Data.Newtype

class (Coercible t a) <= Newtype t a | t -> a

A type class for newtypes 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

• Newtype (Additive a) a
• Newtype (Multiplicative a) a
• Newtype (Conj a) a
• Newtype (Disj a) a
• Newtype (Dual a) a
• Newtype (Endo c a) (c a a)
• Newtype (First a) a
• Newtype (Last a) a

wrap :: forall t a. Newtype t a => a -> t

unwrap :: forall t a. Newtype t a => t -> a

un :: forall t a. Newtype t a => (a -> t) -> t -> a

Given a constructor for a Newtype, this returns the appropriate unwrap function.

ala :: forall f t a s b. Coercible (f t) (f a) => 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 :: forall f g t a s b. Coercible (f t) (f a) => Coercible (g s) (g b) => 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 :: forall t a s b. 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 :: forall f g t a s b. Coercible (f a) (f t) => Coercible (g b) (g s) => 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 :: forall t a s b. Newtype t a => Newtype s b => (a -> t) -> (t -> s) -> a -> b

The opposite of over: lowers a function that operates on Newtyped 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 :: forall f g t a s b. Coercible (f t) (f a) => Coercible (g s) (g b) => 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 :: forall t a s b. 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.

overF2 :: forall f g t a s b. Coercible (f a) (f t) => Coercible (g b) (g s) => Newtype t a => Newtype s b => (a -> t) -> (f a -> f a -> g b) -> f t -> f t -> g s

Much like over2, but where the lifted binary function operates on values in a Functor.

under2 :: forall t a s b. Newtype t a => Newtype s b => (a -> t) -> (t -> t -> s) -> a -> a -> b

The opposite of over2: lowers a binary function that operates on Newtyped values to operate on the wrapped value instead.

underF2 :: forall f g t a s b. Coercible (f t) (f a) => Coercible (g s) (g b) => Newtype t a => Newtype s b => (a -> t) -> (f t -> f t -> g s) -> f a -> f a -> g b

Much like under2, but where the lifted binary function operates on values in a Functor.

traverse :: forall f t a. Coercible (f a) (f t) => Newtype t a => (a -> t) -> (a -> f a) -> t -> f t

Similar to the function from the Traversable class, but operating within a newtype instead.

collect :: forall f t a. Coercible (f a) (f t) => Newtype t a => (a -> t) -> (f a -> a) -> f t -> t

Similar to the function from the Distributive class, but operating within a newtype instead.