Crate pest_consume

Expand description

pest_consume extends pest to make it easy to consume a pest parse tree.

Motivation

When using pest to write a parser, one has to traverse the resulting untyped parse tree by hand to extract the data that will be used by the rest of the application. This usually makes code that is error-prone, difficult to read, and often breaks when the grammar is updated.

pest_consume strives to make this phase of parsing easier, cleaner, and more robust.

Features of pest_consume include:

strong types;
consume parse nodes using an intuitive syntax;
easy error handling;
you won’t ever need to write .into_inner().next().unwrap() again.

Implementing a parser

Let’s start with a pest grammar for parsing CSV files:

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

and the corresponding pest parser:

use pest_consume::Parser;
// Construct the first half of the parser using pest as usual.
#[derive(Parser)]
#[grammar = "../examples/csv/csv.pest"]
struct CSVParser;

To complete the parser, define an impl block with the pest_consume::parser attribute, and for each (non-silent) rule of the grammar a method with the same name. Note how we chose an output type for each rule.

use pest_consume::Error;
type Result<T> = std::result::Result<T, Error<Rule>>;
type Node<'i> = pest_consume::Node<'i, Rule, ()>;

// This is the other half of the parser, using pest_consume.
#[pest_consume::parser]
impl CSVParser {
    fn EOI(_input: Node) -> Result<()> {
        Ok(())
    }
    fn field(input: Node) -> Result<f64> {
        ...
    }
    fn record(input: Node) -> Result<Vec<f64>> {
        ...
    }
    fn file(input: Node) -> Result<Vec<Vec<f64>>> {
        ...
    }
}

This will implement Parser for your type, so that Parser::parse can be called on it. We can now define a complete parser that returns a structured result:

fn parse_csv(input_str: &str) -> Result<Vec<Vec<f64>>> {
    // Parse the input into `Nodes`
    let inputs = CSVParser::parse(Rule::file, input_str)?;
    // There should be a single root node in the parsed tree
    let input = inputs.single()?;
    // Consume the `Node` recursively into the final value
    CSVParser::file(input)
}

It only remains to implement parsing for each rule. The simple cases are when the rule has no children. In this case, we usually only care about the captured string, accessible using Node::as_str.

    fn field(input: Node) -> Result<f64> {
        // Get the string captured by this node
        input.as_str()
            // Convert it into the type we want
            .parse::<f64>()
            // In case of  an error, we use `Node::error` to link the error
            // with the part of the input that caused it
            .map_err(|e| input.error(e))
    }

When the rule has children, the match_nodes! macro provides a typed way to parse the children. match_nodes! uses a syntax similar to slice patterns, and allows for several branches like in a match expression.

We specify for each branch the expected rules of the children, and the macro will recursively consume the children and make the result accessible to the body of the branch. A special .. syntax indicates a variable-length pattern: it will match zero or more children with the given rule, and provide an iterator with the result.

use pest_consume::match_nodes;
...
    fn record(input: Node) -> Result<Vec<f64>> {
        // Checks that the children all match the rule `field`, and captures
        // the parsed children in an iterator. `fds` implements
        // `Iterator<Item=f64>` here.
        Ok(match_nodes!(input.into_children();
            [field(fds)..] => fds.collect(),
        ))
    }

The case of the file rule is similar.

Examples

Some toy examples can be found in the examples/ directory. A real-world example can be found in dhall-rust.

How it works

The main types of this crate (Node, Nodes and Parser) are mostly wrappers around corresponding pest types, respectively Pair, Pairs and Parser. If needed, the wrapped type can be accessed, but that should rarely be necessary.

The pest_consume::parser macro implements the Parser trait for your type, and enables some advanced features, like precedence climbing and rule aliasing. A lot of the magic actually happens in match_nodes!; see there for details.