define a grammar in Antlr

Question

I have defined the following grammar.

grammar Sample_1;


@header {
  package a;
}

@lexer::header {
  package a;
}

program
    :   
        define*
        implement*
    ;


define
    :   IDENT '=(' INTEGER',' INTEGER ')'
    ;

implement
    :IDENT '=(' (IDENT ','?)* ')'
    ;

fragment LETTER : ('a'..'z' | 'A'..'Z') ;
fragment DIGIT : '0'..'9';
INTEGER : DIGIT+ ;
IDENT : LETTER (LETTER | DIGIT)*;
WS : (' ' | '\t' | '\n' | '\r' | '\f')+ {$channel = HIDDEN;};
COMMENT : '//' .* ('\n'|'\r') {$channel = HIDDEN;};

How to check in this grammar so that when I have the example

A=(1,1)
B=(1,2)

G=(A,B)

the result is successful but if I write

A=(1,1)
B=(1,2)

G=(A,E)

it gives an error that E is not defined thanks

the result: i got it working thanks a lot:

grammar Sample_1;

@members{
    int level=0;
}

@header {
  package a;
}

@lexer::header {
  package a;
}

program
    :   
        block
    ;
block   
scope {
    List symbols;
}
@init {
    $block::symbols=new ArrayList();
    level++;
}
@after { 
     System.err.println("Hello");
     level--;
 }
    : (define* implement+)
    ;

define
    :   IDENT {$block::symbols.add($IDENT.text);} '=(' INTEGER',' INTEGER ')' 
    ;

implement
    :IDENT '=(' (a=IDENT 
    {if (!$block::symbols.contains($a.text)){
    System.err.println("undefined");
    }}','?)* ')'
    ;

fragment LETTER : ('a'..'z' | 'A'..'Z') ;
fragment DIGIT : '0'..'9';
INTEGER : DIGIT+ ;
IDENT : LETTER (LETTER | DIGIT)*;
WS : (' ' | '\t' | '\n' | '\r' | '\f')+ {$channel = HIDDEN;};
COMMENT : '//' .* ('\n'|'\r') {$channel = HIDDEN;};

Answer 1

Antlr supports actions , little snippets of code embedded in the grammar file.

An action for an assignment could store into a map. An action for a right-hand-side IDENT could try to pull a value from the map, and throw an exception if it fails.

Chapter 6 in Terrence Parr's "The Definitive ANTLR Reference" covers actions.