Type Definition nom_supreme::error::ErrorTree
source · [−]pub type ErrorTree<I> = GenericErrorTree<I, &'static str, &'static str, Box<dyn Error + Send + Sync + 'static>>;
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"))
});
});
});
});
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TODO WRITE THIS SECTION