Struct texlang::command::Tag

pub struct Tag(/* private fields */);

A tag is a piece of metadata that is optionally attached to a command.

Tags are used to implement certain TeX language semantics. An example is TeX conditionals. When a TeX conditional statement evaluates to false, the \if command must scan the input stream until it finds either an \else or \fi command. (The tokens scanned in this process are in the true branch of the conditional, and must thus be discarded.) Tags are the mechanism by which the scanning algorithm can determine if a token corresponds to an \else of \fi command. Concretely, both \else of \fi command have unique tags associated to them. When scanning the stream, if a token is a command token then the tag for the associated command is compared to the known tags for \else and \fi. If the tags match, the true branch is finished.

In general, TeX commands interface with the VM in two ways. The first most common way is when the main VM loop or expansion loop encounters a command. The loop invokes the command’s associated Rust function. One can think of the Rust function as providing the behavior of the command in this context.

The second way is when a different command, like a conditional command, performs some operation that is dependent on the commands it reads out of the input stream. In this context the commands in the input stream provide behavior using tags. The \else command having the specific else tag results in the conditional branch processing completing.

Note that the same tag can be used for multiple commands, but each command can only have one tag.

Implementation details

Tags are non-zero 32 bit integers. The first tag created has value 1, the second tag has value 2, and so on. A global mutex is used to store the next tag value. Tags have the property that Option<Tag> takes up 4 bytes in memory.

Implementations

impl Tag

pub fn new() -> Tag

Creates a new unique tag.

let tag_1 = Tag::new();
let tag_2 = Tag::new();
assert_ne!(tag_1, tag_2);

Trait Implementations

impl Clone for Tag

fn clone(&self) -> Tag

Returns a copy of the value.

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

Performs copy-assignment from source.

impl Debug for Tag

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

Formats the value using the given formatter.

impl Hash for Tag

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

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 Ord for Tag

fn cmp(&self, other: &Tag) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Selfwhere Self: Sized,

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Selfwhere Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Selfwhere Self: Sized + PartialOrd<Self>,

Restrict a value to a certain interval.

impl PartialEq<Tag> for Tag

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

This method tests for self and other values to be equal, and is used by ==.

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

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

impl PartialOrd<Tag> for Tag

fn partial_cmp(&self, other: &Tag) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

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

This method tests less than (for self and other) and is used by the < operator.

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

This method tests less than or equal to (for self and other) and is used by the <= operator.

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

This method tests greater than (for self and other) and is used by the > operator.

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

This method tests greater than or equal to (for self and other) and is used by the >= operator.

impl Copy for Tag

impl Eq for Tag

impl StructuralEq for Tag

impl StructuralPartialEq for Tag