Enum Undef 

pub enum Undef {
    UnaryOperation {
        op: &'static str,
        val: Box<Value>,
        pos: usize,
    },
    BinaryOperation {
        op: &'static str,
        lhv: Box<Value>,
        rhv: Box<Value>,
        pos: usize,
    },
    Termination {
        expected: char,
        pos: usize,
    },
    Token {
        token: String,
        pos: usize,
    },
    Name {
        pos: usize,
    },
    Value(Box<Value>),
}

Why a value is undefined: the payload of Value::Undefined.

Positions (pos) are byte offsets in the evaluated source. Built-in functions use position 0. Two Undefs are equal when they have the same variant and contents, regardless of position.

The Swan built-in undef.inspect exposes these fields to Swan code.

Variants

UnaryOperation

The result of a unary operation that is not defined for the operand type.

Fields

op: &'static str

Operation name (always a &'static str for built-in ops). If dunder-method dispatch (__neg__, etc.) is added later and user-defined error labels are needed, change this to Rc<str>.

val: Box<Value>

The operand.

pos: usize

Source position of the operation.

BinaryOperation

The result of a binary operation that is not defined for the operand types.

Fields

op: &'static str

Operation name, such as "sum" or "application".

lhv: Box<Value>

The left-hand operand.

rhv: Box<Value>

The right-hand operand.

pos: usize

Source position of the operation.

Termination

Syntax error: a group (...), [...] or {...} without its closing bracket.

Fields

expected: char

The missing closing bracket.

pos: usize

Source position where the bracket was expected.

Token

Syntax error: a token that is not valid where it appears.

Fields

token: String

A description of the token.

pos: usize

Source position of the token.

Name

A name was expected (left of =, : or ->, or after $), but another expression was found.

Fields

pos: usize

Source position of the offending expression.

Value(Box<Value>)

An undefined value created explicitly, for example by the Swan built-in undef.from. It wraps an arbitrary value.

Trait Implementations

impl Clone for Undef

fn clone(&self) -> Undef

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl Debug for Undef

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

Formats the value using the given formatter.

impl PartialEq for Undef

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

Equality operator ==.

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

Inequality operator !=.