lrpar

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. More details can be found in the grmtools book; the quickstart guide is a good place to start.

Example

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 build.rs file to our project which tells lrpar to statically compile the lexer and parser files:

use cfgrammar::yacc::YaccKind;
use lrlex::LexerBuilder;
use lrpar::CTParserBuilder;

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let lex_rule_ids_map = CTParserBuilder::new()
        .yacckind(YaccKind::Grmtools)
        .process_file_in_src("calc.y")?;
    LexerBuilder::new()
        .rule_ids_map(lex_rule_ids_map)
        .process_file_in_src("calc.l")?;
    Ok(())
}

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, ()>:
      Term '+' Expr { Ok($1? + $3?) }
    | Term { $1 }
    ;

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

Factor -> Result<u64, ()>:
      '(' Expr ')' { $2 }
    | 'INT'
      {
          let v = $1.map_err(|_| ())?;
          parse_int($lexer.lexeme_str(&v))
      }
    ;
%%
// 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);
            Err(())
        }
    }
}

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/main.rs 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.
lrlex_mod!(calc_l);
// Using `lrpar_mod!` brings the lexer for `calc.l` into scope.
lrpar_mod!(calc_y);

fn main() {
    // We need to get a `LexerDef` for the `calc` language in order that we can lex input.
    let lexerdef = calc_l::lexerdef();
    let stdin = io::stdin();
    loop {
        print!(">>> ");
        io::stdout().flush().ok();
        match stdin.lock().lines().next() {
            Some(Ok(ref l)) => {
                if l.trim().is_empty() {
                    continue;
                }
                // Now we create a lexer with the `lexer` method with which we can lex an input.
                let mut lexer = lexerdef.lexer(l);
                // Pass the lexer to the parser and lex and parse the input.
                let (res, errs) = calc_y::parse(&mut 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