Important: I do not actively maintain this anymore. I use it in https://github.com/Nadrieril/dhall-rust but otherwise don't plan do add features or other such things. Think of it as a proof-of-concept.
pest_consume extends [pest] to make it easy to consume a pest parse tree.
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.
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:
```rust use pest_consume::Parser; // Construct the first half of the parser using pest as usual.
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.
```rust
use pestconsume::Error;
type Result
// This is the other half of the parser, using pest_consume.
impl CSVParser {
fn EOI(_input: Node) -> Result<()> {
Ok(())
}
fn field(input: Node) -> Result
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:
rust
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].
rust
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.
rust
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.
Some toy examples can be found in the examples/ directory.
A real-world example can be found in dhall-rust.
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][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.
See here for precedence climbing, passing custom data through the parser, and more.
Works with rust >= 1.45 (because it exports a proc-macro in expression position).
Licensed under either of
at your option.
Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.
License: MIT OR Apache-2.0