Option<T> class / persistent Library / Dart Documentation

Option<T> class

class Option<T> {
 static final _none = new Option._internal(false, null);

 final T _value;
 final bool isDefined;

 Option._internal(this.isDefined, this._value);

 factory Option.none() => _none;

 factory Option.some(T value) => new Option._internal(true, value);

 factory Option.fromNullable(T nullableValue) =>
     nullableValue == null ? _none : new Option.some(nullableValue);

 T get value {
   if (isDefined) return _value;
   throw new StateError('Option.none() has no value');
 }

 T get asNullable => isDefined ? _value : null;

 T orElse(T defaultValue) => isDefined ? _value : defaultValue;

 T orElseCompute(T defaultValue()) => isDefined ? _value : defaultValue();

 /// [:forall U, Option<U> map(U f(T value)):]
 Option map(f(T value)) =>
     isDefined ? new Option.some(f(_value)) : this;

 /// [:forall U, Option<U> map(Option<U> f(T value)):]
 Option expand(Option f(T value)) =>
     isDefined ? f(_value) : this;

 /// Precondition: [:this is Option<Option>:]
 Option get flattened {
   assert(isDefined ? _value is Option : true);
   return orElse(_none);
 }

 bool operator ==(Option<T> other) =>
     (isDefined && other.isDefined && _value == other._value)
  || (!isDefined && !other.isDefined);

 int get hashCode => asNullable.hashCode;

 String toString() =>
     isDefined ? "Option.some($_value)" : "Option.none()";
}

Constructors

factory Option.fromNullable(T nullableValue) #

factory Option.fromNullable(T nullableValue) =>
   nullableValue == null ? _none : new Option.some(nullableValue);

factory Option.none() #

factory Option.none() => _none;

factory Option.some(T value) #

factory Option.some(T value) => new Option._internal(true, value);

Properties

final T asNullable #

T get asNullable => isDefined ? _value : null;

final Option flattened #

Precondition: this is Option<Option>

Option get flattened {
 assert(isDefined ? _value is Option : true);
 return orElse(_none);
}

final int hashCode #

Get a hash code for this object.

All objects have hash codes. Hash codes are guaranteed to be the same for objects that are equal when compared using the equality operator ==. Other than that there are no guarantees about the hash codes. They will not be consistent between runs and there are no distribution guarantees.

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

docs inherited from Object

int get hashCode => asNullable.hashCode;

final bool isDefined #

final bool isDefined

final T value #

T get value {
 if (isDefined) return _value;
 throw new StateError('Option.none() has no value');
}

Operators

bool operator ==(Option<T> other) #

The equality operator.

The default behavior for all Objects is to return true if and only if this 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 or return null.
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 equality of two objects should not change over time, or at least only change if one of the objects was modified.

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

docs inherited from Object

bool operator ==(Option<T> other) =>
   (isDefined && other.isDefined && _value == other._value)
|| (!isDefined && !other.isDefined);

Methods

Option expand(Option f(T value)) #

forall U, Option<U> map(Option<U> f(T value))

Option expand(Option f(T value)) =>
   isDefined ? f(_value) : this;

Option map(f(T value)) #

forall U, Option<U> map(U f(T value))

Option map(f(T value)) =>
   isDefined ? new Option.some(f(_value)) : this;

T orElse(T defaultValue) #

T orElse(T defaultValue) => isDefined ? _value : defaultValue;

T orElseCompute(T defaultValue()) #

T orElseCompute(T defaultValue()) => isDefined ? _value : defaultValue();

String toString() #

Returns a string representation of this object.

docs inherited from Object

String toString() =>
   isDefined ? "Option.some($_value)" : "Option.none()";

Option<T> class

Constructors

Code factory Option.fromNullable(T nullableValue) #

Code factory Option.none() #

Code factory Option.some(T value) #

Properties

Code final T asNullable #

Code final Option flattened #

Code final int hashCode #

Code final bool isDefined #

Code final T value #

Operators

Code bool operator ==(Option<T> other) #

Methods

Code Option expand(Option f(T value)) #

Code Option map(f(T value)) #

Code T orElse(T defaultValue) #

Code T orElseCompute(T defaultValue()) #

Code String toString() #

factory Option.fromNullable(T nullableValue) #

factory Option.none() #

factory Option.some(T value) #

final T asNullable #

final Option flattened #

final int hashCode #

final bool isDefined #

final T value #

bool operator ==(Option<T> other) #

Option expand(Option f(T value)) #

Option map(f(T value)) #

T orElse(T defaultValue) #

T orElseCompute(T defaultValue()) #

String toString() #