Enum parser_compose::ErrorTree

source ·

pub enum ErrorTree {
    Or {
        pos: usize,
        left: Box<ErrorTree>,
        right: Box<ErrorTree>,
    },
    Predicate {
        pos: usize,
    },
    Repeat {
        pos: usize,
        child: Box<ErrorTree>,
        matched: usize,
        lower_bound: usize,
        upper_bound: Option<usize>,
    },
    Peeked {
        pos: usize,
        child: Box<ErrorTree>,
    },
    NotPeeked {
        pos: usize,
    },
    AndThen {
        pos: usize,
        callback_error: Box<dyn Error>,
    },
    Sequence {
        pos: usize,
        position_in_sequence: usize,
        child: Box<ErrorTree>,
    },
    Label {
        pos: usize,
        label: &'static str,
        child: Box<ErrorTree>,
    },
    Mismatch {
        expected: String,
        pos: usize,
    },
    Custom {
        pos: usize,
        child: Box<ErrorTree>,
        err: Box<dyn Error>,
    },
}

Expand description

The error type returned when parsers fail.

This is a tree-like structure where nodes represent specific parsers or combinators that failed.

Here is an example:

use parser_compose::Parser;

let msg = "ABCD";

let parser = "B".or("C").or("A");

let result = parser.repeated(4).try_parse(msg.into());

assert!(result.is_err());

println!("{}", result.err().unwrap());
// stdout:
// expected 4 match(es). matched 3 time(s)
//    both branches failed:
//       both branches failed:
//          expected 'B' at position 3
//          expected 'C' at position 3
//       expected 'A' at position 3

In this case, the parser failed because it could not match “A”, “B,” or “C” at position 3 in the input.

You can augment the information in this tree by adding custom errors using err() or labelling a parser using label(). Both those methods insert additional nodes into the tree.

use parser_compose::Parser;

let msg = "A";

let parser = "B".or("C").label("failed to parse C or B");

let result = parser.try_parse(msg.into());

assert!(result.is_err());
println!("{}", result.err().unwrap());

// stdout:
// [failed to parse C or B]
//    both branches failed:
//       expected 'B' at position 0
//       expected 'C' at position 0