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Syntax

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.
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.
  • shift ( &mut bool ) : revoke the shift action if set to false. See Resolving Ambiguities section.

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
            }
            _ => {}
        }
    }
}

LALR parser generation

%lalr;

Switch to LALR parser generation.

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.