Quickstart Guide

Most users will probably be interested in the compile-time Yacc feature of grmtools, which allows traditional .y files to be used mostly unchanged in Rust. This page is a short guide to get you up and running with this feature as quickly as possible.

grmtools includes both a Yacc-style LR parser (lrpar) and a lex-style lexer (lrlex). The lexer breaks input up into individual lexemes and the parser checks to see if the lexemes conform to a grammar. As the parser executes, it can either create a generic parse tree, or execute user-specified Rust code.

A calculator evaluator

Let's assume we want to create a simple calculator which can evaluate expressions such as 2 + 3 * 4. Assuming a fresh Rust project, we first create a Cargo.toml file with the following dependencies:

name = "calc"
version = "0.0.1"
authors = ["<authors>"]
edition = "2021"

doc = false
name = "calc"

cfgrammar = "0.12"
lrlex = "0.12"
lrpar = "0.12"

cfgrammar = "0.12"
lrlex = "0.12"
lrpar = "0.12"

In this situation we want to statically compile the .y grammar and .l lexer into Rust code. We thus need to create a build.rs file inside the root of our project which can process the lexer and grammar. lrlex provides a simple interface which does both jobs for us in one go. Our build.rs file thus looks as follows:

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

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

grmtools accepts several different Yacc variants as input. In our case, we want to execute Rust code as the input is parsed (rather than creating a generic parse tree which we traverse later), so we specified that the yacckind (i.e. what variant of Yacc file we're using) is YaccKind::Grmtools. The grammar file is stored in src/calc.y, but we only specify calc.y as the filename to lrpar, since it searches relative to src/ automatically. Similarly, we only needed to specify calc.l to lrlex.

The lexer

While Yacc-style parsing is powerful, lex-style lexing is less powerful. grmtools allows you to use whatever lexer you want with lrpar. Fortunately, in this case, lrlex is powerful enough for us. calc.l looks as follows:

[0-9]+ "INT"
\+ "PLUS"
\* "MUL"
[\t ]+ ;

Roughly speaking, each line after the %% line is a regular expression (we use the regex crate), a space character, and a quoted lexeme type name. For example, if the user gives us input such as 234 we will create a single lexeme with a value (234) and a type (INT).

The one exception is the final line: if a lexeme type name is replaced with ‘;’ then any matching input is discarded. In this case, whitespace (tabs and spaces) is lexed, but no lexemes are created from it.

The grammar

Our initial version of calc.y looks as follows:

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

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

Factor -> Result<u64, ()>:
      'LBRACK' Expr 'RBRACK' { $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);

The grammar is in 3 parts, separated by the %% lines.

The first part specifies general settings for the grammar, at a minimum the start rule (%start Expr).

The second part is the Yacc grammar. It consists of 3 rules (Expr, Term, and Factor) and 6 productions (2 for each rule, separated by | characters). Because we are using the Grmtools Yacc variant, each rule has a Rust type associated with it (after ->) which specifies the type that each production’s action must return. A production (sometimes called an “alternative”) consists of zero or more symbols. Symbols either reference rules or lexemes. If a production matches text, its ”action” (the Rust code between curly brackets at the end of the production) is executed.

lrpar's actions are subtly different to Yacc. The $x variables refer to the respective symbol in the production, numbered from 1 (i.e. $1 refers to the first symbol in the production). If the symbol references a rule R then an instance of R's type will be stored in the $x variable; if the symbol references a lexeme then an Option<Lexeme> instance is returned. A special $lexer variable allows access to the lexer. The most commonly used function that $lexer exposes is the span_str function, which allows us to extract &'input strs from a Span (e.g. to extract the string represented by a Lexeme, we would use $lexer.span_str(lexeme.span())). As this may suggest, actions may also reference the special lifetime 'input (without any $ prefix), which allows strings to be returned / stored by the grammar without copying memory.

The third part is arbitrary Rust code which can be called by productions’ actions. In our case we have a simple function which converts integers as strings into integers as u64s: if the user provides an invalid number (e.g. one that is too big) the system panics.

This example uses a common grmtools idiom: making use of Result types. This allows us to deal with two different issues that prevent evaluation. First is the “obvious” issue of integers which are too big to represent as u64s: these cause Err to be percolated upwards, preventing evaluation. Second is the issue of error recovery telling us that the user should have inserted an integer: since it would be confusing for us to insert a default value in such cases, we map_err such cases to Err, preventing evaluation. See the section below on error recovery for more details about error recovery.

Putting everything together

The build.rs file will statically compile the lexer and grammar into Rust code that we can then call. The src/main.rs file below provides a simple Python-esque REPL to the user into which they can write calculator expressions:

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(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

Error recovery

Because powerful error recovery is built into lrpar, we can even make minor errors and have the system recover automatically:

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

Note that we didn't have to do anything clever in order for error recovery to happen: it happens by default, and it works with whatever grammar we throw at it. The way to read the resulting error messages are that each numbered repair sequence is a way that the error recovery system found to make sense of the input. For example, for the input 2 + + 3, an error is detected at the second +: we could either delete the second + (option 1 above) or insert an integer. In all cases, error recovery applies repair sequence 1, and continues parsing. 2 + + 3 was thus parsed as if the user had written 2 + 3, hence why it evaluated to 5. Similarly, 2 + 3 4 5 was parsed as if the user had written 2 + 3 * 5.

Error recovery opens up a number of possibilities to customise and streamline the user experience. For example, the simple approach above causes a panic if the user provides a non-u64 number or if error recovery inserts an integer. For more details about the possibilities, see the section on error recovery.