Re: Give me an example please!!!

[Hans Aberg]

At 11:51 -0500 2002/06/06, Javier Andres Mena Zapata wrote:
>Thank you, but I need to know how can I evaluate the expressions, that is,
>what are the ACTIONs. I've thinking about it, but i'm not sure...

You might write something like (including only the changes, actions in C++
pseudo-code):

%%
exp: prim '(' list_of_exp ')' { $$ = op_eval($1, $3); }
   | number                   { $$ = $1; }
...
list_of_exp:
    exp             { $$ = list($1); }
  | list_of_exp exp { $$ = $1.append($2); }
%%

Here, "op_eval" is function that takes the operator as the first argument,
the list of numbers as the second argument, evaluates it, and returns the
result. One can write like you suggested:
  %union {
   int number;
   int operator;  /* 0 to sum, 1 to substract, and * to multiply */
  }
  ...
  %%
  prim:
      '+' { $$ = 0; }
    | '-' { $$ = 1; }
    | '*' { $$ = 2; }
But it would mean that whenever you add a new operator, the function
op_eval must be made to recognize the new operator.

>> Your YYSTYPE must be able to store both a number and the operator type. So
>> you might use the %union feature (see Bison manual), one field of number
>> type X, and then perhaps your operators will have type (C++ pseudo-code)
>>     X (*)(const &list<X>)
>
>I didn't understand this part (especially I don't know so much about
>the STL from C++).

Therefore, one might turn "op_eval" into something like the "foldr" type of
functions in Haskell <http://haskell.org>. The foldr function takes an
operator, an initial value, and by that extends the operator to all lists.
You then write functions that operate on lists:
  int add(X (*)(const &list<X>));
  ...
and change to
  %union {
   int number;
   int (*)(const &list<int>);  /* operator on list */
  }
and your code then becomes
  exp: prim '(' list_of_exp ')' { $$ = (*$1)($3); }
     | number                   { $$ = $1; }

  prim:
      '+' { $$ = &add; }
    | '-' { $$ = &sub; }
    | '*' { $$ = &mult; }

  Hans Aberg