Enum nom_supreme::error::ErrorTree

pub enum ErrorTree<I> {
    Base {
        location: I,
        kind: BaseErrorKind,
    },
    Stack {
        base: Box<Self>,
        contexts: Vec<(I, StackContext)>,
    },
    Alt(Vec<Self>),
}

A comprehensive tree of nom errors describing a parse failure.

This Error type is designed to be VerboseError++. While VerboseError can represent a stack of errors, this type can represent a full tree. In addition to representing a particular specific parse error, it can also represent a stack of nested error contexts (for instance, as provided by context), or a list of alternatives that were all tried individually by alt and all failed.

In general, the design goal for this type is to discard as little useful information as possible. That being said, many ErrorKind variants add very little useful contextual information to error traces; for example, ErrorKind::Alt doesn't add any interesting context to an ErrorTree::Alt, and its presence in a stack precludes merging together adjacent sets of ErrorTree::Alt siblings.

Examples

Base parser errors

An ErrorTree::Base is an error that occurred at the "bottom" of the stack, from a parser looking for 1 specific kind of thing.

use cool_asserts::assert_matches;
use nom::{Parser, Err};
use nom::character::complete::{digit1, char};
use nom_supreme::error::{ErrorTree, BaseErrorKind, StackContext, Expectation};
use nom_supreme::parser_ext::ParserExt;

let err: Err<ErrorTree<&str>> = digit1.parse("abc").unwrap_err();

assert_matches!(err, Err::Error(ErrorTree::Base{
    location: "abc",
    kind: BaseErrorKind::Expected(Expectation::Digit),
}));

let err: Err<ErrorTree<&str>> = char('a').and(char('b')).parse("acb").unwrap_err();

assert_matches!(err, Err::Error(ErrorTree::Base{
    location: "cb",
    kind: BaseErrorKind::Expected(Expectation::Char('b')),
}));

Stacks

An ErrorTree::Stack is created when a parser combinator—typically context—attaches additional error context to a subparser error. It can have any ErrorTree at the base of the stack.

use cool_asserts::assert_matches;
use nom::{Parser, Err};
use nom::character::complete::{alpha1, space1, char,};
use nom::sequence::{separated_pair, delimited};
use nom_supreme::parser_ext::ParserExt;
use nom_supreme::error::{ErrorTree, BaseErrorKind, StackContext, Expectation};

// Parse a single identifier, defined as just a string of letters.
let identifier = alpha1.context("identifier");

// Parse a pair of identifiers, separated by whitespace
let identifier_pair = separated_pair(identifier, space1, identifier)
    .context("identifier pair");

// Parse a pair of identifiers in parenthesis.
let mut parenthesized = delimited(char('('), identifier_pair, char(')'))
    .context("parenthesized");

let err: Err<ErrorTree<&str>> = parenthesized.parse("(abc 123)").unwrap_err();

assert_matches!(err, Err::Error(ErrorTree::Stack {
    base,
    contexts,
}) => {
    assert_matches!(*base, ErrorTree::Base {
        location: "123)",
        kind: BaseErrorKind::Expected(Expectation::Alpha)
    });

    assert_eq!(contexts, [
        ("123)", StackContext::Context("identifier")),
        ("abc 123)", StackContext::Context("identifier pair")),
        ("(abc 123)", StackContext::Context("parenthesized")),
    ]);
});

Alternatives

An ErrorTree::Alt is created when a series of parsers are all tried, and all of them fail. Most commonly this will happen via the alt combinator or the equivalent .or postfix combinator. When all of these subparsers fail, their errors (each individually their own ErrorTree) are aggregated into an ErrorTree::Alt, indicating that "any one of these things were expected."

use cool_asserts::assert_matches;
use nom::{Parser, Err};
use nom::branch::alt;
use nom_supreme::error::{ErrorTree, BaseErrorKind, StackContext, Expectation};
use nom_supreme::parser_ext::ParserExt;
use nom_supreme::tag::complete::tag;

let parse_bool = alt((
    tag("true").value(true),
    tag("false").value(true),
));

let mut parse_null_bool = alt((
    parse_bool.map(Some),
    tag("null").value(None),
));

let err: Err<ErrorTree<&str>> = parse_null_bool.parse("123").unwrap_err();

// This error communicates to the caller that any one of "true", "false",
// or "null" was expected at that location.
assert_matches!(err, Err::Error(ErrorTree::Alt(choices)) => {
    assert_matches!(choices.as_slice(), [
        ErrorTree::Base {
            location: "123",
            kind: BaseErrorKind::Expected(Expectation::Tag("true"))},
        ErrorTree::Base {
            location: "123",
            kind: BaseErrorKind::Expected(Expectation::Tag("false"))},
        ErrorTree::Base {
            location: "123",
            kind: BaseErrorKind::Expected(Expectation::Tag("null"))},
    ])
});

Contexts and Alternatives

Because Stack and Alt recursively contain ErrorTree errors from subparsers, they can be can combined to create error trees of arbitrary complexity.

use cool_asserts::assert_matches;
use nom::{Parser, Err};
use nom::branch::alt;
use nom_supreme::error::{ErrorTree, BaseErrorKind, StackContext, Expectation};
use nom_supreme::parser_ext::ParserExt;
use nom_supreme::tag::complete::tag;

let parse_bool = alt((
    tag("true").value(true),
    tag("false").value(true),
)).context("bool");

let mut parse_null_bool = alt((
    parse_bool.map(Some),
    tag("null").value(None).context("null"),
)).context("null or bool");

let err: Err<ErrorTree<&str>> = parse_null_bool.parse("123").unwrap_err();

assert_matches!(err, Err::Error(ErrorTree::Stack{base, contexts}) => {
    assert_eq!(contexts, [("123", StackContext::Context("null or bool"))]);
    assert_matches!(*base, ErrorTree::Alt(choices) => {
        assert_matches!(&choices[0], ErrorTree::Stack{base, contexts} => {
            assert_eq!(contexts, &[("123", StackContext::Context("bool"))]);
            assert_matches!(&**base, ErrorTree::Alt(choices) => {
                assert_matches!(&choices[0], ErrorTree::Base {
                    location: "123",
                    kind: BaseErrorKind::Expected(Expectation::Tag("true"))
                });
                assert_matches!(&choices[1], ErrorTree::Base {
                    location: "123",
                    kind: BaseErrorKind::Expected(Expectation::Tag("false"))
                });
           });
        });
        assert_matches!(&choices[1], ErrorTree::Stack{base, contexts} => {
            assert_eq!(contexts, &[("123", StackContext::Context("null"))]);
            assert_matches!(&**base, ErrorTree::Base {
                location: "123",
                kind: BaseErrorKind::Expected(Expectation::Tag("null"))
            });
        });
    });
});

Display formatting

TODO WRITE THIS SECTION

Variants

Base

A specific error event at a specific location. Often this will indicate that something like a tag or character was expected at that location. When used as part of a stack, it indicates some additional context for the root error of the stack.

Fields of Base

location: I

The location of this error in the input

kind: BaseErrorKind

The specific error that occurred

Stack

A stack indicates a chain of error contexts was provided. The stack should be read "backwards"; that is, errors earlier in the Vec occurred "sooner" (deeper in the call stack).

Fields of Stack

base: Box<Self>

The original error

contexts: Vec<(I, StackContext)>

The stack of contexts attached to that error

Alt(Vec<Self>)

A series of parsers were tried in order at the same location (for instance, via the alt combinator) and all of them failed. All of the errors in this set are "siblings".

Implementations

impl<I> ErrorTree<I>

pub fn map_locations<T>(
    self,
    convert_location: impl FnMut(I) -> T
) -> ErrorTree<T>

Convert all of the locations in this error using some kind of mapping function. This is intended to help add additional context that may not have been available when the nom parsers were running, such as line and column numbers.

Trait Implementations

impl<I> ContextError<I> for ErrorTree<I>

pub fn add_context(location: I, ctx: &'static str, other: Self) -> Self

Similar to append: Create a new error with some added context

impl<I: Debug> Debug for ErrorTree<I>

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

Formats the value using the given formatter.

impl<I: Display> Display for ErrorTree<I>

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

Formats the value using the given formatter.

impl<I: Display + Debug> Error for ErrorTree<I>

pub fn source(&self) -> Option<&(dyn Error + 'static)>

The lower-level source of this error, if any.

pub fn backtrace(&self) -> Option<&Backtrace>

🔬 This is a nightly-only experimental API. (backtrace)

Returns a stack backtrace, if available, of where this error occurred.

pub fn description(&self) -> &str

👎 Deprecated since 1.42.0:

use the Display impl or to_string()

pub fn cause(&self) -> Option<&dyn Error>

👎 Deprecated since 1.33.0:

replaced by Error::source, which can support downcasting

impl<I, T> ExtractContext<I, ErrorTree<T>> for ErrorTree<I> where
    I: Clone,
    T: RecreateContext<I>, 

pub fn extract_context(self, original_input: I) -> ErrorTree<T>

Given the context attached to a nom error, and given the original input to the nom parser, extract more the useful context information.

impl<I, E: Error + Send + Sync + 'static> FromExternalError<I, E> for ErrorTree<I>

pub fn from_external_error(location: I, _kind: NomErrorKind, e: E) -> Self

Create an error from a given external error, such as from FromStr

impl<I: InputLength> ParseError<I> for ErrorTree<I>

pub fn from_error_kind(location: I, kind: NomErrorKind) -> Self

Create a new error at the given position. Interpret kind as an Expectation if possible, to give a more informative error message.

pub fn append(location: I, kind: NomErrorKind, other: Self) -> Self

Combine an existing error with a new one. This is how error context is accumulated when backtracing. "other" is the original error, and the inputs new error from higher in the call stack.

If other is already an ErrorTree::Stack, the context is added to the stack; otherwise, a new stack is created, with other at the root.

pub fn from_char(location: I, character: char) -> Self

Create an error indicating an expected character at a given position

pub fn or(self, other: Self) -> Self

Combine two errors from branches of alt. If either or both errors are already ErrorTree::Alt, the different error sets are merged; otherwise, a new ErrorTree::Alt is created, containing both self and other.

impl<I> TagError<I, &'static str> for ErrorTree<I>

pub fn from_tag(location: I, tag: &'static str) -> Self

Create an error from an expected tag at a location.

pub fn from_tag_no_case(input: I, tag: T) -> Self

As above, but for a case insensitive tag. By default this just calls from_tag