Recuperación de Errores en Listas

Aunque no existe un método exacto para decidir como ubicar las reglas de recuperación de errores, en general, los sımbolos de error deben ubicarse intentado satisfacer las siguientes reglas:

• Tan cerca como sea posible del sımbolo de arranque.
• Tan cerca como sea posible de los sımbolos terminales.
• Sin introducir nuevos conflictos.

Esquema 36.6.1   En el caso particular de las listas, se recomienda seguir el siguiente esquema:

$$\begin{array}{lll} Construct & EBNF & yacc input\\ \hline ... ...\\ & & \verb\vert \vert x T error \vert\\ \hline \end{array}$$

Programa 36.6.1   Para comprobar el funcionamiento y la validez de la metodología esquematizada en el esquema 36.6.1, consideremos los contenidos del fichero error.y. En el se muestra el tercer caso x:y{Ty} con x = list, T = , e y = NUMBER:

%{
#include <stdio.h>
void put(double x);
void err(int code); 
%}

%union { 
  double val; 
}
%token <val>NUMBER
%%
command 
  : 
  | command list '\n' { yyerrok; } 
  ;

list 
    : NUMBER            { put($1); }
    | list ',' NUMBER   { put($3); yyerrok; }
    | error             { err(1); }
    | list error        { err(2); }
    | list error NUMBER { err(3); put($3); yyerrok; }
    | list ',' error    { err(4); }
    ;
    
%%    
void put(double x) {   
  printf("%2.1lf\n",x);
}

void err(int code) {
  printf("err %d\n",code);
}

main() {
  yydebug = 1;
  yyparse();
}

yyerror(char *s) {
  printf("%s\n",s);
}

Listado 36.6.1   La compilación con yacc da lugar a una tabla ligeramente diferente de la producida por bison. El fichero y.output contiene la tabla:

   0  $accept : command $end

   1  command :
   2          | command list '\n'

   3  list : NUMBER
   4       | list ',' NUMBER
   5       | error
   6       | list error
   7       | list error NUMBER
   8       | list ',' error

state 0
	$accept : . command $end  (0)
	command : .  (1)

	.  reduce 1

	command  goto 1


state 1
	$accept : command . $end  (0)
	command : command . list '\n'  (2)

	$end  accept
	error  shift 2
	NUMBER  shift 3
	.  error

	list  goto 4


state 2
	list : error .  (5)

	.  reduce 5


state 3
	list : NUMBER .  (3)

	.  reduce 3


state 4
	command : command list . '\n'  (2)
	list : list . ',' NUMBER  (4)
	list : list . error  (6)
	list : list . error NUMBER  (7)
	list : list . ',' error  (8)

	error  shift 5
	'\n'  shift 6
	','  shift 7
	.  error


state 5
	list : list error .  (6)
	list : list error . NUMBER  (7)

	NUMBER  shift 8
	error  reduce 6
	'\n'  reduce 6
	','  reduce 6


state 6
	command : command list '\n' .  (2)

	.  reduce 2


state 7
	list : list ',' . NUMBER  (4)
	list : list ',' . error  (8)

	error  shift 9
	NUMBER  shift 10
	.  error


state 8
	list : list error NUMBER .  (7)

	.  reduce 7


state 9
	list : list ',' error .  (8)

	.  reduce 8


state 10
	list : list ',' NUMBER .  (4)

	.  reduce 4


5 terminals, 3 nonterminals
9 grammar rules, 11 states

Ejecución 36.6.1   La ejecución del programa generado por yacc es como sigue:

> error
yydebug: state 0, reducing by rule 1 (command :)
yydebug: after reduction, shifting from state 0 to state 1
10 20
yydebug: state 1, reading 257 (NUMBER)
yydebug: state 1, shifting to state 3
yydebug: state 3, reducing by rule 3 (list : NUMBER)
10.0
yydebug: after reduction, shifting from state 1 to state 4
yydebug: state 4, reading 257 (NUMBER)
syntax error
yydebug: state 4, error recovery shifting to state 5
yydebug: state 5, shifting to state 8
yydebug: state 8, reducing by rule 7 (list : list error NUMBER)
err 3
20.0
yydebug: after reduction, shifting from state 1 to state 4
yydebug: state 4, reading 10 ('\n')
yydebug: state 4, shifting to state 6
yydebug: state 6, reducing by rule 2 (command : command list '\n')
yydebug: after reduction, shifting from state 0 to state 1
10;20 30
yydebug: state 1, reading 257 (NUMBER)
yydebug: state 1, shifting to state 3
yydebug: state 3, reducing by rule 3 (list : NUMBER)
10.0
yydebug: after reduction, shifting from state 1 to state 4
yydebug: state 4, reading 59 (illegal-symbol)
syntax error
yydebug: state 4, error recovery shifting to state 5
yydebug: state 5, error recovery discards token 59 (illegal-symbol)
yydebug: state 5, reading 257 (NUMBER)
yydebug: state 5, shifting to state 8
yydebug: state 8, reducing by rule 7 (list : list error NUMBER)
err 3
20.0
yydebug: after reduction, shifting from state 1 to state 4
yydebug: state 4, reading 257 (NUMBER)
syntax error
yydebug: state 4, error recovery shifting to state 5
yydebug: state 5, shifting to state 8
yydebug: state 8, reducing by rule 7 (list : list error NUMBER)
err 3
30.0
yydebug: after reduction, shifting from state 1 to state 4
yydebug: state 4, reading 10 ('\n')
yydebug: state 4, shifting to state 6
yydebug: state 6, reducing by rule 2 (command : command list '\n')
yydebug: after reduction, shifting from state 0 to state 1
3,
yydebug: state 1, reading 257 (NUMBER)
yydebug: state 1, shifting to state 3
yydebug: state 3, reducing by rule 3 (list : NUMBER)
3.0
yydebug: after reduction, shifting from state 1 to state 4
yydebug: state 4, reading 44 (',')
yydebug: state 4, shifting to state 7
yydebug: state 7, reading 10 ('\n')
syntax error
yydebug: state 7, error recovery shifting to state 9
yydebug: state 9, reducing by rule 8 (list : list ',' error)
err 4
yydebug: after reduction, shifting from state 1 to state 4
yydebug: state 4, shifting to state 6
yydebug: state 6, reducing by rule 2 (command : command list '\n')
yydebug: after reduction, shifting from state 0 to state 1
#
yydebug: state 1, reading 35 (illegal-symbol)
syntax error
yydebug: state 1, error recovery shifting to state 2
yydebug: state 2, reducing by rule 5 (list : error)
err 1
yydebug: after reduction, shifting from state 1 to state 4
yydebug: state 4, error recovery discards token 35 (illegal-symbol)
yydebug: state 4, reading 10 ('\n')
yydebug: state 4, shifting to state 6
yydebug: state 6, reducing by rule 2 (command : command list '\n')
yydebug: after reduction, shifting from state 0 to state 1
yydebug: state 1, reading 0 (end-of-file)