Basic expression Antlr4 grammar

Question

I am trying to write a basic grammar that starts with '{' and ends with '}' . It has 'IF' and 'Else' only. And some basic expressions like a = (arithmetic operation). There is no need for variable initialisation and declaration check. Can someone guide me, I am writing grammar, but it is not working properly. I am writing code below of my grammar.

prog: stat_block+ EOF;

stat_block
: OBRACE block CBRACE
;

block
: stat*
;

stat: expr
;

expr: expr ('*'|'/') expr
| expr ('+'|'-') expr
| expr ('<'|'<='|'>='|'>'|'=') expr
| expr ( '&&'|'||') expr
| expr '(' exprList? ')'
| IF condition_block  (ELSE stat_block)?
;

exprList : expr (',' expr)* ;

condition_block
: OPAR expr CPAR stat_block
;

IF : 'IF';
ELSE : 'ELSE';
OPAR : '(';
CPAR : ')';
OBRACE : '{';
CBRACE : '}';
ID : [a-zA-Z]+ ;
INT : [0-9]+ ;
NEWLINE:'\r'? '\n' ;
WS : [ \t]+ -> skip ;

Sample Code for what i am trying to write grammar for

{
 IF ( a > 10 && funcName(param) = Found ) {
    b = 10;
 }
 ELSE {
    b=20;
 }
}

If statement can be nested. IF some one can provide me with grammar or tell me, what i am doing wrong. that will be huge help. thanks.

Answer 1

With the following changes, I get an error free parse and a parse tree:

1.

NEWLINE:'\r'? '\n' ;

You're not using the NEWLINE token, so, presumably, you want to hide that token from the parser rules. -> skip is one simple way to accomplish that.

NEWLINE:'\r'? '\n' -> skip;

expr
    : expr ('*' | '/') expr
    | expr ('+' | '-') expr
    | expr ('<' | '<=' | '>=' | '>' | '=') expr
    | expr ( '&&' | '||') expr
    | expr '(' exprList? ')'
    | IF condition_block (ELSE stat_block)?
    ;

ID s and NUMBER s should also be considered expressions

expr
    : expr ('*' | '/') expr
    | expr ('+' | '-') expr
    | expr ('<' | '<=' | '>=' | '>' | '=') expr
    | expr ( '&&' | '||') expr
    | expr '(' exprList? ')'
    | IF condition_block (ELSE stat_block)? 
    | ID
    | INT
    ;

block
: stat*
;

stat: expr
;

You haven't accounted for the possibility/requirement of the ; after an expression

It's not clear you intention. so

To simply say that each expression may be followed by a ;

stat: expr ';'?;

This would allow for some possible confusion when expressions appear adjacent to one another without a ; , so consider making it non-optional.

stat: expr ';';

Many language treat ; as a expression/statement separator.

block: (stat (stat ';')*)?;

---

The sum of the changes:

grammar Basic
    ;

prog: stat_block+ EOF;

stat_block: OBRACE block CBRACE;

block: (stat (stat ';')*)?;

stat: expr ';'?;

expr
    : expr ('*' | '/') expr
    | expr ('+' | '-') expr
    | expr ('<' | '<=' | '>=' | '>' | '=') expr
    | expr ( '&&' | '||') expr
    | expr '(' exprList? ')'
    | IF condition_block (ELSE stat_block)?
    | ID
    | INT
    ;

exprList: expr (',' expr)*;

condition_block: OPAR expr CPAR stat_block;

IF:      'IF';
ELSE:    'ELSE';
OPAR:    '(';
CPAR:    ')';
OBRACE:  '{';
CBRACE:  '}';
ID:      [a-zA-Z]+;
INT:     [0-9]+;
NEWLINE: '\r'? '\n' -> skip;
WS:      [ \t]+     -> skip;