Enum nom_supreme::error::ErrorTree [−][src]
pub enum ErrorTree<I> {
Base {
location: I,
kind: BaseErrorKind,
},
Stack {
base: Box<Self>,
contexts: Vec<(I, StackContext)>,
},
Alt(Vec<Self>),
}
Expand description
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(false),
));
let mut parse_null_bool = alt((
parse_bool.map(Some),
tag("null").value(None),
));
assert_eq!(parse_null_bool.parse("true").unwrap(), ("", Some(true)));
assert_eq!(parse_null_bool.parse("false").unwrap(), ("", Some(false)));
assert_eq!(parse_null_bool.parse("null").unwrap(), ("", 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(false),
)).context("bool");
let mut parse_null_bool = alt((
parse_bool.map(Some),
tag("null").value(None).context("null"),
)).context("null or bool");
assert_eq!(parse_null_bool.parse("true").unwrap(), ("", Some(true)));
assert_eq!(parse_null_bool.parse("false").unwrap(), ("", Some(false)));
assert_eq!(parse_null_bool.parse("null").unwrap(), ("", None));
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
A specific error event at a specific location. Often this will indicate that something like a tag or character was expected at that location.
Fields of Base
location: I
The location of this error in the input
kind: BaseErrorKind
The specific error that occurred
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
A series of parsers were tried at the same location (for instance, via
the alt
combinator) and all of them failed. All
of the errors in this set are “siblings”.
Tuple Fields of Alt
0: Vec<Self>
Implementations
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
Similar to append: Create a new error with some added context
impl<I, T> ExtractContext<I, ErrorTree<T>> for ErrorTree<I> where
I: Clone,
T: RecreateContext<I>,
impl<I, T> ExtractContext<I, ErrorTree<T>> for ErrorTree<I> where
I: Clone,
T: RecreateContext<I>,
Given the context attached to a nom error, and given the original input to the nom parser, extract more the useful context information. Read more
Create an error from a given external error, such as from FromStr
Create a new error at the given position. Interpret kind
as an
Expectation
if possible, to give a more informative error message.
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.
Create an error indicating an expected character at a given position
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
.
Auto Trait Implementations
impl<I> !RefUnwindSafe for ErrorTree<I>
impl<I> !UnwindSafe for ErrorTree<I>
Blanket Implementations
Mutably borrows from an owned value. Read more
Given the context attached to a nom error, and given the original input to the nom parser, extract more the useful context information. Read more
Given the original input, as well as the context reported by nom, recreate a context in the original string where the error occurred. Read more