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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
...
%}

...

%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

%%
prog:
...
exp:
...
  | 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) }


;


...