pub trait Parser<I, O, E> {
Show 21 methods
// Required method
fn parse_next(&mut self, input: &mut I) -> PResult<O, E>;
// Provided methods
fn parse(&mut self, input: I) -> Result<O, ParseError<I, E>>
where Self: Sized,
I: Stream + StreamIsPartial + Clone,
E: ParserError<I> { ... }
fn parse_peek(&mut self, input: I) -> IResult<I, O, E> { ... }
fn by_ref(&mut self) -> ByRef<'_, Self>
where Self: Sized { ... }
fn value<O2>(self, val: O2) -> Value<Self, I, O, O2, E>
where Self: Sized,
O2: Clone { ... }
fn void(self) -> Void<Self, I, O, E>
where Self: Sized { ... }
fn output_into<O2>(self) -> OutputInto<Self, I, O, O2, E>
where Self: Sized,
O: Into<O2> { ... }
fn recognize(self) -> Recognize<Self, I, O, E>
where Self: Sized,
I: Stream { ... }
fn with_recognized(self) -> WithRecognized<Self, I, O, E>
where Self: Sized,
I: Stream { ... }
fn span(self) -> Span<Self, I, O, E>
where Self: Sized,
I: Stream + Location { ... }
fn with_span(self) -> WithSpan<Self, I, O, E>
where Self: Sized,
I: Stream + Location { ... }
fn map<G, O2>(self, map: G) -> Map<Self, G, I, O, O2, E>
where G: Fn(O) -> O2,
Self: Sized { ... }
fn try_map<G, O2, E2>(self, map: G) -> TryMap<Self, G, I, O, O2, E, E2>
where Self: Sized,
G: FnMut(O) -> Result<O2, E2>,
I: Stream,
E: FromExternalError<I, E2> { ... }
fn verify_map<G, O2>(self, map: G) -> VerifyMap<Self, G, I, O, O2, E>
where Self: Sized,
G: FnMut(O) -> Option<O2>,
I: Stream,
E: ParserError<I> { ... }
fn flat_map<G, H, O2>(self, map: G) -> FlatMap<Self, G, H, I, O, O2, E>
where Self: Sized,
G: FnMut(O) -> H,
H: Parser<I, O2, E> { ... }
fn and_then<G, O2>(self, inner: G) -> AndThen<Self, G, I, O, O2, E>
where Self: Sized,
G: Parser<O, O2, E>,
O: StreamIsPartial,
I: Stream { ... }
fn parse_to<O2>(self) -> ParseTo<Self, I, O, O2, E>
where Self: Sized,
I: Stream,
O: ParseSlice<O2>,
E: ParserError<I> { ... }
fn verify<G, O2>(self, filter: G) -> Verify<Self, G, I, O, O2, E>
where Self: Sized,
G: Fn(&O2) -> bool,
I: Stream,
O: Borrow<O2>,
O2: ?Sized,
E: ParserError<I> { ... }
fn context<C>(self, context: C) -> Context<Self, I, O, E, C>
where Self: Sized,
I: Stream,
E: AddContext<I, C>,
C: Clone + Debug { ... }
fn complete_err(self) -> CompleteErr<Self>
where Self: Sized { ... }
fn err_into<E2>(self) -> ErrInto<Self, I, O, E, E2>
where Self: Sized,
E: Into<E2> { ... }
}Expand description
Core trait for parsing
The simplest way to implement a Parser is with a function
use winnow::prelude::*;
fn success(input: &mut &str) -> PResult<()> {
let output = ();
Ok(output)
}
let (input, output) = success.parse_peek("Hello").unwrap();
assert_eq!(input, "Hello"); // We didn't consume any inputwhich can be made stateful by returning a function
use winnow::prelude::*;
fn success<O: Clone>(output: O) -> impl FnMut(&mut &str) -> PResult<O> {
move |input: &mut &str| {
let output = output.clone();
Ok(output)
}
}
let (input, output) = success("World").parse_peek("Hello").unwrap();
assert_eq!(input, "Hello"); // We didn't consume any input
assert_eq!(output, "World");Additionally, some basic types implement Parser as well, including
u8andchar, seewinnow::token::one_of&[u8]and&str, seewinnow::token::tag
Required Methods§
Provided Methods§
sourcefn parse(&mut self, input: I) -> Result<O, ParseError<I, E>>
fn parse(&mut self, input: I) -> Result<O, ParseError<I, E>>
Parse all of input, generating O from it
sourcefn parse_peek(&mut self, input: I) -> IResult<I, O, E>
fn parse_peek(&mut self, input: I) -> IResult<I, O, E>
sourcefn by_ref(&mut self) -> ByRef<'_, Self>where
Self: Sized,
fn by_ref(&mut self) -> ByRef<'_, Self>where
Self: Sized,
Treat &mut Self as a parser
This helps when needing to move a Parser when all you have is a &mut Parser.
§Example
Because parsers are FnMut, they can be called multiple times. This prevents moving f
into length_data and g into
Parser::complete_err:
ⓘ
pub fn length_value<'i, O, E: ParserError<&'i [u8]>>(
mut f: impl Parser<&'i [u8], usize, E>,
mut g: impl Parser<&'i [u8], O, E>
) -> impl Parser<&'i [u8], O, E> {
move |i: &mut &'i [u8]| {
let mut data = length_data(f).parse_next(i)?;
let o = g.complete_err().parse_next(&mut data)?;
Ok(o)
}
}By adding by_ref, we can make this work:
pub fn length_value<'i, O, E: ParserError<&'i [u8]>>(
mut f: impl Parser<&'i [u8], usize, E>,
mut g: impl Parser<&'i [u8], O, E>
) -> impl Parser<&'i [u8], O, E> {
move |i: &mut &'i [u8]| {
let mut data = length_data(f.by_ref()).parse_next(i)?;
let o = g.by_ref().complete_err().parse_next(&mut data)?;
Ok(o)
}
}sourcefn value<O2>(self, val: O2) -> Value<Self, I, O, O2, E>
fn value<O2>(self, val: O2) -> Value<Self, I, O, O2, E>
Produce the provided value
§Example
use winnow::ascii::alpha1;
let mut parser = alpha1.value(1234);
assert_eq!(parser.parse_peek("abcd"), Ok(("", 1234)));
assert_eq!(parser.parse_peek("123abcd;"), Err(ErrMode::Backtrack(InputError::new("123abcd;", ErrorKind::Slice))));sourcefn void(self) -> Void<Self, I, O, E>where
Self: Sized,
fn void(self) -> Void<Self, I, O, E>where
Self: Sized,
Discards the output of the Parser
§Example
use winnow::ascii::alpha1;
let mut parser = alpha1.void();
assert_eq!(parser.parse_peek("abcd"), Ok(("", ())));
assert_eq!(parser.parse_peek("123abcd;"), Err(ErrMode::Backtrack(InputError::new("123abcd;", ErrorKind::Slice))));sourcefn output_into<O2>(self) -> OutputInto<Self, I, O, O2, E>
fn output_into<O2>(self) -> OutputInto<Self, I, O, O2, E>
Convert the parser’s output to another type using std::convert::From
§Example
use winnow::ascii::alpha1;
fn parser1<'s>(i: &mut &'s str) -> PResult<&'s str, InputError<&'s str>> {
alpha1(i)
}
let mut parser2 = parser1.output_into();
// the parser converts the &str output of the child parser into a Vec<u8>
let bytes: IResult<&str, Vec<u8>> = parser2.parse_peek("abcd");
assert_eq!(bytes, Ok(("", vec![97, 98, 99, 100])));sourcefn recognize(self) -> Recognize<Self, I, O, E>
fn recognize(self) -> Recognize<Self, I, O, E>
Produce the consumed input as produced value.
§Example
use winnow::ascii::{alpha1};
use winnow::combinator::separated_pair;
let mut parser = separated_pair(alpha1, ',', alpha1).recognize();
assert_eq!(parser.parse_peek("abcd,efgh"), Ok(("", "abcd,efgh")));
assert_eq!(parser.parse_peek("abcd;"),Err(ErrMode::Backtrack(InputError::new(";", ErrorKind::Verify))));sourcefn with_recognized(self) -> WithRecognized<Self, I, O, E>
fn with_recognized(self) -> WithRecognized<Self, I, O, E>
Produce the consumed input with the output
Functions similarly to recognize except it returns the parser output as well.
This can be useful especially in cases where the output is not the same type as the input, or the input is a user defined type.
Returned tuple is of the format (produced output, consumed input).
§Example
use winnow::ascii::{alpha1};
use winnow::token::tag;
use winnow::combinator::separated_pair;
fn inner_parser<'s>(input: &mut &'s str) -> PResult<bool, InputError<&'s str>> {
"1234".value(true).parse_next(input)
}
let mut consumed_parser = separated_pair(alpha1, ',', alpha1).value(true).with_recognized();
assert_eq!(consumed_parser.parse_peek("abcd,efgh1"), Ok(("1", (true, "abcd,efgh"))));
assert_eq!(consumed_parser.parse_peek("abcd;"),Err(ErrMode::Backtrack(InputError::new(";", ErrorKind::Verify))));
// the second output (representing the consumed input)
// should be the same as that of the `recognize` parser.
let mut recognize_parser = inner_parser.recognize();
let mut consumed_parser = inner_parser.with_recognized().map(|(output, consumed)| consumed);
assert_eq!(recognize_parser.parse_peek("1234"), consumed_parser.parse_peek("1234"));
assert_eq!(recognize_parser.parse_peek("abcd"), consumed_parser.parse_peek("abcd"));sourcefn span(self) -> Span<Self, I, O, E>
fn span(self) -> Span<Self, I, O, E>
Produce the location of the consumed input as produced value.
§Example
use winnow::stream::Located;
use winnow::ascii::alpha1;
use winnow::combinator::separated_pair;
let mut parser = separated_pair(alpha1.span(), ',', alpha1.span());
assert_eq!(parser.parse(Located::new("abcd,efgh")), Ok((0..4, 5..9)));
assert_eq!(parser.parse_peek(Located::new("abcd;")),Err(ErrMode::Backtrack(InputError::new(Located::new("abcd;").peek_slice(4).0, ErrorKind::Verify))));sourcefn with_span(self) -> WithSpan<Self, I, O, E>
fn with_span(self) -> WithSpan<Self, I, O, E>
Produce the location of consumed input with the output
Functions similarly to Parser::span except it
returns the parser output as well.
This can be useful especially in cases where the output is not the same type as the input, or the input is a user defined type.
Returned tuple is of the format (produced output, consumed input).
§Example
use winnow::stream::Located;
use winnow::ascii::alpha1;
use winnow::token::tag;
use winnow::combinator::separated_pair;
fn inner_parser<'s>(input: &mut Located<&'s str>) -> PResult<bool, InputError<Located<&'s str>>> {
"1234".value(true).parse_next(input)
}
let mut consumed_parser = separated_pair(alpha1.value(1).with_span(), ',', alpha1.value(2).with_span());
assert_eq!(consumed_parser.parse(Located::new("abcd,efgh")), Ok(((1, 0..4), (2, 5..9))));
assert_eq!(consumed_parser.parse_peek(Located::new("abcd;")),Err(ErrMode::Backtrack(InputError::new(Located::new("abcd;").peek_slice(4).0, ErrorKind::Verify))));
// the second output (representing the consumed input)
// should be the same as that of the `span` parser.
let mut recognize_parser = inner_parser.span();
let mut consumed_parser = inner_parser.with_span().map(|(output, consumed)| consumed);
assert_eq!(recognize_parser.parse_peek(Located::new("1234")), consumed_parser.parse_peek(Located::new("1234")));
assert_eq!(recognize_parser.parse_peek(Located::new("abcd")), consumed_parser.parse_peek(Located::new("abcd")));sourcefn map<G, O2>(self, map: G) -> Map<Self, G, I, O, O2, E>
fn map<G, O2>(self, map: G) -> Map<Self, G, I, O, O2, E>
Maps a function over the output of a parser
§Example
use winnow::{error::ErrMode,error::ErrorKind, error::InputError, Parser};
use winnow::ascii::digit1;
let mut parser = digit1.map(|s: &str| s.len());
// the parser will count how many characters were returned by digit1
assert_eq!(parser.parse_peek("123456"), Ok(("", 6)));
// this will fail if digit1 fails
assert_eq!(parser.parse_peek("abc"), Err(ErrMode::Backtrack(InputError::new("abc", ErrorKind::Slice))));sourcefn try_map<G, O2, E2>(self, map: G) -> TryMap<Self, G, I, O, O2, E, E2>
fn try_map<G, O2, E2>(self, map: G) -> TryMap<Self, G, I, O, O2, E, E2>
Applies a function returning a Result over the output of a parser.
§Example
use winnow::ascii::digit1;
let mut parse = digit1.try_map(|s: &str| s.parse::<u8>());
// the parser will convert the result of digit1 to a number
assert_eq!(parse.parse_peek("123"), Ok(("", 123)));
// this will fail if digit1 fails
assert_eq!(parse.parse_peek("abc"), Err(ErrMode::Backtrack(InputError::new("abc", ErrorKind::Slice))));
// this will fail if the mapped function fails (a `u8` is too small to hold `123456`)
assert_eq!(parse.parse_peek("123456"), Err(ErrMode::Backtrack(InputError::new("123456", ErrorKind::Verify))));sourcefn verify_map<G, O2>(self, map: G) -> VerifyMap<Self, G, I, O, O2, E>
fn verify_map<G, O2>(self, map: G) -> VerifyMap<Self, G, I, O, O2, E>
Apply both Parser::verify and Parser::map.
§Example
use winnow::ascii::digit1;
let mut parse = digit1.verify_map(|s: &str| s.parse::<u8>().ok());
// the parser will convert the result of digit1 to a number
assert_eq!(parse.parse_peek("123"), Ok(("", 123)));
// this will fail if digit1 fails
assert_eq!(parse.parse_peek("abc"), Err(ErrMode::Backtrack(InputError::new("abc", ErrorKind::Slice))));
// this will fail if the mapped function fails (a `u8` is too small to hold `123456`)
assert_eq!(parse.parse_peek("123456"), Err(ErrMode::Backtrack(InputError::new("123456", ErrorKind::Verify))));sourcefn flat_map<G, H, O2>(self, map: G) -> FlatMap<Self, G, H, I, O, O2, E>
fn flat_map<G, H, O2>(self, map: G) -> FlatMap<Self, G, H, I, O, O2, E>
Creates a parser from the output of this one
§Example
use winnow::token::take;
use winnow::binary::u8;
fn length_data<'s>(input: &mut &'s [u8]) -> PResult<&'s [u8], InputError<&'s [u8]>> {
u8.flat_map(take).parse_next(input)
}
assert_eq!(length_data.parse_peek(&[2, 0, 1, 2][..]), Ok((&[2][..], &[0, 1][..])));
assert_eq!(length_data.parse_peek(&[4, 0, 1, 2][..]), Err(ErrMode::Backtrack(InputError::new(&[0, 1, 2][..], ErrorKind::Slice))));which is the same as
use winnow::token::take;
use winnow::binary::u8;
fn length_data<'s>(input: &mut &'s [u8]) -> PResult<&'s [u8], InputError<&'s [u8]>> {
let length = u8.parse_next(input)?;
let data = take(length).parse_next(input)?;
Ok(data)
}
assert_eq!(length_data.parse_peek(&[2, 0, 1, 2][..]), Ok((&[2][..], &[0, 1][..])));
assert_eq!(length_data.parse_peek(&[4, 0, 1, 2][..]), Err(ErrMode::Backtrack(InputError::new(&[0, 1, 2][..], ErrorKind::Slice))));sourcefn and_then<G, O2>(self, inner: G) -> AndThen<Self, G, I, O, O2, E>
fn and_then<G, O2>(self, inner: G) -> AndThen<Self, G, I, O, O2, E>
Applies a second parser over the output of the first one
§Example
use winnow::ascii::digit1;
use winnow::token::take;
let mut digits = take(5u8).and_then(digit1);
assert_eq!(digits.parse_peek("12345"), Ok(("", "12345")));
assert_eq!(digits.parse_peek("123ab"), Ok(("", "123")));
assert_eq!(digits.parse_peek("123"), Err(ErrMode::Backtrack(InputError::new("123", ErrorKind::Slice))));sourcefn parse_to<O2>(self) -> ParseTo<Self, I, O, O2, E>
fn parse_to<O2>(self) -> ParseTo<Self, I, O, O2, E>
Apply std::str::FromStr to the output of the parser
§Example
use winnow::{error::ErrMode,error::ErrorKind, error::InputError, Parser};
use winnow::ascii::digit1;
fn parser<'s>(input: &mut &'s str) -> PResult<u64, InputError<&'s str>> {
digit1.parse_to().parse_next(input)
}
// the parser will count how many characters were returned by digit1
assert_eq!(parser.parse_peek("123456"), Ok(("", 123456)));
// this will fail if digit1 fails
assert_eq!(parser.parse_peek("abc"), Err(ErrMode::Backtrack(InputError::new("abc", ErrorKind::Slice))));sourcefn verify<G, O2>(self, filter: G) -> Verify<Self, G, I, O, O2, E>
fn verify<G, O2>(self, filter: G) -> Verify<Self, G, I, O, O2, E>
Returns the output of the child parser if it satisfies a verification function.
The verification function takes as argument a reference to the output of the parser.
§Example
let mut parser = alpha1.verify(|s: &str| s.len() == 4);
assert_eq!(parser.parse_peek("abcd"), Ok(("", "abcd")));
assert_eq!(parser.parse_peek("abcde"), Err(ErrMode::Backtrack(InputError::new("abcde", ErrorKind::Verify))));
assert_eq!(parser.parse_peek("123abcd;"),Err(ErrMode::Backtrack(InputError::new("123abcd;", ErrorKind::Slice))));sourcefn context<C>(self, context: C) -> Context<Self, I, O, E, C>
fn context<C>(self, context: C) -> Context<Self, I, O, E, C>
If parsing fails, add context to the error
This is used mainly to add user friendly information to errors when backtracking through a parse tree.
sourcefn complete_err(self) -> CompleteErr<Self>where
Self: Sized,
fn complete_err(self) -> CompleteErr<Self>where
Self: Sized,
Transforms Incomplete into Backtrack
§Example
let mut parser = take(5u8).complete_err();
assert_eq!(parser.parse_peek(Partial::new("abcdefg")), Ok((Partial::new("fg"), "abcde")));
assert_eq!(parser.parse_peek(Partial::new("abcd")), Err(ErrMode::Backtrack(InputError::new(Partial::new("abcd"), ErrorKind::Complete))));Trait Implementations§
source§impl<'a, I, O, E> Parser<I, O, E> for Box<dyn Parser<I, O, E> + 'a>
impl<'a, I, O, E> Parser<I, O, E> for Box<dyn Parser<I, O, E> + 'a>
source§fn parse_next(&mut self, i: &mut I) -> PResult<O, E>
fn parse_next(&mut self, i: &mut I) -> PResult<O, E>
source§fn parse_peek(&mut self, input: I) -> IResult<I, O, E>
fn parse_peek(&mut self, input: I) -> IResult<I, O, E>
source§fn by_ref(&mut self) -> ByRef<'_, Self>where
Self: Sized,
fn by_ref(&mut self) -> ByRef<'_, Self>where
Self: Sized,
Treat
&mut Self as a parser Read moresource§fn void(self) -> Void<Self, I, O, E>where
Self: Sized,
fn void(self) -> Void<Self, I, O, E>where
Self: Sized,
Discards the output of the
Parser Read moresource§fn output_into<O2>(self) -> OutputInto<Self, I, O, O2, E>
fn output_into<O2>(self) -> OutputInto<Self, I, O, O2, E>
Convert the parser’s output to another type using
std::convert::From Read moresource§fn map<G, O2>(self, map: G) -> Map<Self, G, I, O, O2, E>
fn map<G, O2>(self, map: G) -> Map<Self, G, I, O, O2, E>
Maps a function over the output of a parser Read more
source§fn flat_map<G, H, O2>(self, map: G) -> FlatMap<Self, G, H, I, O, O2, E>
fn flat_map<G, H, O2>(self, map: G) -> FlatMap<Self, G, H, I, O, O2, E>
Creates a parser from the output of this one Read more
source§fn complete_err(self) -> CompleteErr<Self>where
Self: Sized,
fn complete_err(self) -> CompleteErr<Self>where
Self: Sized,
Implementations on Foreign Types§
source§impl<'a, I, O, E> Parser<I, O, E> for Box<dyn Parser<I, O, E> + 'a>
impl<'a, I, O, E> Parser<I, O, E> for Box<dyn Parser<I, O, E> + 'a>
fn parse_next(&mut self, i: &mut I) -> PResult<O, E>
source§impl<'s, I, E: ParserError<I>> Parser<I, <I as Stream>::Slice, E> for &'s str
impl<'s, I, E: ParserError<I>> Parser<I, <I as Stream>::Slice, E> for &'s str
This is a shortcut for tag.
§Example
fn parser<'s>(s: &mut &'s str) -> PResult<&'s str, InputError<&'s str>> {
alt(("Hello", take(5usize))).parse_next(s)
}
assert_eq!(parser.parse_peek("Hello, World!"), Ok((", World!", "Hello")));
assert_eq!(parser.parse_peek("Something"), Ok(("hing", "Somet")));
assert_eq!(parser.parse_peek("Some"), Err(ErrMode::Backtrack(InputError::new("Some", ErrorKind::Slice))));
assert_eq!(parser.parse_peek(""), Err(ErrMode::Backtrack(InputError::new("", ErrorKind::Slice))));source§impl<'s, I, E: ParserError<I>> Parser<I, <I as Stream>::Slice, E> for &'s [u8]
impl<'s, I, E: ParserError<I>> Parser<I, <I as Stream>::Slice, E> for &'s [u8]
This is a shortcut for tag.
§Example
fn parser<'s>(s: &mut &'s [u8]) -> PResult<&'s [u8], InputError<&'s [u8]>> {
alt((&"Hello"[..], take(5usize))).parse_next(s)
}
assert_eq!(parser.parse_peek(&b"Hello, World!"[..]), Ok((&b", World!"[..], &b"Hello"[..])));
assert_eq!(parser.parse_peek(&b"Something"[..]), Ok((&b"hing"[..], &b"Somet"[..])));
assert_eq!(parser.parse_peek(&b"Some"[..]), Err(ErrMode::Backtrack(InputError::new(&b"Some"[..], ErrorKind::Slice))));
assert_eq!(parser.parse_peek(&b""[..]), Err(ErrMode::Backtrack(InputError::new(&b""[..], ErrorKind::Slice))));source§impl<'s, I, E: ParserError<I>, const N: usize> Parser<I, <I as Stream>::Slice, E> for &'s [u8; N]
impl<'s, I, E: ParserError<I>, const N: usize> Parser<I, <I as Stream>::Slice, E> for &'s [u8; N]
This is a shortcut for tag.
§Example
fn parser<'s>(s: &mut &'s [u8]) -> PResult<&'s [u8], InputError<&'s [u8]>> {
alt((b"Hello", take(5usize))).parse_next(s)
}
assert_eq!(parser.parse_peek(&b"Hello, World!"[..]), Ok((&b", World!"[..], &b"Hello"[..])));
assert_eq!(parser.parse_peek(&b"Something"[..]), Ok((&b"hing"[..], &b"Somet"[..])));
assert_eq!(parser.parse_peek(&b"Some"[..]), Err(ErrMode::Backtrack(InputError::new(&b"Some"[..], ErrorKind::Slice))));
assert_eq!(parser.parse_peek(&b""[..]), Err(ErrMode::Backtrack(InputError::new(&b""[..], ErrorKind::Slice))));source§impl<I, E> Parser<I, <I as Stream>::Token, E> for char
impl<I, E> Parser<I, <I as Stream>::Token, E> for char
This is a shortcut for one_of.
§Example
fn parser<'s>(i: &mut &'s str) -> PResult<char, InputError<&'s str>> {
'a'.parse_next(i)
}
assert_eq!(parser.parse_peek("abc"), Ok(("bc", 'a')));
assert_eq!(parser.parse_peek(" abc"), Err(ErrMode::Backtrack(InputError::new(" abc", ErrorKind::Verify))));
assert_eq!(parser.parse_peek("bc"), Err(ErrMode::Backtrack(InputError::new("bc", ErrorKind::Verify))));
assert_eq!(parser.parse_peek(""), Err(ErrMode::Backtrack(InputError::new("", ErrorKind::Token))));source§impl<I, E> Parser<I, u8, E> for u8
impl<I, E> Parser<I, u8, E> for u8
This is a shortcut for one_of.
§Example
fn parser<'s>(i: &mut &'s [u8]) -> PResult<u8, InputError<&'s [u8]>> {
b'a'.parse_next(i)
}
assert_eq!(parser.parse_peek(&b"abc"[..]), Ok((&b"bc"[..], b'a')));
assert_eq!(parser.parse_peek(&b" abc"[..]), Err(ErrMode::Backtrack(InputError::new(&b" abc"[..], ErrorKind::Verify))));
assert_eq!(parser.parse_peek(&b"bc"[..]), Err(ErrMode::Backtrack(InputError::new(&b"bc"[..], ErrorKind::Verify))));
assert_eq!(parser.parse_peek(&b""[..]), Err(ErrMode::Backtrack(InputError::new(&b""[..], ErrorKind::Token))));