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To: Paul G <paul@rusko.us>
CC: internals@lists.php.net
References: <20040729021337.73894.qmail@pb1.pair.com> <200407291117.09573.lpedretti@suserver.com> <200407291037.12396.ilia@prohost.org> <41098F91.3000606@hristov.com> <004801c475bd$812f3580$0200a8c0@rusko>
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Subject: Re: [PHP-DEV] GOTO operator
From: sthomas@townnews.com (Shaun Thomas)

Paul G wrote:

> function pseudocode()
> {
>     $a=allocate_resource_z();
>     $b=allocate_resource_y();
> 
>     $res=do_stuff();
>     if(!$res)
>         goto err_out;
> 
>     $c=allocate_resource_x();
>     $res=do_more_stuff();
>     if(!$res)
>         goto err_out;
> 
>     $d=allocate_resource_foo();
>     $res=do_even_more_stuff();
>     if(!$res)
>         goto err_out;
> 
>     return true;
> 
> err_out:
>     free_resouce($a);
>     free_resouce($b);
>     if(isset($c)) free_resouce($c);
>     if(isset($d)) free_resouce($c);
> 
>     return false;
> }

While this is valid, it's not really a good example of pseudocode that 
requires a goto to be efficient.  You could do the same thing with 
try/catch/throw:

function pseudocode()
{
   $a=allocate_resource_z();
   $b=allocate_resource_y();

   try {
     $res=do_stuff();
     if(!$res)
       throw SOME_ERROR;

     $c=allocate_resource_x();
     $res=do_more_stuff();
     if(!$res)
       throw SOME_ERROR;

     $d=allocate_resource_foo();
     $res=do_even_more_stuff();
     if(!$res)
       throw SOME_ERROR;

     return true;
   }

   catch SOME_ERROR {
     free_resouce($a);
     free_resouce($b);
     if(isset($c)) free_resouce($c);
     if(isset($d)) free_resouce($c);

     return false;
   }
}

This is also one of the reasons languages like Ada have named code 
blocks for exceptions.  Here's some ada-like pseudo-code:

BEGIN

   a := allocate_resource_z();
   b := allocate_resource_y();

   res := do_stuff();

   if (!res)
     raise some_exception;

   c := allocate_resource_x();
   res := =do_more_stuff();

   if (!res)
     raise some_exception;

   d := allocate_resource_foo();
   res := do_even_more_stuff();

   if(!res)
     raise unknown;

   return TRUE;

EXCEPTION

   free_resouce(a);
   free_resouce(b);
   if (c) free_resouce(c);
   if (d) free_resouce(c);

   return FALSE;

END

The case that Ilia and the other main developers is not for cases like 
this.  If you look through the Linux kernel, you can see very good 
examples of goto usage that is not related to this kind of error 
handling.  It's mainly to jump around inside parsing code while 
maintaining a stack, so the same chunk of code can continuously handle 
the same source of input without hundreds of if statements and indented 
blocks.

-- 
Shaun M. Thomas                INN Database Administrator
Phone: (309) 743-0812          Fax  : (309) 743-0830
Email: sthomas@townnews.com    Web  : www.townnews.com