ANTLR 4

Cool Features

by Alexander Vasiltsov

What features?

● Recursive rules

● Precedence

● Associativity

● Actions

● Attributes

● Semantic predicates

● Commands

Recursive rules

expr : expr ‘*’ expr

| expr ‘+’ expr

| INT

| ‘(‘ expr ‘)’

;

expr : addExpr;

addExpr : multExpr (‘+’ multExpr)*;

multExpr : atom (‘*’ atom)*;

atom : INT;

Precedence

Rule or alternative described earlier has higher

priority than others

Associativity

By default, ANTLR

associates operators left to

right

It’s possible to change the

direction of associativity

expr : <assoc=right> expr ‘^’ expr

| INT

;

expr : expr ‘^’ expr

| INT

;

assoc option

expr : <assoc=right> expr ‘^’ expr

| expr ‘*’ expr

| expr ‘+’ expr

| INT

| ‘(‘ expr ‘)’

;

Actions

rule : subrule {<some code in target language> }

;

rule2 : alternative1 {<some code for alternative1> }

| alternative2 {<some code for alternative2> }

;

rule3 : subrule1 {<some code after subrule1 matched> }

TOKEN {<some code after TOKEN matched> }

subrule2 {<some code after whole rule matched> }

;

Actions and attributes

<rulename> [<arguments list>]returns [<return values>]locals [<local members>]@init {

<init code>}@after {

<finalization code>}: <rule description>;

Actions and attributes (2)rule [int i, string s] returns [string[] vals, int max]locals [int loc=0]@init {

//some init code here}@after {

//some post-processing code}: 'TOKENS'+;

public partial class RuleContext : ParserRuleContext {public int i;public string s;public string[] vals;public int max;public int loc;

public RuleContext(ParserRuleContext parent, intinvokingState, int i, string s) : base(parent, invokingState)

{this.i = i;this.s = s;

}...

}

public RuleContext rule(int i, string s) {//some init code here... //parsing logic here//some post-processing code

}

Custom attributes

parent_rule : rule[0, "test"] {Console.WriteLine("max: "+ $rule.max);};

rule [int i, string s] returns [string[] vals, int max]locals [int loc=0]@init {

$ctx.vals = new[] {s};$ctx.loc = i + 10;

}@after {

$ctx.max *= 10;}

: (TOKEN { if ($ctx.loc > $TOKEN.text.Length) $ctx.max++;})+;

Predefined token attributes

Attribute Typ

e

Description

text string Text matched

type int Token type

line int Line number (counting from 1)

pos int Position in line (counting from 0)

index int Overall token offset inside input stream

channel int Channel where token was emited

int int Token integer value

Predefined rule attributes

Attribute Type Description

$ctx ParserRuleContext Context object

$ctx.GetText() string Text matched

$ctx.start IToken First token

$ctx.stop IToken Last token

Where else to place code

grammar MyGrammar;

@header {

using System;

}

@members {

private int myField;

public int MyProperty {get; set;}

public void MyMethod() {

//do something

}

}

rule : subrule TOKEN+ ;

grammar MyGrammar;

@lexer::header { ... }

@parser::header { ... }

@lexer::members { ... }

@parser::members { ... }

rule : subrule TOKEN+ ;

Bonus for C#

● MyGrammar.g4.lexer.csnamespace MyNamespace{

partial class SimpleLexer{}

}

● MyGrammar.g4.parser.csnamespace MyNamespace{

partial class SimpleParser{}

}

Semantic predicates

Lexer commands

TokenName: alternative -> command-name[(parameter)] ;

skip Skips token, doesn’t send it to parser

type(T) Set type T to token

channel(C) Send token in channel C

mode(M) Switch lexer to mode M

pushMode(M) Switch lexer to mode M, put current mode to the

stack

popMode Switch lexer to mode taken from the stack’s top

more Match token but continue searching

type(T)

Sets required type to the token

channel(C)

Sends token in desired channel

Predefined channels:

● Token.DEFAULT_CHANNEL

● Token.HIDDEN_CHANNEL

mode(M), pushMode(M), popMode

Switch lexer to defined mode.

Each mode contains its own set of rules

Default mode: DEFAULT_MODE

more

Matches token but continues searching

For those who like hardcoreIt’s possible to override any method in generated lexer and parser classes. Chose one that fits your

needs and act!