Debug parser by printing useful information

I would like to parse a set of expressions, for instance:X[3], X[-3], XY[-2], X[4]Y[2], etc.

In my parser.mly, index (which is inside []) is defined as follows:

index:
| INTEGER { $1 }
| MINUS INTEGER { 0 - $2 }

The token INTEGER, MINUS etc. are defined in lexer as normal.

I try to parse an example, it fails. However, if I comment | MINUS INTEGER { 0 - $2 }, it works well. So the problem is certainly related to that. To debug, I want to get more information, in other words I want to know what is considered to be MINUS INTEGER. I tried to add print:

index:
| INTEGER { $1 }
| MINUS INTEGER { Printf.printf "%n" $2; 0 - $2 }

But nothing is printed while parsing.

Could anyone tell me how to print information or debug that?

I tried coming up with an example of what you describe and was able to get output of 8 with what I show below. [This example is completely stripped down so that it only works for [1] and [- 1 ], but I believe it's equivalent logically to what you said you did.]

However, I also notice that your example's debug string in your example does not have an explicit flush with %! at the end, so that the debugging output might not be flushed to the terminal until later than you expect.

Here's what I used:

Test.mll:

{
   open Ytest
   open Lexing
}
rule test =
parse
"-" { MINUS }
| "1" { ONE 1 }
| "[" { LB }
| "]" { RB }
| [ ' ' '\t' '\r' '\n'  ] { test lexbuf } 
| eof { EOFTOKEN } 

Ytest.mly:

%{
%}
%token <int> ONE 
%token MINUS LB RB EOFTOKEN
%start item
%type <int> index item
%%
index:
    ONE { 2 }
    | MINUS ONE { Printf.printf "%n" 8; $2 }
item : LB index RB EOFTOKEN { $2 } 

Parse.ml

open Test;;
open Ytest;;
open Lexing;;
let lexbuf = Lexing.from_channel stdin in
ignore (Ytest.item Test.test lexbuf)