Parser stop mid-parse - ocaml

Question

I am completely out of ideas. I spend every free minute this day on this, but I am completely out of ideas.
This is my Ocamlyacc grammar:
input: /* empty */ { }
| input stmt { }
stmt:
extern { print_endline "Got an extern import" }
| func { print_endline "Got function definition" }
| call { print_endline "Got function call" }
extern:
EXTERN proto { Extern $2 }
func:
DEF proto expr { Function ($2, $3) }
proto:
IDENTIFIER LPAREN id_list RPAREN { print_endline "Got prototype definition"; Prototype ($1, $3) }
id_list:
/* empty */ { [] }
| IDENTIFIER { [$1] }
| id_list COMMA IDENTIFIER { $3 :: $1 }
expr_list:
/* empty */ { [] }
| expr { [$1] }
| expr_list COMMA expr { $3 :: $1 }
expr:
call { $1 }
| expr OP expr { Binary ($2, $1, $3) }
| IDENTIFIER { Variable $1 }
| NUMBER { Number $1 }
| LPAREN expr RPAREN { $2 }
call:
IDENTIFIER LPAREN expr_list RPAREN { Call ($1, $3) }
When I start parsing def foo(a,b) a+b it should tell me it got a function and a prototype declaration, according to debug messages. But instead, I only get the message on parsing the proto rule.
Further debug messages show that the parser comes as far as to the a of the expression a+b and then stops. No error message, nothing else. It just stops as if the entire text hat been parsed completely without meeting any of the rules in stmt.
There are no shift/reduce error or similar. The AST types are also not the problem. I have no idea any more, maybe someone else can help. Surely it is something obvious but I cannot see it.
EDIT: Lexer by popular demand:
{
open Parser
}
rule token = parse
| [' ' '\t' '\n'] { token lexbuf }
| "def" { DEF }
| "extern" { EXTERN }
| "if" { IF }
| "then" { THEN }
| "else" { ELSE }
| ['+' '-' '*' '/'] as c { OP c }
| ['A'-'Z' 'a'-'z'] ['A'-'Z' 'a'-'z' '0'-'9' '_']* as id { IDENTIFIER id }
| ['0'-'9']*'.'['0'-'9']+ as num { NUMBER (float_of_string num) }
| '(' { LPAREN }
| ')' { RPAREN }
| ',' { COMMA }
| '#' { comment lexbuf }
| _ { raise Parsing.Parse_error }
| eof { raise End_of_file }
and comment = parse
| '\n' { token lexbuf }
| _ { comment lexbuf }

Answer 1

First point: I hated you a bit for not giving a compilable source code. I had to reinvent the AST types, the %token declarations etc. to test your code.
The problem is a delicate interplay between the
| eof { raise End_of_file }
lexing rule, and your grammar.
Raising Enf_of_file on EOF in the lexer is a good idea if your grammar never naturally encounters the end of the file. For example, grammars that are naturally \n-terminated or ;;-terminated will stop parsing at this point, and never get to the EOF token.
But your grammar isn't one of those. When the parser gets to DEF proto expr ., it asks for the next token to see if it weren't, by chance, and OP, so it calls the lexer, which finds EOF, and blows.
Here is my fix:
In lex.mll:
| eof { EOF }
In parse.mly:
%token EOF
%start stmt_eof
%type <Types.stmt> stmt_eof
[...]
stmt_eof: stmt EOF { $1 }
Finally, you should seriously consider Menhir as a replacement for ocamlyacc. It does everything ocamlyacc does, only better, with clearer grammar files (eg. you wouldn't have to reinvent the foo_list nonterminal each time), better error messages, debugging features...