Enum Number 

pub enum Number {
    Singular,
    Dual,
    Plural,
    Unknown,
}

Grammatical number (singular, dual, plural).

Number indicates how many entities are being referred to. While English has a simple singular/plural distinction, many languages (including Arabic, Hebrew, Sanskrit, and Ancient Greek) have a dual form for exactly two.

The Dual Number

The dual is particularly important for Arabic NLP:

كِتَابٌ      (kitābun)    - one book (singular)
كِتَابَانِ   (kitābāni)   - two books (dual)
كُتُبٌ      (kutubun)    - three+ books (plural)

For coreference, we treat dual as compatible with plural (a pair of entities can be referred to with plural pronouns in many contexts).

Cross-Linguistic Distribution

Language	Has Dual?	Example
Arabic	Yes	كِتَابَانِ (two books)
Hebrew	Yes	יָדַיִם (two hands)
Sanskrit	Yes	द्वौ (two)
English	No*	“both” (lexical, not grammatical)
Japanese	No	本 (hon) + counter

*English lost the dual around 1000 CE; Old English had it.

Variants

Singular

Singular: exactly one entity

The default for most zero pronouns in Arabic, where the dropped subject typically refers to a single previously-mentioned entity.

Dual

Dual: exactly two entities

Important for Arabic, Hebrew, Sanskrit, and other Semitic/Indo-European languages. In Arabic, verb conjugations and noun endings differ for dual vs plural.

Plural

Plural: more than one (or more than two in dual languages)

In languages with dual, plural specifically means “three or more”. In languages without dual, plural means “two or more”.

Unknown

Unknown or ambiguous number

Used when number cannot be determined, e.g., “you” in English (ambiguous singular/plural), or “they” (singular they vs plural).

Implementations

impl Number

pub fn is_compatible(&self, other: &Number) -> bool

Check if this number is compatible with another for coreference.

Unknown is compatible with anything (permissive). Dual is compatible with plural in some contexts.

Trait Implementations

impl Clone for Number

fn clone(&self) -> Number

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for Number

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

impl Default for Number

fn default() -> Number

Returns the “default value” for a type.

impl<'de> Deserialize<'de> for Number

fn deserialize<__D>( __deserializer: __D, ) -> Result<Number, <__D as Deserializer<'de>>::Error>

where __D: Deserializer<'de>,

Deserialize this value from the given Serde deserializer.

impl Display for Number

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

impl FromStr for Number

type Err = String

The associated error which can be returned from parsing.

fn from_str(s: &str) -> Result<Number, <Number as FromStr>::Err>

Parses a string s to return a value of this type.

impl Hash for Number

fn hash<__H>(&self, state: &mut __H)

where __H: Hasher,

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl PartialEq for Number

fn eq(&self, other: &Number) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl Serialize for Number

fn serialize<__S>( &self, __serializer: __S, ) -> Result<<__S as Serializer>::Ok, <__S as Serializer>::Error>

where __S: Serializer,

Serialize this value into the given Serde serializer.

impl Copy for Number

impl Eq for Number

impl StructuralPartialEq for Number