Data.Nullable

data Nullable :: Type -> Type

A nullable type. This type constructor is intended to be used for interoperating with JavaScript functions which accept or return null values.

The runtime representation of Nullable T is the same as that of T, except that it may also be null. For example, the JavaScript values null, [], and [1,2,3] may all be given the type Nullable (Array Int). Similarly, the JavaScript values [], [null], and [1,2,null,3] may all be given the type Array (Nullable Int).

There is one pitfall with Nullable, which is that values of the type Nullable T will not function as you might expect if the type T happens to itself permit null as a valid runtime representation.

In particular, values of the type Nullable (Nullable T) will ‘collapse’, in the sense that the PureScript expressions notNull null and null will both leave you with a value whose runtime representation is just null. Therefore it is important to avoid using Nullable T in situations where T itself can take null as a runtime representation. If in doubt, use Maybe instead.

Nullable does not permit lawful Functor, Applicative, or Monad instances as a result of this pitfall, which is why these instances are not provided.

Instances

• (Show a) => Show (Nullable a)
• (Eq a) => Eq (Nullable a)
• Eq1 Nullable
• (Ord a) => Ord (Nullable a)
• Ord1 Nullable

null :: forall a. Nullable a

The null value.

notNull :: forall a. a -> Nullable a

Wrap a non-null value.

toMaybe :: forall a. Nullable a -> Maybe a

Represent null using Maybe a as Nothing. Note that this function can violate parametricity, as it inspects the runtime representation of its argument (see the warning about the pitfall of Nullable above).

toNullable :: forall a. Maybe a -> Nullable a

Takes Nothing to null, and Just a to a.