Recuperación de Errores en Listas

Programa 11.6.1 Para comprobar el funcionamiento y la validez de la metodología esquematizada en el esquema 11.6.1, 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 11.6.1 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 11.6.1 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)