RustyLR

LR(1) and LALR(1) Deterministic Finite Automata (DFA) generator from Context Free Grammar (CFGs).

[dependencies]
rusty_lr = "0.9.0"

Features

• pure Rust implementation
• readable error messages, both for grammar building and parsing
• compile-time DFA construction from CFGs ( with proc-macro )
• customizable reduce action
• resolving conflicts of ambiguous grammar
• tracing parser action with callback

Why proc-macro, not external executable?

• Decent built-in lexer, with consideration of unicode and comments.
• Can generate pretty error messages, by just passing Span data.
• With modern IDE, can see errors in real-time with specific location.

Sample

Please refer to the example directory for the full example.

In example/calculator/parser.rs,

use rusty_lr::lr1;
use rusty_lr::lalr1;

enum Token {
    // token definitions
    ...
}

// this define struct `EParser`
// where 'E' is the start symbol
lalr1! {
    // type of userdata
    %userdata i32;
    // type of token ( as Terminal symbol )
    %tokentype Token;

    // start symbol
    %start E;
    // eof symbol; for augmented rule generation
    %eof Token::Eof;

    // define tokens
    %token num Token::Num(0); // `num` maps to `Token::Num(0)`
    %token plus Token::Plus;
    %token star Token::Star;
    %token lparen Token::LParen;
    %token rparen Token::RParen;

    // resolving shift/reduce conflict
    %left plus;
    %left star;

    // data that each token holds can be accessed by its name
    // s is slice of shifted terminal symbols captured by current rule
    // userdata can be accessed by `data` ( &mut i32, for this situation )
    A(i32) : A plus a2=A {
            println!("{:?} {:?} {:?}", A.slice, *plus, a2.slice );
            //                         ^        ^      ^
            //                         |        |      |- slice of 2nd 'A'
            //                         |        |- &Token
            //                         |- slice of 1st 'A'
            println!( "{:?}", s );
            *data += 1;
            A.value + a2.value // --> this will be new value of current 'A'
        //  ^         ^
        //  |         |- value of 2nd 'A'
        //  |- value of 1st 'A'
        }
      | M { M.value }
      ;

    M(i32) : M star m2=M { *M * *m2 }
      | P { *P }
      ;

    P(i32) : num {
        if let Token::Num(n) = *num { *n }
        else { return Err(format!("{:?}", num)); }
        //            ^^^^^^^^^^^^^^^^^^^^^^^^^^
        //             reduce action returns Result<(), String>
    }
      | lparen E rparen { *E }
      ;

    E(i32) : A  { *A };
}

In example/calculator/src/main.rs,

mod parser;

fn main() {
    use parser::Token;
    // 1 + 2 * ( 3 + 4 ) = 15
    let input = vec![
        Token::Num(1),
        Token::Plus,
        Token::Num(2),
        Token::Star,
        Token::LParen,
        Token::Num(3),
        Token::Plus,
        Token::Num(4),
        Token::RParen,
        Token::Eof,
    ];

    let parser = parser::EParser::new();
    let mut context = parser.begin();
    let mut userdata: i32 = 0;
    for token in input {
        match parser.feed(&mut context, token, &mut userdata) {
            //                          ^^^^^   ^^^^^^^^^^^^ userdata passed here as `&mut i32`
            //                           |- feed token
            Ok(_) => {}
            Err(e) => {
                println!("{:?}", e);
                return;
            }
        }
    }
    // res = value of start symbol ( E(i32) )
    let res = context.accept();
    println!("{}", res);
    println!("userdata: {}", userdata);
}

The result will be:

[Num(3)] Plus [Num(4)]
[Num(3), Plus, Num(4)]
[Num(1)] Plus [Num(2), Star, LParen, Num(3), Plus, Num(4), RParen]
[Num(1), Plus, Num(2), Star, LParen, Num(3), Plus, Num(4), RParen]
15
userdata: 2

Build Deterministic Finite Automata (DFA) from Context Free Grammar (CFG)

This section will describe how to build DFA from CFGs, on runtime.

1. Define terminal and non-terminal symbols

#[derive(Clone, Hash, PartialEq, Eq, PartialOrd, Ord)] // must implement these traits
pub enum Term {
    Num,
    Plus,
    Mul,
    LeftParen,
    RightParen,
    Eof,
}

#[derive(Clone, Hash, PartialEq, Eq)] // must implement these traits
pub enum NonTerm {
    E,
    A,
    M,
    P,
    Augmented,
}

/// impl Display for TermType, NonTermType will make related ProductionRule, error message Display-able
impl Display for TermType { ... }
impl Display for NonTermType { ... }

Or simply, you can use char or u8 as terminal, and &'static str or String as non-terminal. Any type that implements traits above can be used as terminal and non-terminal symbols.

2. Define production rules

Consider the following context free grammar.

A -> A + A  (reduce left)
A -> M

This grammar can be written as:

/// type alias
type Token = rusty_lr::Token<Term, NonTerm>;

/// create grammar
let mut grammar = rusty_lr::Grammar::<Term,NonTerm>::new();

grammar.add_rule(
    NonTerm::A,
    vec![Token::NonTerm(NonTerm::A), Token::Term(Term::Plus), Token::NonTerm(NonTerm::A)],
);
grammar.add_rule(
    NonTerm::A,
    vec![Token::NonTerm(NonTerm::M)],
);

/// set reduce type
grammar.set_reduce_type( Term::Plus, ReduceType::Left );

Note that the production rule A -> A + A has a shift/reduce conflict. To resolve this conflict, the precedence of shift/reduce is given to terminal symbol Plus. Left means that for Plus token, the parser will reduce the rule instead of shifting the token.

reduce/reduce conflict (e.g. duplicated rules) will be always an error.

3. Build DFA

Calling grammar.build() for LR(1) or grammar.build_lalr() for LALR(1) will build the DFA from the CFGs.

let parser:rusty_lr::Parser<Term,NonTerm> = match grammar.build(NonTerm::Augmented) {
    Ok(parser) => parser,
    Err(err) => {
        // error is Display if Term, NonTerm is Display
        eprintln!("{}", err);
        return;
    }
};

You must explicitly specify the Augmented non-terminal symbol, and the Augmented production rule must be defined in the grammar.

Augmented -> StartSymbol $

The returned Parser struct contains the DFA states and the production rules(cloned). It is completely independent from the Grammar, so you can drop the Grammar struct, or export the Parser struct to another module.

4. Error messages

The Error type returned from Grammar::build() will contain the error information. You can manually match the error type for custom error message, but for most cases, using println!("{}", err) will be enough to see the detailed errors. Error is Display if both Term and NonTerm is Display, and It is Debug if both Term and NonTerm is Debug.

Sample error messages

For Shift/Reduce conflicts,

Build failed: Shift/Reduce Conflict
NextTerm: '0'
Reduce Rule:
"Num" -> "Digit"
Shift Rules:
"Digit" -> '0' • /Lookaheads: '\0', '0'
Try rearanging the rules or set ReduceType to Terminal '0' to resolve the conflict.

For Reduce/Reduce conflicts,

Build failed: Reduce/Reduce Conflict with lookahead: '\0'
Production Rule1:
"Num" -> "Digit"
Production Rule2:
"Num" -> "Digit"

Parse input sequence with generated DFA

For given input sequence, you can start parsing with Parser::begin() method. Once you get the Context from begin(), you can feed the input sequence one by one with Parser::feed() method.

let terms = vec![ Term::Num, Term::Plus, Term::Num, Term::Mul, Term::LeftParen, Term::Num, Term::Plus, Term::Num, Term::RightParen, Term::Eof];

// start parsing
let mut context = parser.begin();

// feed input sequence
for term in terms {
    match parser.feed(&mut context, term) {
        Ok(_) => (),
        Err(err) => {
            eprintln!("{:?}", err);
            return;
        }
    }
}

EOF token is feeded at the end of sequence, and the augmented rule Augmented -> StartSymbol $ will not be reduced since there are no lookahead symbols.

Parse with callback

For tracing parser action, you can implement Callback trait and pass it to parser.feed_callback().

struct ParserCallback {}

impl rusty_lr::Callback<Term, NonTerm> for ParserCallback {
    /// Error type for callback
    type Error = String;

    fn reduce(
        &mut self,
        rules: &[rusty_lr::ProductionRule<char, String>],
        //                                  ^     |- NonTerm
        //                                  |- Term
        states: &[rusty_lr::State<char, String>],
        //                         ^     |- NonTerm
        //                         |- Term
        state_stack: &[usize],
        rule: usize,
    ) -> Result<(), Self::Error> {
        // `Rule` is Display if Term, NonTerm is Display
        println!("Reduce by {}", rules[rule]);
        Ok(())
    }
    fn shift_and_goto(
        &mut self,
        rules: &[rusty_lr::ProductionRule<char, String>],
        states: &[rusty_lr::State<char, String>],
        state_stack: &[usize],
        term: &char,
    ) -> Result<(), Self::Error> {
        Ok(())
    }
    fn shift_and_goto_nonterm(
        &mut self,
        rules: &[rusty_lr::ProductionRule<char, String>],
        states: &[rusty_lr::State<char, String>],
        state_stack: &[usize],
        nonterm: &String,
    ) -> Result<(), Self::Error> {
        Ok(())
    }
}

// Num + Num * ( Num + Num )
let terms = vec![ Term::Num, Term::Plus, Term::Num, Term::Mul, Term::LeftParen, Term::Num, Term::Plus, Term::Num, Term::RightParen, Term::Eof];

// start parsing
let mut context = parser.begin();
let mut callback = ParserCallback {};

// feed input sequence
for term in terms {
    match parser.feed_callback(&mut context, &mut callback, term) {
        Ok(_) => (),
        Err(err) => {
            eprintln!("{:?}", err);
            return;
        }
    }
}

The result will be:

Reduce by P -> Num
Reduce by M -> P
Reduce by A -> M
Reduce by P -> Num
Reduce by M -> P
Reduce by P -> Num
Reduce by M -> P
Reduce by A -> M
Reduce by P -> Num
Reduce by M -> P
Reduce by A -> M
Reduce by A -> A + A
Reduce by E -> A
Reduce by P -> ( E )
Reduce by M -> P
Reduce by M -> M * M
Reduce by A -> M
Reduce by A -> A + A
Reduce by E -> A

proc-macro lr1! and lalr1!

lr1! and lalr1! are procedural macros that will build Parser struct from CFGs at compile time. Please refer to the Sample section for actual usage.

Every line in the macro must follow the syntax below.

Token type (must defined)

'%tokentype' <RustType> ';'

Define the type of terminal symbols. <RustType> must be accessible at the point where the macro is called.

Token definition (must defined)

'%token' <Ident> <RustExpr> ';'

Map terminal symbol's name <Ident> to the actual value <RustExpr>. <RustExpr> must be accessible at the point where the macro is called.

Start symbol (must defined)

'%start' <Ident> ';'

Define the start symbol of the grammar.

Eof symbol (must defined)

'%eof' <RustExpr> ';'

Define the eof terminal symbol. <RustExpr> must be accessible at the point where the macro is called. 'eof' terminal symbol will be automatically added to the grammar.

Userdata type (optional)

'%userdata' <RustType> ';'

Define the type of userdata passed to feed() function.

Reduce type (optional)

// reduce first
'%left' <Ident> ';'
'%l' <Ident> ';'
'%reduce' <Ident> ';'

// shift first
'%right' <Ident> ';'
'%r' <Ident> ';'
'%shift' <Ident> ';'

Set the shift/reduce precedence for terminal symbols. <Ident> must be defined in %token.

Production rules

<Ident><RuleType>
  ':' <Token>* <ReduceAction>
  '|' <Token>* <ReduceAction>
  ...
  ';'

Define the production rules.

<Token> : <Ident as Term or Non-Term>                                ... (1)
        | <Ident as variable name> '=' <Ident as Term or Non-Term>   ... (2)
        ;

For (1), <Ident> must be valid terminal or non-terminal symbols. In this case, the data of the token will be mapped to the variable with the same name as <Ident>. For (2), the data of the token will be mapped to the variable on the left side of '='. For more information about the token data and variable, refer to the reduce action below.

RuleType (optional)

<RuleType> : '(' <RustType> ')'
           |
           ;

Define the type of value that this production rule holds.

ReduceAction

<ReduceAction> : '{' <RustExpr> '}'
               |
               ;

Define the action to be executed when the rule is matched and reduced. If <RuleType> is defined, <ReduceAction> itself must be the value of <RuleType> (i.e. no semicolon at the end of the statement).

predefined variables can be used in <ReduceAction>:

• s : slice of shifted terminal symbols &[<TermType>] captured by current rule.
• data : userdata passed to feed() function.

To access the data of each token, you can directly use the name of the token as a variable. For non-terminal symbols, the type of data is rusty_lr::NonTermData<'a, TermType, RuleType>. For terminal symbols, the type of data is rusty_lr::TermData<'a, TermType>. If multiple variables are defined with the same name, the variable on the front-most will be used.

For example, following code will print the value of each A, and the slice of each A and plus token in the production rule E -> A plus A.

%token plus ...;

E : A plus a2=A
  {
    println!("Value of 1st A: {}", A.value);  // A.value or *A
    println!("Slice of 1st A: {:?}", A.slice);
    println!("Value of 2nd A: {}", a2.value); // a2.value or *a2
    println!("Slice of 2nd A: {:?}", a2.slice);

    if let Token::Plus(plus) = *plus {
        println!( "Plus token: {:?}", plus );
    }
  }
  ;

A(i32): ... ;

Result<(), String> can be returned from <ReduceAction>. Returned Err will be delivered to the caller of feed() function.

Error type (optional)

'%err' <RustType> ';'
'%error' <RustType> ';'

Define the type of Err variant in Result<(), Err> returned from <ReduceAction>. If not defined, String will be used.

Start Parsing

lr1! and lalr1! will generate struct <StartSymbol>Parser.

The struct has the following functions:

• new() : create new parser
• begin(&self) : create new context
• feed(&self, &mut Context, TermType, &mut UserData) -> Result<(), ParseError> : feed token to the parser
• feed_callback(&self, &mut Context, &mut C: Callback, TermType, &mut UserData) -> Result<(), ParseError> : feed token with callback

Note that the parameter &mut UserData is omitted if %userdata is not defined. Once the input sequence (including eof token) is feeded, without errors, you can get the value of start symbol by calling context.accept().

let parser = parser::EParser::new();
// create context
let mut context = parser.begin();
// define userdata
let mut userdata: i32 = 0;

// start feeding tokens
for token in input_sequence {
    match parser.feed(&mut context, token, &mut userdata) {
        //                          ^^^^^   ^^^^^^^^^^^^ userdata passed here as `&mut i32`
        //                           |- feed token
        Ok(_) => {}
        Err(e) => {
            println!("{:?}", e);
            return;
        }
    }
}
// res = value of start symbol
let res = context.accept();
println!("{}", res);
println!("userdata: {}", userdata);