ANTLR 4

Parser Structure

Error RecoveringBy Alexander Vasiltsov

class Grammar_NameParser : Parser {

class RuleContext : ParserRuleContext {...}

RuleContext rule();

class SubruleContext : ParserRuleContext {...}

class LabelContext : SubruleContext {...}

class Label_altContext : SubruleContext {...}

SubruleContext : subrule();

class LoopingContext : ParserRuleContext {...}

LoopingContext looping();

}

grammar Grammar_Name;

rule: subrule TOKEN;

subrule: looping+ # label

| TOKEN # label_alt

;

looping: looping

| TOKEN;

TOKEN: ‘TOKEN’;

Structure of generated parser

grammar Grammar_Name;

rule: subrule TOKEN;

subrule: looping+ # label

| TOKEN # label_alt

;

looping: looping

| TOKEN;

TOKEN: ‘TOKEN’;

class Grammar_NameParser : Parser {

class RuleContext : ParserRuleContext {...}

RuleContext rule();

class SubruleContext : ParserRuleContext {...}

class LabelContext : SubruleContext {...}

class Label_altContext : SubruleContext {...}

SubruleContext : subrule();

class LoopingContext : ParserRuleContext {...}

LoopingContext looping();

}

Structure of Context classes (1)

rule: subrule TOKEN;

class RuleContext : ParserRuleContext {

RuleContext(ParserRuleContext parent, int invokingState) {...}

SubruleContext subrule() {...}

ITerminalNode TOKEN() {...}

void EnterRule(IParseTreeListener listener) {...}

void ExitRule(IParseTreeListener listener) {...}

TResult Accept<TResult>(IParseTreeVisitor<TResult> visitor) {...}

}

Structure of Context classes (2)subrule: looping+ # label

| TOKEN # label_alt

;

class SubruleContext : ParserRuleContext {

SubruleContext(ParserRuleContext parent, int invokingState) {...}

}

class LabelContext : SubruleContext {

LabelContext(ParserRuleContext parent, int invokingState) {...}

IReadOnlyList<LoopingContext> looping() {...}

LoopingContext looping(int i) {...}

void EnterRule(IParseTreeListener listener) {...}

void ExitRule(IParseTreeListener listener) {...}

TResult Accept<TResult>(IParseTreeVisitor<TResult> visitor) {...}

}

class Label_altContext : SubruleContext {

Label_altContext(ParserRuleContext parent, int invokingState) {...}

ITerminalNode TOKEN() {...}

void EnterRule(IParseTreeListener listener) {...}

void ExitRule(IParseTreeListener listener) {...}

TResult Accept<TResult>(IParseTreeVisitor<TResult> visitor) {...}

}

Structure of Context classes (3)

looping: looping

| TOKEN;

class LoopingContext : ParserRuleContext {

LoopingContext(ParserRuleContext parent, int invokingState) {...}

LoopingContext loopig() {...}

ITerminalNode TOKEN() {...}

void EnterRule(IParseTreeListener listener) {...}

void ExitRule(IParseTreeListener listener) {...}

TResult Accept<TResult>(IParseTreeVisitor<TResult> visitor) {...}

}

ITree

ParserRuleContext

IToken

ANTLR namespaces

Antlr4.Runtime

org.antlr.v4.runtime

Contains the most commonly used classes and interfaces, such

as the hierarchies for input streams, character and token

buffers, error handling, token construction, lexing, and parsing

Antlr4.Runtime.Atn

org.antlr.v4.atn

This is used internally for ANTLR’s Adaptive LL(*) lexing and

parsing strategy1

Antlr4.Runtime.Dfa

org.antlr.v4.dfa

Holds all of the DFA2 implementation classes

Antlr4.Runtime.Misc

org.antlr.v4.misc

Holds miscellaneous data structures but also the commonly

used TestRig class

Antlr4.Runtime.Sharpen Holds different helpers for C# target language

Antlr4.Runtime.Tree

org.antlr.v4.tree

This package holds all of the classes and interfaces needed to

implement parser trees

org.antlr.v4.runtime.tree.gui A basic parse tree viewer

1. http://en.wikipedia.org/wiki/Augmented_transition_network

2. http://en.wikipedia.org/wiki/Deterministic_finite_automaton

Recognizer

IIntStream

ITokenFactory

IAntlrErrorListener class diagram

IAntlrErrorStrategy

RecognitionException

Let’s look at example

Error Recovery

Recovery process

● Each generated method is wrapped by try-

catch block

● reoprtError() - reports ErrorListener about

parsing error

● recover() - consumes tokens until it finds one

present in resynchronization set

Resynchronization set

Resynchronization set is the union of rule

reference following sets for all the rules on the

invocation stack

Invocation Stack: [group, expr, atom]

Resynchronization Set: { ‘^’, ‘]’ }

Recovering from mismatched tokens

class 9 T { int i; } class { int i; }

Recovering from errors in subrules

Subrule start: ● Parsers attempt single token

deletion

● Parsers don’t attempt single

token insertion

Looping subrule continuation

test:● Acts inside subrule that is looping

construct (...)+

● Parser tries to stay in the loop after

recovery

● Consumes tokens until it finds a token

consistent with one of following sets:

a) another iteration of the loop

b) what follows the loop

c) the resynchronization set of the

current