Let's break down the issues in your current grammar first, then walk through a corrected version that supports your desired syntax (including nested IFs, assignments, arithmetic, comparisons, logical ops, and function calls).

Key Issues in Your Original Grammar #

• No Separation Between Statements and Expressions: You've mashed IF statements directly into the expr rule, but IF is a statement (it doesn't return a value in your use case), while assignments/arithmetic/comparisons are expressions. This confuses the parser's structure.
• Missing Assignment Rule: Your example uses b = 10 but there's no rule to handle assignment operations—your current = is treated as a comparison operator, which is not what you want.
• Unstructured Operator Precedence: All operators (arithmetic, comparison, logical) are in a single expr rule, which means the parser won't respect proper precedence (e.g., * should evaluate before +, && before ||). This leads to parsing errors or incorrect parse trees.
• No Statement Termination: Your example uses semicolons (;) to end statements, but your grammar doesn't account for this.
• Left Recursion Ambiguity: While Antlr4 supports left recursion, combining all expression types into one rule creates ambiguity that the parser can't resolve cleanly.

Corrected Grammar #

Here's a revised grammar that fixes these issues and supports your sample code (including nested IFs):

grammar ExprGrammar;

prog: stat_block EOF; // Top-level is a single block (matches your sample's outer {})

stat_block : OBRACE block CBRACE ;
block : stat* ;

// Statements: assignment, IF statement, or expression statement
stat: assign_stat ';'
    | if_stat
    ;

// Assignment rule: ID = expr
assign_stat: ID '=' expr ;

// IF statement with optional ELSE, supports nested blocks
if_stat: IF OPAR expr CPAR stat_block (ELSE stat_block)? ;

// Expression hierarchy with proper precedence (from lowest to highest)
expr: expr '||' expr               # LogicalOr
    | expr '&&' expr               # LogicalAnd
    | expr ('<'|'<='|'>'|'>='|'==') expr # Comparison
    | expr ('+'|'-') expr          # AddSub
    | expr ('*'|'/') expr          # MulDiv
    | func_call                    # FuncCallExpr
    | ID                           # Id
    | INT                          # Int
    | OPAR expr CPAR               # ParenExpr
    ;

// Function call rule: ID(expr, expr, ...)
func_call: ID OPAR exprList? CPAR ;
exprList : expr (',' expr)* ;

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

What's Changed & Why #

1. Statement-Expression Separation:
   • We split stat into explicit statement types: assign_stat (for assignments like b = 10) and if_stat (for IF/ELSE blocks). This makes the parser's structure clear and aligns with your sample code.
2. Proper Operator Precedence:
   • The expr rule is layered from lowest precedence (||) to highest ((), IDs, literals). This ensures the parser evaluates operations in the correct order (e.g., multiplication before addition, logical AND before OR).
3. Assignment Handling:
   • The assign_stat rule explicitly defines assignment, separating it from comparison (we use == for equality checks instead of reusing =—this avoids ambiguity, though if you do want = as equality, you can adjust, but assignment is usually a separate operator).
4. Statement Termination:
   • We added ; to terminate assignment statements, matching your sample code's syntax.
5. Cleaner Function Calls:
   • func_call is a separate rule under expr, so the parser correctly identifies function calls as atomic expressions (e.g., funcName(param) in your sample).

Testing Your Sample Code #

Your example:

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

This will parse correctly with the revised grammar. Nested IFs (e.g., { IF (x > 5) { IF (y < 3) { z = 0; } } }) will also work, since stat_block can contain any number of statements, including other if_stat entries.

Additional Notes #

• If you want to allow = as a comparison operator (instead of ==), you can add '=' to the Comparison alternative, but be aware that this might create ambiguity with assignment. It's better to use distinct operators for assignment and equality.
• The grammar uses labeled alternatives (like # LogicalOr) which makes it easier to generate visitor/listener classes if you want to implement evaluation later.

内容的提问来源于stack exchange，提问作者ahsan ayub