rusty_lr_core 2.7.0

core library for rusty_lr
Documentation

RustyLR

crates.io docs.rs

GLR, LR(1) and LALR(1) parser generator for Rust.

RustyLR provides procedural macros and buildscript tools to generate GLR, 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, and the error messages are readable and detailed. For huge and complex grammars, it is recommended to use the buildscipt.

features in Cargo.toml

  • build : Enable buildscript tools.
  • fxhash : In parser table, replace std::collections::HashMap with FxHashMap from rustc-hash.

Example

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

    // 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 }
        | space* lparen E rparen space* { E }
        ;

    E(f32) : A ;
}

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

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;
        }
    }
}
parser.feed(&mut context, '\0', &mut userdata).unwrap();    // feed `eof` token

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

Readable error messages (with codespan)

images/error1.png images/error2.png

  • This error message is generated by the buildscript tool, not the procedural macros.

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
  • tools for integrating with build.rs

Contents

proc-macro

Below procedural macros are provided:

  • lr1! : generate LR(1) parser
  • lalr1! : generate 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.

Integrating with build.rs

This buildscripting tool 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 buildscript than the procedural macros. Generated code will contain the same structs and functions as the procedural macros. In your actual source code, you can include! the generated file.

Unlike the procedural macros, the 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. And the context-free grammar must be followed by %%.

// parser.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;

You must enable the feature build to use in the build script.

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

// build.rs
use rusty_lr::build;

fn main() {
    println!("cargo::rerun-if-changed=src/parser.rs");

    let output = format!("{}/parser.rs", std::env::var("OUT_DIR").unwrap());
    build::Builder::new()
        .file("src/parser.rs") // path to the input file
    //  .lalr()                // to generate LALR(1) parser
        .build(&output);       // path to the output file
}

In your source code, include the generated file.

include!(concat!(env!("OUT_DIR"), "/parser.rs"));

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();

GLR Parser

The GLR (Generalized LR parser) can be generated by %glr; directive in the grammar.

// generate GLR parser;
// from now on, shift/reduce, reduce/reduce conflicts will not be treated as errors
%glr; 
...

GLR parser can handle ambiguous grammars that LR(1) or LALR(1) parser cannot. When it encounters any kind of conflict during parsing, the parser will diverge into multiple states, and will try every paths until it fails. Of course, there must be single unique path left at the end of parsing (the point where you feed eof token).

Resolving Ambiguities

You can resolve the ambiguties through the reduce action. Simply, returning Result::Err(Error) from the reduce action will revoke current path. The Error variant type can be defined by %err directive.

Note on GLR Parser

  • Still in development, not have been tested enough (patches are welcome!).
  • Since there are multiple paths, the reduce action can be called multiple times, even if the result will be thrown away in the future.
    • Every RuleType and Term must implement Clone trait.
    • clone() will be called carefully, only when there are multiple paths.
  • User must be aware of the point where shift/reduce or reduce/reduce conflicts occur. Every time the parser diverges, the calculation cost will increase.

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.

Quick Reference

Production rules

Every production rules have the base form:

NonTerminalName
    : Pattern1 Pattern2 ... PatternN { ReduceAction }
    | Pattern1 Pattern2 ... PatternN { ReduceAction }
   ...
    ;

Each Pattern follows the syntax:

  • name : Non-terminal or terminal symbol name defined in the grammar.
  • [term1 term_start-term_last], [^term1 term_start-term_last] : Set of terminal symbols. eof will be automatically removed from the terminal set.
  • P* : Zero or more repetition of P.
  • P+ : One or more repetition of P.
  • P? : Zero or one repetition of P.
  • (P1 P2 P3) : Grouping of patterns.
  • P / term, P / [term1 term_start-term_last], P / [^term1 term_start-term_last] : Lookaheads; P followed by one of given terminal set. Lookaheads are not consumed.

Notes

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)

You can assign a value for each non-terminal symbol. In reduce action, you can access the value of each pattern holds, and can assign new value to current non-terminal symbol. Please refer to the ReduceAction and Accessing token data in ReduceAction section below. At the end of parsing, the value of the start symbol will be the result of the parsing. By default, terminal symbols hold the value of %tokentype passed by feed() function.

struct MyType<T> {
    ...
}

E(MyType<i32>) : ... Patterns ... { <This will be new value of E> } ;

ReduceAction (optional)

Reduce action can be written in Rust code. It is executed when the rule is matched and reduced.

  • If RuleType is defined for current non-terminal symbol, 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.

  • 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.
NoRuleType: ... ;

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

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

// 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 ( &mut UserData ) : userdata passed to the feed() function.
  • lookahead ( &Term ) : lookahead token that caused the reduce action.

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.
  • You can remap the variable name by using = operator.
E(i32) : A plus a2=A {
    println!("Value of A: {:?}", A);
    println!("Value of plus: {:?}", plus);
    println!("Value of a2: {:?}", a2);

    A + a2 // new value of E
};

For some regex pattern, the type of variable will be modified as follows:

  • P* : Vec<P>
  • P+ : Vec<P>
  • P? : Option<P>

You can still access the Vec or Option by using the base name of the pattern.

E(i32) : A* {
    println!( "Value of A: {:?}", A ); // Vec<A>
};

For terminal set [term1 term_start-term_end], [^term1 term_start-term_end], there is no predefined variable name. You must explicitly define the variable name.

E: digit=[zero-nine] {
    println!( "Value of digit: {:?}", digit ); // %tokentype
};

For group (P1 P2 P3):

  • If none of the patterns hold value, the group itself will not hold any value.
  • If only one of the patterns holds value, the group will hold the value of the very pattern. And the variable name will be same as the pattern. (i.e. If P1 holds value, and others don't, then (P1 P2 P3) will hold the value of P1, and can be accessed via name P1)
  • If there are multiple patterns holding value, the group will hold Tuple of the values. There is no default variable name for the group, you must define the variable name explicitly by = operator.
NoRuleType: ... ;

I(i32): ... ;

// I will be chosen
A: (NoRuleType I NoRuleType) {
    println!( "Value of I: {:?}", I ); // can access by 'I'
    I
};

// ( i32, i32 )
B: i2=( I NoRuleType I ) {
    println!( "Value of I: {:?}", i2 ); // must explicitly define the variable name
};

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.

A(i32) : ... ;

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

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 name <RustExpr> ;

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

%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 NonTerminalName ;

Set the start symbol of the grammar as NonTerminalName.

%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.

%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 { ... }

...

%userdata MyUserData;

...

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

Reduce type (optional)

// reduce first
%left term1 ;
%left [term1 term_start-term_last] ;

// shift first
%right term1 ;
%right [term1 term_start-term_last] ;

Set the shift/reduce precedence for terminal symbols. %left can be abbreviated as %reduce or %l, and %right can be abbreviated as %shift or %r.

// define tokens
%token plus '+';
%token hat '^';


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

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

Error type (optional)

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

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

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

...


%err MyErrorType<GenericType> ;

...

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

Derive (optional)

Specify the derive attributes for the generated Context struct. By default, the generated Context does not implement any traits. But in some cases, you may want to derive traits like Clone, Debug, or Serialize, Deserialize of serde.

In this case, user must ensure that every member of the Context must implement the trait. Currently, Context is holding the stack data, which is Vec<usize> for state stack and Vec<T> for every RuleType in the grammar.

%derive Clone, Debug, serde::Serialize ;

// here, #[derive(Clone,Debug)] will be added to the generated `Context` struct
%derive Clone, Debug;

...

let mut context = parser.begin();
// do something with context...

println!( "{:?}", context );          // debug-print context
let cloned_context = context.clone(); // clone context, you can re-feed the input sequence using cloned context

GLR parser generation

%glr;

Swith to GLR parser generation.

If you want to generate GLR parser, add %glr; directive in the grammar. With this directive, any Shift/Reduce, Reduce/Reduce conflicts will not be treated as errors.

See GLR Parser section for more details.