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None final ​

final class None extends Option<Never>

An Option that signifies the absence of a value.

Inheritance

Object → Option<A> → None

Constructors ​

None() const ​

const None()
Implementation
dart
const None() : super._();

Properties ​

hashCode no setter override ​

int get hashCode

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
dart
@override
int get hashCode => 0;

isDefined no setter inherited ​

bool get isDefined

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

Inherited from Option.

Implementation
dart
bool get isDefined => this is Some;

isEmpty no setter inherited ​

bool get isEmpty

Whether this collection contains no elements.

Inherited from Option.

Implementation
dart
@override
bool get isEmpty => this is None;

isNotEmpty no setter inherited ​

bool get isNotEmpty

Whether this collection contains at least one element.

Inherited from RIterableOnce.

Implementation
dart
bool get isNotEmpty => !isEmpty;

isTraversableAgain no setter inherited ​

bool get isTraversableAgain

Whether this collection can be traversed more than once.

Always false for a bare RIterableOnce; overridden to true by RIterable and its subtypes.

Inherited from RIterableOnce.

Implementation
dart
bool get isTraversableAgain => false;

iterator no setter override ​

RIterator<Never> get iterator

Returns an RIterator over the elements of this collection.

Implementation
dart
@override
RIterator<Never> get iterator => RIterator.empty();

knownSize no setter inherited ​

int get knownSize

Returns the number of elements in this collection, if that number is already known. If not, -1 is returned.

Inherited from RIterableOnce.

Implementation
dart
int get knownSize => -1;

nonEmpty no setter inherited ​

bool get nonEmpty

Whether this collection contains at least one element.

Inherited from Option.

Implementation
dart
@override
bool get nonEmpty => this is Some;

runtimeType no setter inherited ​

Type get runtimeType

A representation of the runtime type of the object.

Inherited from Object.

Implementation
dart
external Type get runtimeType;

size no setter inherited ​

int get size

Returns the number of elements in this collection.

Inherited from RIterableOnce.

Implementation
dart
int get size {
  if (knownSize >= 0) {
    return knownSize;
  } else {
    final it = iterator;
    var len = 0;

    while (it.hasNext) {
      len += 1;
      it.next();
    }

    return len;
  }
}

Methods ​

collect() inherited ​

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

Returns a new collection by applying f to each element an only keeping results of type Some.

Inherited from Option.

Implementation
dart
@override
Option<B> collect<B>(Function1<A, Option<B>> f) => flatMap(f);

collectFirst() inherited ​

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

Applies f to each element of this collection, returning the first element that results in a Some, if any.

Inherited from RIterableOnce.

Implementation
dart
Option<B> collectFirst<B>(Function1<A, Option<B>> f) {
  final it = iterator;

  while (it.hasNext) {
    final x = f(it.next());
    if (x.isDefined) return x;
  }

  return none();
}

copyToArray() inherited ​

int copyToArray(Array<Never> xs, [int start = 0, int? n])

Copies elements into xs starting at start, writing at most n elements (or all remaining capacity when n is omitted).

Returns the number of elements actually copied.

Inherited from RIterableOnce.

Implementation
dart
int copyToArray(Array<A> xs, [int start = 0, int? n]) {
  final it = iterator;
  final end = start + min(n ?? Integer.maxValue, xs.length - start);
  var i = start;

  while (i < end && it.hasNext) {
    xs[i] = it.next();
    i += 1;
  }

  return i - start;
}

corresponds() inherited ​

bool corresponds<B>(RIterable<B> that, bool Function(Never, B) p)

Returns true if this collection has the same size as that and each corresponding element from this and that satisfies the given predicate p.

Inherited from RIterableOnce.

Implementation
dart
bool corresponds<B>(
  covariant RIterable<B> that,
  Function2<A, B, bool> p,
) {
  final a = iterator;
  final b = that.iterator;

  while (a.hasNext && b.hasNext) {
    if (!p(a.next(), b.next())) return false;
  }

  return !a.hasNext && !b.hasNext;
}

count() inherited ​

int count(bool Function(Never) p)

Return the number of elements in this collection that satisfy the given predicate.

Inherited from RIterableOnce.

Implementation
dart
int count(Function1<A, bool> p) {
  var res = 0;
  final it = iterator;

  while (it.hasNext) {
    if (p(it.next())) res += 1;
  }

  return res;
}

drop() inherited ​

Option<Never> drop(int n)

Returns a new collection with the first n elements removed.

Inherited from Option.

Implementation
dart
@override
Option<A> drop(int n) => filter((_) => n <= 0);

dropWhile() inherited ​

Option<Never> dropWhile(bool Function(Never) p)

Returns a new collection with leading elements satisfying p removed.

Inherited from Option.

Implementation
dart
@override
Option<A> dropWhile(Function1<A, bool> p) => filterNot(p);

exists() inherited ​

bool exists(bool Function(Never) p)

Returns true if any element of this collection satisfies the given predicate, false if no elements satisfy it.

Inherited from RIterableOnce.

Implementation
dart
bool exists(Function1<A, bool> p) {
  var res = false;
  final it = iterator;

  while (!res && it.hasNext) {
    res = p(it.next());
  }

  return res;
}

filter() override ​

Option<Never> filter(bool Function(Never) p)

Returns a new collection containing only elements that satisfy p.

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

filterNot() inherited ​

Option<Never> filterNot(bool Function(Never) p)

Returns a new collection containing only elements that do not satisfy p.

Inherited from Option.

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

find() inherited ​

Option<Never> find(bool Function(Never) p)

Returns the first element from this collection that satisfies the given predicate p. If no element satisfies p, None is returned.

Inherited from RIterableOnce.

Implementation
dart
Option<A> find(Function1<A, bool> p) {
  final it = iterator;

  while (it.hasNext) {
    final a = it.next();
    if (p(a)) return Some(a);
  }

  return none();
}

flatMap() override ​

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

Returns a new collection by applying f to each element and concatenating the results.

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

fold() override ​

B fold<B>(B Function() ifEmpty, B Function(Never) f)

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

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

foldLeft() inherited ​

B foldLeft<B>(B z, B Function(B, Never) op)

Returns a summary value by applying op to all elements of this collection, moving from left to right. The fold uses a seed value of z.

Inherited from RIterableOnce.

Implementation
dart
B foldLeft<B>(B z, Function2<B, A, B> op) {
  var result = z;
  final it = iterator;

  while (it.hasNext) {
    result = op(result, it.next());
  }

  return result;
}

foldRight() inherited ​

B foldRight<B>(B z, B Function(Never, B) op)

Returns a summary value by applying op to all elements of this collection, moving from right to left. The fold uses a seed value of z.

Inherited from RIterableOnce.

Implementation
dart
B foldRight<B>(B z, Function2<A, B, B> op) => _reversed().foldLeft(z, (b, a) => op(a, b));

forall() inherited ​

bool forall(bool Function(Never) p)

Returns true if all elements of this collection satisfy the given predicate, false if any elements do not.

Inherited from RIterableOnce.

Implementation
dart
bool forall(Function1<A, bool> p) {
  var res = true;
  final it = iterator;

  while (res && it.hasNext) {
    res = p(it.next());
  }

  return res;
}

foreach() inherited ​

void foreach<U>(U Function(Never) f)

Applies f to each element of this collection, discarding any resulting values.

Inherited from RIterableOnce.

Implementation
dart
void foreach<U>(Function1<A, U> f) {
  final it = iterator;
  while (it.hasNext) {
    f(it.next());
  }
}

map() override ​

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

Returns a new collection by applying f to each element.

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

maxByOption() inherited ​

Option<Never> maxByOption<B>(B Function(Never) f, Order<B> order)

Finds the largest element in this collection by applying f to each element and using the given Order to find the greatest.

If this collection is empty, None is returned.

Inherited from RIterableOnce.

Implementation
dart
Option<A> maxByOption<B>(Function1<A, B> f, Order<B> order) => _minMaxByOption(f, order.max);

maxOption() inherited ​

Option<Never> maxOption(Order<Never> order)

Finds the largest element in this collection according to the given Order.

If this collection is empty, None is returned.

Inherited from RIterableOnce.

Implementation
dart
Option<A> maxOption(Order<A> order) => switch (knownSize) {
  0 => none(),
  _ => _reduceOptionIterator(iterator, order.max),
};

minByOption() inherited ​

Option<Never> minByOption<B>(B Function(Never) f, Order<B> order)

Finds the smallest element in this collection by applying f to each element and using the given Order to find the greatest.

If this collection is empty, None is returned.

Inherited from RIterableOnce.

Implementation
dart
Option<A> minByOption<B>(Function1<A, B> f, Order<B> order) => _minMaxByOption(f, order.min);

minOption() inherited ​

Option<Never> minOption(Order<Never> order)

Finds the largest element in this collection according to the given Order.

If this collection is empty, None is returned.

Inherited from RIterableOnce.

Implementation
dart
Option<A> minOption(Order<A> order) => switch (knownSize) {
  0 => none(),
  _ => _reduceOptionIterator(iterator, order.min),
};

mkString() inherited ​

String mkString({String? start, String? sep, String? end})

Returns a String by using each elements toString(), adding sep between each element. If start is defined, it will be prepended to the resulting string. If end is defined, it will be appended to the resulting string.

Inherited from RIterableOnce.

Implementation
dart
String mkString({String? start, String? sep, String? end}) {
  if (knownSize == 0) {
    return '${start ?? ""}${end ?? ""}';
  } else {
    return _mkStringImpl(StringBuffer(), start ?? '', sep ?? '', end ?? '');
  }
}

noSuchMethod() inherited ​

dynamic noSuchMethod(Invocation invocation)

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:

dart
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:

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

reduce() inherited ​

Never reduce(Never Function(Never, Never) op)

Reduces this collection to a single value by applying op left to right.

Throws if the collection is empty.

Inherited from RIterableOnce.

Implementation
dart
A reduce(Function2<A, A, A> op) => reduceLeft(op);

reduceLeft() inherited ​

Never reduceLeft(Never Function(Never, Never) op)

Reduces from left to right. Throws if empty.

Inherited from RIterableOnce.

Implementation
dart
A reduceLeft(Function2<A, A, A> op) => switch (this) {
  final IndexedSeq<A> seq when seq.length > 0 => _foldl(seq, 1, seq[0], op),
  _ when knownSize == 0 => throw UnsupportedError('empty.reduceLeft'),
  _ => _reduceLeftIterator(() => throw UnsupportedError('empty.reduceLeft'), op),
};

reduceLeftOption() inherited ​

Option<Never> reduceLeftOption(Never Function(Never, Never) op)

Returns a summary values of all elements of this collection by applying f to each element, moving left to right.

If this collection is empty, None will be returned.

Inherited from RIterableOnce.

Implementation
dart
Option<A> reduceLeftOption(Function2<A, A, A> op) => switch (knownSize) {
  0 => none(),
  _ => _reduceOptionIterator(iterator, op),
};

reduceOption() inherited ​

Option<Never> reduceOption(Never Function(Never, Never) op)

Returns a summary values of all elements of this collection by applying f to each element, moving left to right.

If this collection is empty, None will be returned.

Inherited from RIterableOnce.

Implementation
dart
Option<A> reduceOption(Function2<A, A, A> op) => reduceLeftOption(op);

reduceRight() inherited ​

Never reduceRight(Never Function(Never, Never) op)

Reduces from right to left. Throws if empty.

Inherited from RIterableOnce.

Implementation
dart
A reduceRight(Function2<A, A, A> op) => switch (this) {
  final IndexedSeq<A> seq when seq.length > 0 => _foldr(seq, op),
  _ when knownSize == 0 => throw UnsupportedError('empty.reduceLeft'),
  _ => _reversed().reduceLeft((x, y) => op(y, x)),
};

reduceRightOption() inherited ​

Option<Never> reduceRightOption(Never Function(Never, Never) op)

Returns a summary values of all elements of this collection by applying f to each element, moving right to left.

If this collection is empty, None will be returned.

Inherited from RIterableOnce.

Implementation
dart
Option<A> reduceRightOption(Function2<A, A, A> op) => switch (knownSize) {
  -1 => _reduceOptionIterator(_reversed().iterator, (x, y) => op(y, x)),
  0 => none(),
  _ => Some(reduceRight(op)),
};

scan() inherited ​

RIterableOnce<B> scan<B>(B z, B Function(B, Never) op)

Alias for scanLeft.

Inherited from RIterableOnce.

Implementation
dart
RIterableOnce<B> scan<B>(B z, Function2<B, A, B> op) => scanLeft(z, op);

scanLeft() inherited ​

RIterableOnce<B> scanLeft<B>(B z, B Function(B, Never) op)

Returns a new collection of running totals starting with z.

The first element of the result is z; each subsequent element is the result of applying op to the previous total and the next element.

Inherited from Option.

Implementation
dart
@override
RIterableOnce<B> scanLeft<B>(B z, Function2<B, A, B> op) => toIList().scanLeft(z, op);

slice() inherited ​

RIterableOnce<Never> slice(int from, int until)

Returns a new collection containing elements in the range [from, until).

Inherited from Option.

Implementation
dart
@override
RIterableOnce<A> slice(int from, int until) => toIList().slice(from, until);

span() inherited ​

Record span(bool Function(Never) p)

Returns two collections: elements before and starting from the first element that does not satisfy p.

Inherited from Option.

Implementation
dart
@override
(RIterableOnce<A>, RIterableOnce<A>) span(Function1<A, bool> p) => toIList().span(p);

splitAt() inherited ​

Record splitAt(int n)

Returns two collections: the first n elements and the remainder.

Inherited from RIterableOnce.

Implementation
dart
(RIterableOnce<A>, RIterableOnce<A>) splitAt(int n) {
  final spanner = _Spanner<A>(n);
  return span(spanner.call);
}

take() inherited ​

RIterableOnce<Never> take(int n)

Returns a new collection containing only the first n elements.

Inherited from Option.

Implementation
dart
@override
RIterableOnce<A> take(int n) => toIList().take(n);

takeWhile() inherited ​

RIterableOnce<Never> takeWhile(bool Function(Never) p)

Returns a new collection of leading elements that satisfy p.

Inherited from Option.

Implementation
dart
@override
RIterableOnce<A> takeWhile(Function1<A, bool> p) => toIList().takeWhile(p);

tapEach() inherited ​

RIterableOnce<Never> tapEach<U>(U Function(Never) f)

Applies f to each element in this collection, discarding any results and returns this collection.

Inherited from RIterableOnce.

Implementation
dart
RIterableOnce<A> tapEach<U>(Function1<A, U> f) {
  foreach(f);
  return this;
}

toIList() inherited ​

IList<Never> toIList()

Returns an IList with the same elements as this collection.

Inherited from RIterableOnce.

Implementation
dart
IList<A> toIList() => IList.from(this);

toIndexedSeq() inherited ​

IndexedSeq<Never> toIndexedSeq()

Returns an IndexedSeq with the same elements as this collection.

Inherited from RIterableOnce.

Implementation
dart
IndexedSeq<A> toIndexedSeq() => IndexedSeq.from(this);

toISet() inherited ​

ISet<Never> toISet()

Returns an ISet with the same elements as this collection, duplicates removed.

Inherited from RIterableOnce.

Implementation
dart
ISet<A> toISet() => ISet.from(this);

toIVector() inherited ​

IVector<Never> toIVector()

Returns an IVector with the same elements as this collection.

Inherited from RIterableOnce.

Implementation
dart
IVector<A> toIVector() => IVector.from(this);

toLeft() override ​

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

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

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

toList() inherited ​

List<Never> toList({bool growable = true})

Returns a new List with the same elements as this collection.

Inherited from RIterableOnce.

Implementation
dart
List<A> toList({bool growable = true}) {
  if (growable) {
    final it = iterator;
    final res = List<A>.empty(growable: true);

    while (it.hasNext) {
      res.add(it.next());
    }

    return res;
  } else {
    final it = iterator;
    return List.generate(size, (_) => it.next());
  }
}

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
dart
@override
Never? toNullable() => null;

toRight() override ​

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

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

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

toSeq() inherited ​

RSeq<Never> toSeq()

Returns a RSeq with the same elements as this collection.

Inherited from RIterableOnce.

Implementation
dart
RSeq<A> toSeq() => RSeq.from(this);

toString() inherited ​

String toString()

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

Operators ​

operator ==() override ​

bool operator ==(Object other)

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