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Problem with precedence declaration in .mly file
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 Date: -- (:) From: Angela Zhu 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.

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) }

;

...

```