Enum logos::FilterResult

pub enum FilterResult<T, E> {
    Emit(T),
    Skip,
    Error(E),
}

Type that can be returned from a callback, either producing a field for a token, skipping it, or emitting an error.

Example

use logos::{Logos, FilterResult};

#[derive(Debug, PartialEq, Clone, Default)]
enum LexingError {
    NumberParseError,
    NumberIsTen,
    #[default]
    Other,
}

impl From<std::num::ParseIntError> for LexingError {
    fn from(_: std::num::ParseIntError) -> Self {
        LexingError::NumberParseError
    }
}

#[derive(Logos, Debug, PartialEq)]
#[logos(error = LexingError)]
enum Token {
    #[regex(r"[ \n\f\t]+", logos::skip)]
    Ignored,

    #[regex("[0-9]+", |lex| {
        let n: u64 = lex.slice().parse().unwrap();

        // Only emit a token if `n` is an even number.
        if n % 2 == 0 {
            // Emit an error if `n` is 10.
            if n == 10 {
                FilterResult::Error(LexingError::NumberIsTen)
            } else {
                FilterResult::Emit(n)
            }
        } else {
            FilterResult::Skip
        }
    })]
    NiceEvenNumber(u64)
}

let tokens: Vec<_> = Token::lexer("20 11 42 23 100 10").collect();

assert_eq!(
    tokens,
    &[
        Ok(Token::NiceEvenNumber(20)),
        // skipping 11
        Ok(Token::NiceEvenNumber(42)),
        // skipping 23
        Ok(Token::NiceEvenNumber(100)),
        // error at 10
        Err(LexingError::NumberIsTen),
    ]
);

Variants

Emit(T)

Emit a token with a given value T. Use () for unit variants without fields.

Skip

Skip current match, analog to Skip.

Error(E)

Emit a <Token as Logos>::ERROR token.