Luciano Mammino PRO
Cloud developer, entrepreneur, fighter, butterfly maker! #nodejs #javascript - Author of https://www.nodejsdesignpatterns.com , Founder of https://fullstackbulletin.com
Quick intro to nom
A parser combinator library for Rust
What is a parser?
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What is a parser?
A program that can turn text (or bytes) into structured infromation
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What is a parser?
A program that can turn text (or bytes) into structured infromation
"Point: (22, 17, -11)"
struct Point3D {
x: i32,
y: i32,
z: i32,
}get these slides at
Naive approach
String split like a mad-man 🤪
[label, remainder] = split(input, ": ", 2)
input = "Point: (22, 17, -11)"
label
remainder
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Naive approach
String split like a mad-man 🤪
[label, remainder] = split(input, ": ", 2)
input = "Point:Â (22, 17, -11)"
[, remainder] = split(remainder, "(", 2)Naive approach
String split like a mad-man 🤪
[label, remainder] = split(input, ": ", 2)
input = "Point: (22, 17, -11)"[, remainder] = split(remainder, "(", 2)[numbers,] = split(remainder, ")", 2)
Naive approach
String split like a mad-man 🤪
[label, remainder] = split(input, ": ", 2)
input = "Point: (22, 17, -11)"
[, remainder] = split(remainder, "(", 2)[numbers,] = split(remainder, ")", 2)
[x, y, z] = split(numbers, ", ", 3)
The problem with this approach
- Not always suitable (you need "delimiters")
- Simple for simple stuff, increasingly hard for "realistic" use cases
- Hard to handle errors consistently
Consuming parser
Tries to "eat and match" the source text (or bytes)
Consuming parser
Tries to "eat and match" the source text (or bytes)
input = "Point: (22, 17, -11)"
1. readLabel(input)
"Point", ": (22, 17, -11)"
Parsed data
Remainder string
Consuming parser
Tries to "eat and match" the source text (or bytes)
input = "Point: (22, 17, -11)"1. readLabel(input)
"Point", ": (22, 17, -11)"
2. readSeparator(remainder)
":", " (22, 17, -11)"
Consuming parser
Tries to "eat and match" the source text (or bytes)
input = "Point:Â (22, 17, -11)"1. readLabel(input)
"Point", ": (22, 17, -11)"
2. readSeparator(remainder)
":", " (22, 17, -11)"
3. readSpaces(remainder)
" ", "(22, 17, -11)"
Consuming parser
Tries to "eat and match" the source text (or bytes)
input = "Point:Â (22, 17, -11)"1. readLabel(input)
"Point", ": (22, 17, -11)"
2. readSeparator(remainder)
":", " (22, 17, -11)"
3. readSpaces(remainder)
" ", "(22, 17, -11)"
4. readIntTuple(remainder)
[22,17,-11], ""Consuming parser
Handling errors: if we fail to match we have to return an error
input = "Hello World"
readNumber(input)
"Cannot match number on 'Hello World'"$ cargo add nom
fn main() {
let s = "Point: (22, 17, -11)";
let (_, point) = parse_point(s).unwrap();
println!("{:?}", point);
}fn parse_point(input: &str) -> IResult<&str, Point3D> {
// ... parsing logic goes here
Ok((input, Point3D { x, y, z }))
}the value to parse from
A nom Result type
Input type
Output type
fn parse_point(input: &str) -> IResult<&str, Point3D> {
let (input, _) = tag("Point: ")(input)?;
// ...
Ok((input, Point3D { x, y, z }))
}nom combinator: exact match
what to match
the value to match against
If it fails to match
we propagate the error
the parsed value
(in this case "Point: ")
the remainder string
fn parse_point(input: &str) -> IResult<&str, Point3D> {
let (input, _) = tag("Point: ")(input)?;
let (input, _) = tag("(")(input)?;
// ...
Ok((input, Point3D { x, y, z }))
}fn parse_point(input: &str) -> IResult<&str, Point3D> {
let (input, _) = tag("Point: ")(input)?;
let (input, _) = tag("(")(input)?;
let (input, x) = i32(input)?;
// ...
Ok((input, Point3D { x, y, z }))
}combinator that parses a sign (+ or -) and sequence of digits and converts them to a i32
first coordinate
fn parse_point(input: &str) -> IResult<&str, Point3D> {
let (input, _) = tag("Point: ")(input)?;
let (input, _) = tag("(")(input)?;
let (input, x) = i32(input)?;
let (input, _) = tag(", ")(input)?;
let (input, y) = i32(input)?;
let (input, _) = tag(", ")(input)?;
let (input, z) = i32(input)?;
let (input, _) = tag(")")(input)?;
Ok((input, Point3D { x, y, z }))
}With nom you generally break parsers into smaller and reusable parsers
/// parses "," followed by 0 or more spaces
fn separator(input: &str) -> IResult<&str, ()> {
let (input, _) = pair(tag(","), space0)(input)?;
Ok((input, ()))
}nom combinator that applies 2 parsers in sequence and returns a tuple with the 2 parsed values
/// parses 3 numbers separated by a separator (e.g. "22, 17, -11")
fn parse_coordinates(input: &str) -> IResult<&str, (i32, i32, i32)> {
let (input, (x, _, y, _, z)) = tuple((
i32,
separator,
i32,
separator,
i32
))(input)?;
Ok((input, (x, y, z)))
}nom combinator that applies a sequence of parsers in sequence and returns a tuple with the parsed values
Note how we are reusing the parser we just created!
fn parse_point2(input: &str) -> IResult<&str, Point3D> {
let (input, _) = tag("Point: ")(input)?;
let (input, (x, y, z)) = delimited(
tag("("),
parse_coordinates,
tag(")")
)(input)?;
Ok((input, Point3D { x, y, z }))
}Links
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Quick Introduction to nom, a parser combinator library for Rust
By Luciano Mammino
Quick Introduction to nom, a parser combinator library for Rust
A quick introduction to nom, a parser combinator library for Rust showing why it's a useful tool and how to get started with it. Live streamed at https://twitch.tv/loige and then published on youtube at https://youtube.com/loige
