "err 2" <|> Left "err 3" == Left "err 3"
```

#### Members

`alt :: forall a. f a -> f a -> f a`

#### Instances

## Re-exports from **Data.**Functor

### #Functor Source

`class Functor f where`

A `Functor`

is a type constructor which supports a mapping operation
`map`

.

`map`

can be used to turn functions `a -> b`

into functions
`f a -> f b`

whose argument and return types use the type constructor `f`

to represent some computational context.

Instances must satisfy the following laws:

- Identity:
`map identity = identity`

- Composition:
`map (f <<< g) = map f <<< map g`

#### Members

`map :: forall a b. (a -> b) -> f a -> f b`

#### Instances

### #void Source

`void :: forall f a. Functor f => f a -> f Unit`

The `void`

function is used to ignore the type wrapped by a
`Functor`

, replacing it with `Unit`

and keeping only the type
information provided by the type constructor itself.

`void`

is often useful when using `do`

notation to change the return type
of a monadic computation:

```
main = forE 1 10 \n -> void do
print n
print (n * n)
```