Comma-Separated Values is a very simple text format. CSV files consist of a list of records, each on a separate line. Each record is a list of fields separated by commas.

For example, here is a CSV file with numeric fields:

65279,1179403647,1463895090
3.1415927,2.7182817,1.618034
-40,-273.15
13,42
65537

Let's write a program that computes the sum of these fields and counts the number of records.

Start by initializing a new project using Cargo:

$ cargo init --bin csv-tool
     Created binary (application) project
$ cd csv-tool

Add the pest and pest_derive crates to the dependencies section in Cargo.toml:

[dependencies]
pest = "2.0"
pest_derive = "2.0"

And finally bring pest and pest_derive into scope in src/main.rs:

# #![allow(unused_variables)]
#fn main() {
extern crate pest;
#[macro_use]
extern crate pest_derive;
#}

The #[macro_use] attribute is necessary to use pest to generate parsing code! This is a very important attribute.

pest works by compiling a description of a file format, called a grammar, into Rust code. Let's write a grammar for a CSV file that contains numbers. Create a new file named src/csv.pest with a single line:

field = { (ASCII_DIGIT | "." | "-")+ }

This is a description of every number field: each character is either an ASCII digit 0 through 9, a full stop ., or a hyphen–minus -. The plus sign + indicates that the pattern can occur one or more times.

Rust needs to know to compile this file using pest:

# #![allow(unused_variables)]
#fn main() {
use pest::Parser;

#[derive(Parser)]
#[grammar = "csv.pest"]
pub struct CSVParser;
#}

If you run cargo doc, you will see that pest has created the function CSVParser::parse and an enum called Rule with a single variant Rule::field.

Let's test it out! Rewrite main:

fn main() {
    let successful_parse = CSVParser::parse(Rule::field, "-273.15");
    println!("{:?}", successful_parse);

    let unsuccessful_parse = CSVParser::parse(Rule::field, "this is not a number");
    println!("{:?}", unsuccessful_parse);
}

$ cargo run
  [ ... ]
Ok([Pair { rule: field, span: Span { str: "-273.15", start: 0, end: 7 }, inner: [] }])
Err(Error { variant: ParsingError { positives: [field], negatives: [] }, location: Pos(0), path: None, line: "this is not a number", continued_line: None, start: (1, 1), end: None })

Yikes! That's a complicated type! But you can see that the successful parse was Ok, while the failed parse was Err. We'll get into the details later.

For now, let's complete the grammar:

field = { (ASCII_DIGIT | "." | "-")+ }
record = { field ~ ("," ~ field)* }
file = { SOI ~ (record ~ ("\r\n" | "\n"))* ~ EOI }

The tilde ~ means "and then", so that "abc" ~ "def" matches abc followed by def. (For this grammar, "abc" ~ "def" is exactly the same as "abcdef", although this is not true in general; see a later chapter about WHITESPACE.)

In addition to literal strings ("\r\n") and built-ins (ASCII_DIGIT), rules can contain other rules. For instance, a record is a field, and optionally a comma , and then another field repeated as many times as necessary. The asterisk * is just like the plus sign +, except the pattern is optional: it can occur any number of times at all (zero or more).

There are two more rules that we haven't defined: SOI and EOI are two special rules that match, respectively, the start of input and the end of input. Without EOI, the file rule would gladly parse an invalid file! It would just stop as soon as it found the first invalid character and report a successful parse, possibly consisting of nothing at all!

Now we're ready to finish the program. We will use File to read the CSV file into memory. We'll also be messy and use expect everywhere.

use std::fs;

fn main() {
    let unparsed_file = fs::read_to_string("numbers.csv").expect("cannot read file");

    // ...
}

Next we invoke the parser on the file. Don't worry about the specific types for now. Just know that we're producing a pest::iterators::Pair that represents the file rule in our grammar.

fn main() {
    // ...

    let file = CSVParser::parse(Rule::file, &unparsed_file)
        .expect("unsuccessful parse") // unwrap the parse result
        .next().unwrap(); // get and unwrap the `file` rule; never fails

    // ...
}

Finally, we iterate over the records and fields, while keeping track of the count and sum, then print those numbers out.

fn main() {
    // ...

    let mut field_sum: f64 = 0.0;
    let mut record_count: u64 = 0;

    for record in file.into_inner() {
        match record.as_rule() {
            Rule::record => {
                record_count += 1;

                for field in record.into_inner() {
                    field_sum += field.as_str().parse::<f64>().unwrap();
                }
            }
            Rule::EOI => (),
            _ => unreachable!(),
        }
    }

    println!("Sum of fields: {}", field_sum);
    println!("Number of records: {}", record_count);
}

If p is a parse result (a Pair) for a rule in the grammar, then p.into_inner() returns an iterator over the named sub-rules of that rule. For instance, since the file rule in our grammar has record as a sub-rule, file.into_inner() returns an iterator over each parsed record. Similarly, since a record contains field sub-rules, record.into_inner() returns an iterator over each parsed field.

Try it out! Copy the sample CSV at the top of this chapter into a file called numbers.csv, then run the program! You should see something like this:

$ cargo run
  [ ... ]
Sum of fields: 2643429302.327908
Number of records: 5