mincaml grammar in antlr4 - grammar

Question

I am trying to write mincaml parser in antlr4. github(https://github.com/esumii/min-caml/blob/master/parser.mly).
Japanese site : http://esumii.github.io/min-caml/ .
here is antlr 4 code.
grammar MinCaml;
simple_exp: #simpleExp
| LPAREN exp RPAREN #parenExp
| LPAREN RPAREN #emptyParen
| BOOL #boolExpr
| INT #intExpr
| FLOAT #floatExpr
| IDENT #identExpr
| simple_exp DOT LPAREN exp RPAREN #arrayGetExpr
;
exp : #programExp
| simple_exp #simpleExpInExp
| NOT exp #notExp
| MINUS exp #minusExp
| MINUS_DOT exp #minusFloatExp
| left = exp op = (AST_DOT | SLASH_DOT) right = exp #astSlashExp
| left = exp op = (PLUS | MINUS | MINUS_DOT | PLUS_DOT) right = exp #addSubExp
| left = exp op = (EQUAL | LESS_GREATER | LESS | GREATER | LESS_EQUAL | GREATER_EQUAL) right = exp #logicExp
| IF condition = exp THEN thenExp = exp ELSE elseExp = exp #ifExp
| LET IDENT EQUAL exp IN exp #letExp
| LET REC fundef IN exp #letRecExp
| exp actual_args #appExp
| exp COMMA exp elems #tupleExp
| LET LPAREN pat RPAREN EQUAL exp IN exp #tupleReadExp
| simple_exp DOT LPAREN exp RPAREN LESS_MINUS exp #putExp
| exp SEMICOLON exp #expSeqExp
| ARRAY_CREATE simple_exp simple_exp #arrayCreateExp
;
fundef:
| IDENT formal_args EQUAL exp
;
formal_args:
| IDENT formal_args
| IDENT
;
actual_args:
| actual_args simple_exp
| simple_exp
;
elems:
| COMMA exp elems
|
;
pat:
| pat COMMA IDENT
| IDENT COMMA IDENT
;
LET : 'let';
REC : 'rec';
IF : 'if';
THEN : 'then';
ELSE : 'else';
IN : 'in';
IDENT : '_' | [a-z][a-zA-Z0-9_]+;
ARRAY_CREATE : 'Array.create';
LPAREN : '(';
RPAREN : ')';
BOOL : 'true' 'false';
NOT : 'not';
INT : ['1'-'9'] (['0'-'9'])*;
FLOAT : (['0'-'9'])+ ('.' (['0'-'9'])*)? (['e', 'E'] (['+', '-'])? (['0'-'9'])+)?;
MINUS : '-';
PLUS : '+';
MINUS_DOT : '-.';
PLUS_DOT : '+.';
AST_DOT : '*.';
SLASH_DOT : '/.';
EQUAL : '=';
LESS_GREATER : '';
LESS_EQUAL : '=';
LESS : '';
DOT : '.';
LESS_MINUS : ' skip ; // toss out whitespace
COMMENT : '(*' .*? '*)' -> skip;
but I get following errors on rules exp and actual args.
error(148): MinCaml.g4:13:0: left recursive rule exp contains a left recursive alternative which can be followed by the empty string
error(148): MinCaml.g4:41:0: left recursive rule actual_args contains a left recursive alternative which can be followed by the empty string
But I don't see a any possibility of empty string on both rules. Or am I wrong?
What is wrong with this code?

Answer 1

The first line of the exp rule (actually of every rule) is the likely problem:
exp : #programExp
The standard rule form is
r: alt1 | alt2 | .... | altN ;
The alt1s in the grammar are all empty. An empty alt "matches an empty string".
Given the elems rule appears to have an intentional empty alt, consider that, in general terms, rules with empty alts can be problematic. Rather than using an empty alt, make the corresponding element in the parent rule optional (either ? or *).