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Problem with precedence declaration in .mly file
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Date: -- (:)
From: Angela Zhu <angela.zhu@c...>
Subject: Re: [Caml-list] Problem with precedence declaration in .mly file

On Oct 30, 2007, at 9:18 AM, Oliver Bandel wrote:

> Zitat von Angela Zhu <angela.zhu@cs.rice.edu>:
>
>> Hi all,
>>
>> I have some problem with precedence declaration in OCaml parser.
>> If I what to say exponential(ATOB) is prior to *(STAR) and /  
>> (DIVIDE),
>>   * and / are prior to +(PLUS)  and -(MINUS),
>> I wrote the following in the parser:
>>
>>
>> /***** Precedence Rules  *****/
>> ...
>> %left PLUS MINUS
>> %left STAR DIVIDE
>> %left ATOB
>> ...
>>
>> But I still have the following problems:
>> (1) It appears that the parser
>> reads "test = 2^2 + 7;" as "test = 2^9" instead of "test = 4+7",  
>> which
>> would follow the conventional order of operations.
>>
>> (2)It also interprets "test = (1^2)/3 + 1;" as "test = (1 ^ 2
>> / (3 + 1));"
>>
>> Can any one help me to see why it happens? Why the precedence rules
>> doesn't work?
> [...]
>
> Precedences also can be created by sophisticated
> organization of the grammar rules.
But I want to avoid this.
>
> So, if your grammar rules may have a contradictory
> meaning, then your parser works not as expected.
>
> In general I would use the precedence-declarations only,
> when you run into parser conflicts, if you don't use them.
> When developing a grammr, I would recommend, first to start
> with the grammar rules, and add precedence-/associatitivity-
> declarations, at the end, if really necessary.
>
>
> What is the rest of your mly-file?
>
> A complete example would be helpful.
Here is part of my .mly file:
Beside the precedence issue, everything works fine.

  %{

open Past
open Parsing
open ParseError

let pi = 4.0 *. atan 1.0;;

let get_range n = {
   pos_start = Parsing.rhs_start_pos n;
   pos_end = Parsing.rhs_end_pos n;
}

let unclosed opening_name opening_num closing_name closing_num =
   raise(Error(Unclosed(get_range opening_num, opening_name,
                        get_range closing_num, closing_name)))
%}

/* List of all tokens the lexer can output */

...

%token PLUS
%token STAR
%token MINUS
%token DIVIDE
%token AND	
%token OR
...

%token ATOB  /* A^B: exponential */
...


/***** Precedence Rules  *****/
%left PLUS MINUS
%left STAR DIVIDE
%left ATOB
%nonassoc prec_unary_minus


%start prog

%type <Past.pprog> prog

%%

/* Rules for parsing. The parsing rules should generally be in a */
/* one-to-one correspondence with the BNF */
/* type prog = Prog of consDeclare list * varDeclare list *  
inpDeclare list * sysDeclare list */




prog:

exp:
    LPAREN exp RPAREN          { $2 }
  | LPAREN exp error           { unclosed "(" 1 ")" 3 }

  | exp PLUS exp					{ Add($1, $3) }
  | MINUS exp  					{ Sub(Value(VFloat(0.0)), $2) }
  | exp MINUS exp             	{ Sub($1, $3) }
  | exp DIVIDE exp				{ Divide($1, $3) }
  | exp STAR exp              	{ Mult($1, $3) }
  | exp ATOB exp              	{ Atob($1, $3) }


  | value PLUS exp				{ Add(Value($1), $3) }
  | value MINUS exp             	{ Sub(Value($1), $3) }
  | value DIVIDE exp				{ Divide(Value($1), $3) }
  | value STAR exp              	{ Mult(Value($1), $3) }
  | value ATOB exp              	{ Atob(Value($1), $3) }


...

  | IDENT						{ Id($1) }
  | value						{ Value($1) }


;


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