Writing parsers in Rust with combinators
Motivation
We're creating a bencode parser for our BitTorrent client, matey.
Potential solutions
regex
manual
generators
Parser combinators
let needle_parser = tag("needle");
needle_parser("needlehaystack"); // => Ok(("haystack", "needle"))
use nom::multi::many0_count;
let count_needles = many0_count(needle_parser);
count_needles("haystack"); // => Ok(("haystack", 0))
count_needles("needleneedleneedleneedlehaystack");
// => Ok(("haystack", 4))use nom::sequence::pair;
use nom::multi::many0_count;
let hello_world = pair(tag("hello"), tag("world"));
hello_world("helloworld"); // => Ok(("", "helloworld"))
many0_count(hello_world)("helloworldhelloworldhelloworld"); // => Ok(("", 3))Bencode
d
8:announce 46:http://torrent.fedoraproject.org:6969/announce
13:creation date i1587996226e
4:infod
5:files l
d
6:length i1030e
4:path l
41:Fedora-Workstation-32-1.6-x86_64-CHECKSUM
e
e
d
6:length i1966178304e
4:path l
41:Fedora-Workstation-Live-x86_64-32-1.6.isoeee
e
e
e
e
eOur parser
use nom::{
bytes::complete::tag, sequence::pair,
multi::many0, branch::alt, combinators::map
};
pub fn parse_bencode(bencode: &[u8]) -> IResult<&[u8], Bencode> {
alt((
map(number, Bencode::Number),
map(string, Bencode::ByteString),
map(list, Bencode::List),
map(dict, Bencode::Dict),
))(bencode)
}
pub fn string(bencode: &[u8]) -> IResult<&[u8], Vec<u8>> { /* ... */ }
pub fn number(bencode: &[u8]) -> IResult<&[u8], i64> { /* ... */ }
pub fn list(bencode: &[u8]) -> IResult<&[u8], Vec<Bencode>> {
let (bencode, _) = tag("l")(bencode)?;
let (bencode, output_list) = many0(parse_bencode)(bencode)?;
let (bencode, _) = tag("e")(bencode)?;
Ok((bencode, output_list))
}
pub fn dict(bencode: &[u8]) -> IResult<&[u8], BTreeMap<Vec<u8>, Bencode>> {
let (bencode, _) = tag("d")(bencode)?;
let (bencode, output_tuple_list) = many0(pair(string, parse_bencode))(bencode)?;
let (bencode, _) = tag("e")(bencode)?;
Ok((bencode, output_tuple_list.into_iter().collect()))
}Torrent {
announce: "http://torrent.fedoraproject.org:6969/announce",
info: TorrentInfo {
name: "Fedora-Workstation-Live-x86_64-32",
files: [
TorrentFile {
length: 1030,
path: "Fedora-Workstation-32-1.6-x86_64-CHECKSUM",
},
TorrentFile {
length: 1966178304,
path: "Fedora-Workstation-Live-x86_64-32-1.6.iso",
},
],
}
}
Writing parsers in Rust with combinators
By Eshan Singh
