rusty_lr_core 2.1.0

yacc-like, LR(1) and LALR(1) parser generator and code generation
Documentation

RustyLR

crates.io docs.rs for proc-macro

crates.io for executable

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

RustyLR provides both executable and procedural macros to generate LR(1) and LALR(1) parser. The generated parser will be a pure Rust code, and the calculation of building DFA will be done at compile time. Reduce action can be written in Rust code, and the error messages are readable and detailed with executable. For huge and complex grammars, it is recommended to use the executable version.

By default, RustyLR uses std::collections::HashMap for the parser tables. If you want to use FxHashMap from rustc-hash, add features=["fxhash"] to your Cargo.toml.

[dependencies]
rusty_lr = { version = "...", features = ["fxhash"] }

Example

// this define `EParser` struct
// where `E` is the start symbol
lr1! {
    // userdata type
    %userdata i32;
    // token type
    %tokentype char;
    // start symbol
    %start E;
    // eof symbol
    %eof '\0';

    // token definition
    %token zero '0';
    %token one '1';
    %token two '2';
    %token three '3';
    %token four '4';
    %token five '5';
    %token six '6';
    %token seven '7';
    %token eight '8';
    %token nine '9';
    %token plus '+';
    %token star '*';
    %token lparen '(';
    %token rparen ')';
    %token space ' ';

    // conflict resolving
    %left [plus star]; // reduce first for token 'plus', 'star'

    // context-free grammars
    Digit(char): [zero-nine]; // character set '0' to '9'

    Number(i32) // type assigned to production rule `Number`
        : space* Digit+ space* // regex pattern
    { Digit.into_iter().collect::<String>().parse().unwrap() }; 
    //    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this will be the value of `Number`
    // reduce action written in Rust code

    A(f32): A plus a2=A {
        *data += 1; // access userdata by `data`
        println!( "{:?} {:?} {:?}", A, plus, a2 );
        A + a2
    }
        | M
        ;

    M(f32): M star m2=M { M * m2 }
        | P
        ;

    P(f32): Number { Number as f32 }
        | WS0 lparen E rparen WS0 { E }
        ;

    E(f32) : A ;
}

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

let input_sequence = "1 + 2 * ( 3 + 4 )";

// start feeding tokens
for token in input_sequence.chars() {
    match parser.feed(&mut context, token, &mut userdata) {
        //                          ^^^^^   ^^^^^^^^^^^^ userdata passed here as `&mut i32`
        //                           |- feed token
        Ok(_) => {}
        Err(e) => {
            match e {
                EParseError::InvalidTerminal(invalid_terminal) => {
                    ...
                }
                EParseError::ReduceAction(error_from_reduce_action) => {
                    ...
                }
            }
            println!("{}", e);
            // println!( "{}", e.long_message( &parser, &context ) );
            return;
        }
    }
}
// feed `eof` token
parser.feed(&mut context, '\0', &mut userdata).unwrap();

// res = value of start symbol
let res = context.accept();
println!("{}", res);
println!("userdata: {}", userdata);

Readable error messages (with codespan)

images/error1.png images/error2.png

Contents

Features

  • pure Rust implementation
  • readable error messages, both for grammar building and parsing
  • compile-time DFA construction from CFGs
  • customizable reduce action
  • resolving conflicts of ambiguous grammar
  • regex patterns partially supported
  • executable for generating parser tables

proc-macro

Below procedural macros are provided:

  • lr1! : LR(1) parser
  • lalr1! : LALR(1) parser

These macros will generate structs:

  • Parser : contains DFA tables and production rules
  • ParseError : type alias for Error returned from feed()
  • Context : contains current state and data stack
  • enum NonTerminals : a list of non-terminal symbols
  • Rule : type alias for production rules
  • State : type alias for DFA states

All structs above are prefixed by <StartSymbol>. In most cases, what you want is the Parser and ParseError structs, and the others are used internally.

Start Parsing

The Parser struct has the following functions:

  • new() : create new parser
  • begin(&self) : create new context
  • feed(&self, &mut Context, TerminalType, &mut UserData) -> Result<(), ParseError> : feed token to the parser

Note that the parameter &mut UserData is omitted if %userdata is not defined. All you need to do is to call new() to generate the parser, and begin() to create a context. Then, you can feed the input sequence one by one with feed() function. Once the input sequence is feeded (including eof token), without errors, you can get the value of start symbol by calling context.accept().

let parser = Parser::new();
let context = parser.begin();
for token in input_sequence {
    match parser.feed(&context, token) {
        Ok(_) => {}
        Err(e) => { // e: ParseError
            println!("{}", e);
            return;
        }
    }
}
let start_symbol_value = context.accept();

Error Handling

There are two error variants returned from feed() function:

  • InvalidTerminal(InvalidTerminalError) : when invalid terminal symbol is fed
  • ReduceAction(ReduceActionError) : when the reduce action returns Err(Error)

For ReduceActionError, the error type can be defined by %err directive. If not defined, String will be used.

When printing the error message, there are two ways to get the error message:

  • e.long_message( &parser, &context ) : get the error message as String, in a detailed format
  • e as Display : briefly print the short message through Display trait.

The long_message function requires the reference to the parser and the context. It will make a detailed error message of what current state was trying to parse, and what the expected terminal symbols were.

Example of long_message

Invalid Terminal: *
Expected one of:  , (, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
-------------------------------Backtracing state--------------------------------
WS0 -> • _RustyLRGenerated0
_RustyLRGenerated1 -> •  
_RustyLRGenerated1 -> • _RustyLRGenerated1  
_RustyLRGenerated0 -> • _RustyLRGenerated1
_RustyLRGenerated0 ->  •
Number -> • WS0 _RustyLRGenerated3 WS0
M -> • M * M
M -> M * • M
M -> • P
P -> • Number
P -> • WS0 ( E ) WS0
-----------------------------------Prev state-----------------------------------
M -> M • * M
-----------------------------------Prev state-----------------------------------
A -> • A + A
A -> A + • A
A -> • M
M -> • M * M
-----------------------------------Prev state-----------------------------------
A -> A • + A
-----------------------------------Prev state-----------------------------------
A -> • A + A
E -> • A
Augmented -> • E

Syntax

To start writing down a context-free grammar, you need to define necessary directives first. This is the syntax of the procedural macros.

lr1! {
// %directives
// %directives
// ...
// %directives

// NonTerminalSymbol(RuleType): ProductionRules
// NonTerminalSymbol(RuleType): ProductionRules
// ...
}

lr1! macro will generate a parser struct with LR(1) DFA tables. If you want to generate LALR(1) parser, use lalr1! macro. Every line in the macro must follow the syntax below.

Bootstrap, Expanded Bootstrap would be a good example to understand the syntax and generated code. It is RustyLR syntax parser written in RustyLR itself.

Token type (must defined)

'%tokentype' <RustType> ';'

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

enum MyTokenType<Generic> {
    Digit,
    Ident,
    ...
    VariantWithGeneric<Generic>
}

lr! {
...
%tokentype MyTokenType<i32>;
}

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.

lr1! {
%tokentype u8;

%token zero b'0';
%token one b'1';

...

// 'zero' and 'one' will be replaced by b'0' and b'1' respectively
E: zero one;
}

Start symbol (must defined)

'%start' <Ident> ';'

Define the start symbol of the grammar.

lr1! {
%start E;
// this internally generate augmented rule <Augmented> -> E eof

E: ... ;
}

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.

lr1! {
%eof b'\0';
// you can access eof terminal symbol by 'eof' in the grammar
// without %token eof ...;
}

Userdata type (optional)

'%userdata' <RustType> ';'

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

struct MyUserData { ... }

lr1! {
...
%userdata MyUserData;
}

...

fn main() {
    ...
    let mut userdata = MyUserData { ... };
    parser.feed( ..., token, &mut userdata); // <-- userdata feed here
}

Reduce type (optional)

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

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

Set the shift/reduce precedence for terminal symbols. <Ident> must be defined in %token. With <TerminalSet>, you can define reduce type to multiple terminals at once. Please refer to the Regex Pattern section below. %left can be abbreviated as %reduce or %l, and %right can be abbreviated as %shift or %r.

lr1! {
// define tokens
%token plus '+';
%token hat '^';


// reduce first for token 'plus'
%left plus;

// shift first for token 'hat'
%right hat;
}

lr1! {
// define tokens
%token zero b'0';
%token one b'1';
...
%token nine b'9';

// shift first for tokens in range 'zero' to 'nine'
%shift [zero-nine];
}

Production rules

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

Define the production rules.

<TokenMapped> : <Ident as var_name> '=' <TokenPattern>
              | <TokenPattern>
              ;

<TokenPattern> : <Ident as terminal or non-terminal>
               | <TerminalSet>
               | <TokenPattern> '*'    (zero or more)
               | <TokenPattern> '+'    (one or more)
               | <TokenPattern> '?'    (zero or one)
               ;

This production rule defines non-terminal E to be A, then zero or more plus, then D mapped to variable d. For more information, please refer to the Accessing token data in ReduceAction section below.

lr1! {
E: A plus* d=D;
}

Regex pattern

Regex patterns are partially supported. You can use *, +, ? to define the number of repetitions, and [] to define the set of terminal symbols.

%token lparen '(';
%token rparen ')';
%token zero '0';
...
%token nine '9';

A: [zero-nine]+; // zero to nine

B: [^lparen rparen]; // any token except lparen and rparen

C: [lparen rparen one-nine]*; // lparen and rparen, and one to nine

Note that when using range pattern [first-last], the range is constructed by the order of the %token directives, not by the actual value of the token. If you define tokens in the following order:

%token one '1';
%token two '2';
...
%token zero '0';
%token nine '9';

The range [zero-nine] will be ['0', '9'], not ['0'-'9'].

RuleType (optional)

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

Define the type of value that this production rule holds.

lr1! {
E(MyType<...>): ... Tokens ... ;
}

ReduceAction (optional)

<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).

  • <ReduceAction> can be omitted if:

    • <RuleType> is not defined
    • Only one token is holding value in the production rule ( Non-terminal symbol with <RuleType> defined, or terminal symbols are considered as holding value )

  • Result<(),Error> can be returned from <ReduceAction>.

    • Returned Error will be delivered to the caller of feed() function.
    • ErrorType can be defined by %err or %error directive. See Error type section.

Omitting ReduceAction:

lr1! {
NoRuleType: ... ;

RuleTypeI32(i32): ... { 0 } ;

// RuleTypeI32 will be chosen
E(i32): NoRuleType NoRuleType RuleTypeI32 NoRuleType;
}

Returning Result<(),String> from ReduceAction:

lr1! {
// set Err variant type to String
%err String;

%token div '/';

E(i32): A div a2=A {
    if a2 == 0 {
        return Err("Division by zero".to_string());
    }

    A / a2
};

A(i32): ... ;
}

Accessing token data in ReduceAction

predefined variables can be used in <ReduceAction>:

  • 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 variable is <RuleType>. For terminal symbols, the type of variable is %tokentype.

If multiple variables are defined with the same name, the variable on the front-most will be used.

For regex pattern, type of variable will be modified by following:

Pattern Non-Terminal<RuleType>=T Non-Terminal<RuleType>=(not defined) TerminalTerminalSet
'*' Vec<T> (not defined) Vec<TermType>
'+' Vec<T> (not defined) Vec<TermType>
'?' Option<T> (not defined) Option<TermType> 
lr1! {
%token plus ...;

// one or more 'A', then optional 'plus', then zero or more 'B'
E(f32) : A+ plus? b=B* minus_or_star=[minus star]
  {
    println!("Value of A: {:?}", A);         // Vec<i32>
    println!("Value of plus: {:?}", plus); // Option<TermType>
    println!("Value of b: {:?}", b);       // Vec<f32>
    println!("Value of minus_or_star: {:?}", minus_or_star); // must explicitly define the variable name

    let first_A = A[0];
    let first_B = b.first(); // Option<&f32>


    // this will be the new value of E
    if let Some(first_B) = first_B {
        let value = first_A as f32 + *first_B;
        value
    } else {
        first_a as f32
    }
  }
  ;

A(i32): ... ;
B(f32): ... ;
}

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.

enum MyErrorType<T> {
    ErrVar1,
    ErrVar2,
    ErrVar3(T),
    ...
}

lr1! {

%err MyErrorType<GenericType> ;

}

...

match parser.feed( ... ) {
    Ok(_) => {}
    Err(err) => {
        match err {
            ParseError::ReduceAction( err ) => {
                // do something with err
            }
            _ => {}
        }
    }
}

Exclamation mark !

An exclamation mark ! can be used right after the token to ignore the value of the token. The token will be treated as if it is not holding any value.

Tip When combining with repeatance pattern *, +, ?, use ! first. It can prevent Vec<T> built from the value of the token internally.

lr1! {
%token plus ...;

A(i32) : ... ;

// A in the middle will be chosen, since other A's are ignored
E(i32) : A! A A!;

B: A*!; // Vec<i32> will be built from the value of A, and then ignored

C: A!*; // A will be ignored first, and then repeatance pattern will be applied
}

executable rustylr

An executable version of lr1! and lalr1! macro. Converts a context-free grammar into a deterministic finite automaton (DFA) tables, and generates a Rust code that can be used as a parser for that grammar.

cargo install rustylr

This executable will provide much more detailed, pretty-printed error messages than the procedural macros. If you are writing a huge, complex grammar, it is recommended to use this executable than the procedural macros. --verbose option is useful for debugging the grammar. It will print where the auto-generated rules are originated from and the resolving process of shift/reduce conflicts. like this

Although it is convenient to use the proc-macros for small grammars, since modern IDEs feature (rust-analyzer's auto completion, inline error messages) could be enabled.

This program searches for %% in the input file. ( Not the lr1!, lalr1! macro )

The contents before %% will be copied into the output file as it is. Context-free grammar must be followed by %%. Each line must follow the syntax of rusty_lr#syntax

// my_grammar.rs
use some_crate::some_module::SomeStruct;

enum SomeTypeDef {
    A,
    B,
    C,
}

%% // <-- input file splitted here

%tokentype u8;
%start E;
%eof b'\0';

%token a b'a';
%token lparen b'(';
%token rparen b')';

E: lparen E rparen
 | P
 ;

P: a;

Calling the command will generate a Rust code my_parser.rs.

$ rustylr my_grammar.rs my_parser.rs --verbose

Possible options can be found by --help.

$ rustylr --help
Usage: rustylr [OPTIONS] <INPUT_FILE> [OUTPUT_FILE]

Arguments:
  <INPUT_FILE>
          input_file to read

  [OUTPUT_FILE]
          output_file to write

          [default: out.tab.rs]

Options:
      --no-format
          do not rustfmt the output

  -l, --lalr
          build LALR(1) parser

  -v, --verbose
          print debug information.
    
          print the auto-generated rules, and where they are originated from.
          print the shift/reduce conflicts, and the resolving process.