# Data.Newtype

- Package
- purescript-newtype
- Repository
- purescript/purescript-newtype

### #Newtype Source

`class Newtype t a | t -> a where`

A type class for `newtype`

s to enable convenient wrapping and unwrapping,
and the use of the other functions in this module.

The compiler can derive instances of `Newtype`

automatically:

```
newtype EmailAddress = EmailAddress String
derive instance newtypeEmailAddress :: Newtype EmailAddress _
```

Note that deriving for `Newtype`

instances requires that the type be
defined as `newtype`

rather than `data`

declaration (even if the `data`

structurally fits the rules of a `newtype`

), and the use of a wildcard for
the wrapped type.

Instances must obey the following laws:

```
unwrap <<< wrap = id
wrap <<< unwrap = id
```

#### Members

### #ala Source

`ala :: forall b s a t f. Functor f => Newtype t a => Newtype s b => (a -> t) -> ((b -> s) -> f t) -> f a`

This combinator is for when you have a higher order function that you want
to use in the context of some newtype - `foldMap`

being a common example:

```
ala Additive foldMap [1,2,3,4] -- 10
ala Multiplicative foldMap [1,2,3,4] -- 24
ala Conj foldMap [true, false] -- false
ala Disj foldMap [true, false] -- true
```

### #alaF Source

`alaF :: forall b s a t g f. Functor f => Functor g => Newtype t a => Newtype s b => (a -> t) -> (f t -> g s) -> f a -> g b`

Similar to `ala`

but useful for cases where you want to use an additional
projection with the higher order function:

```
alaF Additive foldMap String.length ["hello", "world"] -- 10
alaF Multiplicative foldMap Math.abs [1.0, -2.0, 3.0, -4.0] -- 24.0
```

The type admits other possibilities due to the polymorphic `Functor`

constraints, but the case described above works because ((->) a) is a
`Functor`

.

### #over Source

`over :: forall b s a t. Newtype t a => Newtype s b => (a -> t) -> (a -> b) -> t -> s`

Lifts a function operate over newtypes. This can be used to lift a
function to manipulate the contents of a single newtype, somewhat like
`map`

does for a `Functor`

:

```
newtype Label = Label String
derive instance newtypeLabel :: Newtype Label _
toUpperLabel :: Label -> Label
toUpperLabel = over Label String.toUpper
```

But the result newtype is polymorphic, meaning the result can be returned as an alternative newtype:

```
newtype UppercaseLabel = UppercaseLabel String
derive instance newtypeUppercaseLabel :: Newtype UppercaseLabel _
toUpperLabel' :: Label -> UppercaseLabel
toUpperLabel' = over Label String.toUpper
```

### #overF Source

`overF :: forall b s a t g f. Functor f => Functor g => Newtype t a => Newtype s b => (a -> t) -> (f a -> g b) -> f t -> g s`

Much like `over`

, but where the lifted function operates on values in a
`Functor`

:

```
findLabel :: String -> Array Label -> Maybe Label
findLabel s = overF Label (Foldable.find (_ == s))
```

The above example also demonstrates that the functor type is polymorphic
here too, the input is an `Array`

but the result is a `Maybe`

.

### #under Source

`under :: forall b s a t. Newtype t a => Newtype s b => (a -> t) -> (t -> s) -> a -> b`

The opposite of `over`

: lowers a function that operates on `Newtype`

d
values to operate on the wrapped value instead.

```
newtype Degrees = Degrees Number
derive instance newtypeDegrees :: Newtype Degrees _
newtype NormalDegrees = NormalDegrees Number
derive instance newtypeNormalDegrees :: Newtype NormalDegrees _
normaliseDegrees :: Degrees -> NormalDegrees
normaliseDegrees (Degrees deg) = NormalDegrees (deg % 360.0)
asNormalDegrees :: Number -> Number
asNormalDegrees = under Degrees normaliseDegrees
```

As with `over`

the `Newtype`

is polymorphic, as illustrated in the example
above - both `Degrees`

and `NormalDegrees`

are instances of `Newtype`

,
so even though `normaliseDegrees`

changes the result type we can still put
a `Number`

in and get a `Number`

out via `under`

.

### #underF Source

`underF :: forall b s a t g f. Functor f => Functor g => Newtype t a => Newtype s b => (a -> t) -> (f t -> g s) -> f a -> g b`

Much like `under`

, but where the lifted function operates on values in a
`Functor`

:

```
newtype EmailAddress = EmailAddress String
derive instance newtypeEmailAddress :: Newtype EmailAddress _
isValid :: EmailAddress -> Boolean
isValid x = false -- imagine a slightly less strict predicate here
findValidEmailString :: Array String -> Maybe String
findValidEmailString = underF EmailAddress (Foldable.find isValid)
```

The above example also demonstrates that the functor type is polymorphic
here too, the input is an `Array`

but the result is a `Maybe`

.

### #over2 Source

`over2 :: forall b s a t. Newtype t a => Newtype s b => (a -> t) -> (a -> a -> b) -> t -> t -> s`

Lifts a binary function to operate over newtypes.

```
newtype Meter = Meter Int
derive newtype instance newtypeMeter :: Newtype Meter _
newtype SquareMeter = SquareMeter Int
derive newtype instance newtypeSquareMeter :: Newtype SquareMeter _
area :: Meter -> Meter -> SquareMeter
area = over2 Meter (*)
```

The above example also demonstrates that the return type is polymorphic here too.

- Modules
- Data.
Newtype