None final

finalclassNoneextendsOption<Never>

An Option that signifies the absence of a value.

Inheritance

Object → Option<A> → None

Constructors

None() const

constNone()

Implementation

const None() : super._();

Properties

hashCode no setter override

intgethashCode

The hash code for this object.

A hash code is a single integer which represents the state of the object that affects operator == comparisons.

All objects have hash codes. The default hash code implemented by Object represents only the identity of the object, the same way as the default operator == implementation only considers objects equal if they are identical (see identityHashCode).

If operator == is overridden to use the object state instead, the hash code must also be changed to represent that state, otherwise the object cannot be used in hash based data structures like the default Set and Map implementations.

Hash codes must be the same for objects that are equal to each other according to operator ==. The hash code of an object should only change if the object changes in a way that affects equality. There are no further requirements for the hash codes. They need not be consistent between executions of the same program and there are no distribution guarantees.

Objects that are not equal are allowed to have the same hash code. It is even technically allowed that all instances have the same hash code, but if clashes happen too often, it may reduce the efficiency of hash-based data structures like HashSet or HashMap.

If a subclass overrides hashCode, it should override the operator == operator as well to maintain consistency.

Implementation

@override
int get hashCode => 0;

isDefined no setter inherited

boolgetisDefined

Returns true if this Option is a Some, false if it's a None.

Inherited from Option.

Implementation

bool get isDefined => this is Some;

isEmpty no setter inherited

boolgetisEmpty

Returns true if this is a None, false if it's a Some.

Inherited from Option.

Implementation

bool get isEmpty => this is None;

nonEmpty no setter inherited

boolgetnonEmpty

Returns true if this is a Some, false if it's a None.

Inherited from Option.

Implementation

bool get nonEmpty => this is Some;

runtimeType no setter inherited

TypegetruntimeType

A representation of the runtime type of the object.

Inherited from Object.

Implementation

external Type get runtimeType;

Methods

exists() inherited

boolexists(boolFunction(Never)p)

Returns true if this is a Some and the value satisfies p.

Inherited from Option.

Implementation

bool exists(Function1<A, bool> p) => fold(() => false, p);

filter() override

Option<Never>filter(boolFunction(Never)p)

Returns this option if it is non-empty and p returns true for the value. Otherwise returns None.

Implementation

@override
Option<Never> filter(Function1<Never, bool> p) => this;

filterNot() inherited

Option<Never>filterNot(boolFunction(Never)p)

Returns this option if it is empty or p returns false for the value. Otherwise returns None.

Inherited from Option.

Implementation

Option<A> filterNot(Function1<A, bool> p) => filter((a) => !p(a));

flatMap() override

Option<B>flatMap<B>(Option<B>Function(Never)f)

Returns the result of applying f to the contained value if this is a Some, or None if this is a None.

Implementation

@override
Option<B> flatMap<B>(Function1<Never, Option<B>> f) => this;

fold() override

Bfold<B>(BFunction()ifEmpty,BFunction(Never)f)

Returns the result of applying f to this Option value if non-empty. Otherwise, returns the result of ifEmpty.

Implementation

@override
B fold<B>(Function0<B> ifEmpty, Function1<Never, B> f) => ifEmpty();

foldLeft() inherited

BfoldLeft<B>(Bz,BFunction(B,Never)op)

Applies op to the seed z and the contained value, or returns z for None.

Inherited from Option.

Implementation

B foldLeft<B>(B z, Function2<B, A, B> op) => fold(() => z, (a) => op(z, a));

foldRight() inherited

BfoldRight<B>(Bz,BFunction(Never,B)op)

Applies op to the contained value and seed z, or returns z for None.

Inherited from Option.

Implementation

B foldRight<B>(B z, Function2<A, B, B> op) => fold(() => z, (a) => op(a, z));

forall() inherited

boolforall(boolFunction(Never)p)

Returns true if this is a None, or if the value satisfies p.

Inherited from Option.

Implementation

bool forall(Function1<A, bool> p) => fold(() => true, p);

foreach() inherited

voidforeach<U>(UFunction(Never)f)

Applies f to the value if this is a Some; does nothing for None.

Inherited from Option.

Implementation

void foreach<U>(Function1<A, U> f) => fold(() {}, f);

map() override

Option<B>map<B>(BFunction(Never)f)

Applies f to the contained value and wraps the result in Some, or returns None if this is a None.

Implementation

@override
Option<B> map<B>(Function1<Never, B> f) => this;

noSuchMethod() inherited

dynamicnoSuchMethod(Invocationinvocation)

Invoked when a nonexistent method or property is accessed.

A dynamic member invocation can attempt to call a member which doesn't exist on the receiving object. Example:

dynamic object = 1;
object.add(42); // Statically allowed, run-time error

This invalid code will invoke the noSuchMethod method of the integer 1 with an Invocation representing the .add(42) call and arguments (which then throws).

Classes can override noSuchMethod to provide custom behavior for such invalid dynamic invocations.

A class with a non-default noSuchMethod invocation can also omit implementations for members of its interface. Example:

class MockList<T> implements List<T> {
  noSuchMethod(Invocation invocation) {
    log(invocation);
    super.noSuchMethod(invocation); // Will throw.
  }
}
void main() {
  MockList().add(42);
}

This code has no compile-time warnings or errors even though the MockList class has no concrete implementation of any of the List interface methods. Calls to List methods are forwarded to noSuchMethod, so this code will log an invocation similar to Invocation.method(#add, [42]) and then throw.

If a value is returned from noSuchMethod, it becomes the result of the original invocation. If the value is not of a type that can be returned by the original invocation, a type error occurs at the invocation.

The default behavior is to throw a NoSuchMethodError.

Inherited from Object.

Implementation

@pragma("vm:entry-point")
@pragma("wasm:entry-point")
external dynamic noSuchMethod(Invocation invocation);

toLeft() override

Either<Never,X>toLeft<X>(XFunction()ifEmpty)

If this is a Some a Left is returned with the value. If this is a None, a Right is returned with the result of evaluating ifEmpty.

Implementation

@override
Either<Never, X> toLeft<X>(Function0<X> ifEmpty) => Either.right(ifEmpty());

toNullable() override

Never?toNullable()

Returns a nullable value, which is the value itself if this is a Some, or null if this is a None.

Implementation

@override
Never? toNullable() => null;

toRight() override

Either<X,Never>toRight<X>(XFunction()ifEmpty)

If this is a Some a Right is returned with the value. If this is a None, a Left is returned with the result of evaluating ifEmpty.

Implementation

@override
Either<X, Never> toRight<X>(Function0<X> ifEmpty) => Either.left(ifEmpty());

toString() inherited

StringtoString()

A string representation of this object.

Some classes have a default textual representation, often paired with a static parse function (like int.parse). These classes will provide the textual representation as their string representation.

Other classes have no meaningful textual representation that a program will care about. Such classes will typically override toString to provide useful information when inspecting the object, mainly for debugging or logging.

Inherited from Option.

Implementation

@override
String toString() => fold(() => 'None', (a) => 'Some($a)');

Operators

operator ==() override

booloperator ==(Objectother)

The equality operator.

The default behavior for all Objects is to return true if and only if this object and other are the same object.

Override this method to specify a different equality relation on a class. The overriding method must still be an equivalence relation. That is, it must be:

• Total: It must return a boolean for all arguments. It should never throw.

• Reflexive: For all objects o, o == o must be true.

• Symmetric: For all objects o1 and o2, o1 == o2 and o2 == o1 must either both be true, or both be false.

• Transitive: For all objects o1, o2, and o3, if o1 == o2 and o2 == o3 are true, then o1 == o3 must be true.

The method should also be consistent over time, so whether two objects are equal should only change if at least one of the objects was modified.

If a subclass overrides the equality operator, it should override the hashCode method as well to maintain consistency.

Implementation

@override
bool operator ==(Object other) => other is None;