Crate lrpar

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lrpar provides a Yacc-compatible parser (where grammars can be generated at compile-time or run-time). It can take in traditional .y files and convert them into an idiomatic Rust parser.

If you’re new to lrpar, please read the “quick start guide”. The “grmtools book” and API reference have more detailed information. You can find the appropriate documentation for the version of lrpar you are using here:

Documentation for all past and present releases


Let’s assume we want to statically generate a parser for a simple calculator language (and let’s also assume we are able to use lrlex for the lexer). We need to add a file to our project which statically compiles both the lexer and parser. While we can perform both steps individually, it’s easiest to use lrlex which does both jobs for us in one go. Our file thus looks as follows:

use cfgrammar::yacc::YaccKind;
use lrlex::CTLexerBuilder;

fn main() -> Result<(), Box<dyn std::error::Error>> {
        .lrpar_config(|ctp| {

where src/calc.l is as follows:

[0-9]+ "INT"
\+ "+"
\* "*"
\( "("
\) ")"
[\t ]+ ;

and src/calc.y is as follows:

%start Expr
%avoid_insert "INT"
Expr -> Result<u64, ()>:
      Expr '+' Term { Ok($1? + $3?) }
    | Term { $1 }

Term -> Result<u64, ()>:
      Term '*' Factor { Ok($1? * $3?) }
    | Factor { $1 }

Factor -> Result<u64, ()>:
      '(' Expr ')' { $2 }
    | 'INT'
          let v = $1.map_err(|_| ())?;
// Any functions here are in scope for all the grammar actions above.

fn parse_int(s: &str) -> Result<u64, ()> {
    match s.parse::<u64>() {
        Ok(val) => Ok(val),
        Err(_) => {
            eprintln!("{} cannot be represented as a u64", s);

Because we specified that our Yacc file is in Grmtools format, each rule has a separate Rust type to which all its functions conform (in this case, all the rules have the same type, but that’s not a requirement).

A simple src/ is as follows:

use std::io::{self, BufRead, Write};

use lrlex::lrlex_mod;
use lrpar::lrpar_mod;

// Using `lrlex_mod!` brings the lexer for `calc.l` into scope. By default the module name
// will be `calc_l` (i.e. the file name, minus any extensions, with a suffix of `_l`).
// Using `lrpar_mod!` brings the parser for `calc.y` into scope. By default the module name
// will be `calc_y` (i.e. the file name, minus any extensions, with a suffix of `_y`).

fn main() {
    // Get the `LexerDef` for the `calc` language.
    let lexerdef = calc_l::lexerdef();
    let stdin = io::stdin();
    loop {
        print!(">>> ");
        match stdin.lock().lines().next() {
            Some(Ok(ref l)) => {
                if l.trim().is_empty() {
                // Now we create a lexer with the `lexer` method with which we can lex an input.
                let lexer = lexerdef.lexer(l);
                // Pass the lexer to the parser and lex and parse the input.
                let (res, errs) = calc_y::parse(&lexer);
                for e in errs {
                    println!("{}", e.pp(&lexer, &calc_y::token_epp));
                match res {
                    Some(Ok(r)) => println!("Result: {}", r),
                    _ => eprintln!("Unable to evaluate expression.")
            _ => break

We can now cargo run our project and evaluate simple expressions:

>>> 2 + 3
Result: 5
>>> 2 + 3 * 4
Result: 14
>>> (2 + 3) * 4
Result: 20

lrpar also comes with advanced error recovery built-in:

>>> 2 + + 3
Parsing error at line 1 column 5. Repair sequences found:
   1: Delete +
   2: Insert INT
Result: 5
>>> 2 + 3 3
Parsing error at line 1 column 7. Repair sequences found:
   1: Insert *
   2: Insert +
   3: Delete 3
Result: 11
>>> 2 + 3 4 5
Parsing error at line 1 column 7. Repair sequences found:
   1: Insert *, Delete 4
   2: Insert +, Delete 4
   3: Delete 4, Delete 5
   4: Insert +, Shift 4, Delete 5
   5: Insert +, Shift 4, Insert +
   6: Insert *, Shift 4, Delete 5
   7: Insert *, Shift 4, Insert *
   8: Insert *, Shift 4, Insert +
   9: Insert +, Shift 4, Insert *
Result: 17


A convenience macro for including statically compiled .y files. A file src/a/b/c.y processed by CTParserBuilder::grammar_in_src_dir can then be used in a crate with lrpar_mod!("a/b/c.y").


An interface to the result of CTParserBuilder::build().
A CTParserBuilder allows one to specify the criteria for building a statically generated parser.
Records a single parse error.
A run-time parser builder.
A Span records what portion of the user’s input something (e.g. a lexeme or production) references (i.e. the Span doesn’t hold a reference / copy of the actual input).


A lexing or parsing error. Although the two are quite distinct in terms of what can be reported to users, both can (at least conceptually) occur at any point of the intertwined lexing/parsing process.
A generic parse tree.
After a parse error is encountered, the parser attempts to find a way of recovering. Each entry in the sequence of repairs is represented by a ParseRepair.
What recovery algorithm should be used when a syntax error is encountered?
Specifies the Rust Edition that will be emitted during code generation.
Specify the visibility of the module generated by CTBuilder.


A lexing error.
A lexeme represents a segment of the user’s input that conforms to a known type: this trait captures the common behaviour of all lexeme structs.
The base trait which all lexers which want to interact with lrpar must implement.
A NonStreamingLexer is one that takes input in one go, and is then able to hand out substrings to that input and calculate line and column numbers from a Span.