[% setvar title All Perl core functions should return objects %]

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TITLE

All Perl core functions should return objects

VERSION

   Maintainer: Nathan Wiger <nate@wiger.org>
   Date: 8 Aug 2000
   Last-Modified: 15 Sep 2000
   Mailing List: perl6-language-objects@perl.org
   Number: 73
   Version: 2
   Status: Frozen
   Requires: RFC 159

ABSTRACT

In the past, Perl has only provided two return options for its builtin functions: scalars or lists. In a scalar context, only one select value was returned, greatly impacting the functionality of the function (unless you like pulling apart long lists, or pain, or both).

The reason this was done was to allow easy access to string/numeric data, and because polymorphic objects were not a reality. However, objects can use the mechanisms described in RFC 159: True Polymorphic Objects to become strings, numbers, and other data types on-demand in Perl 6. As such, we should make all Perl functions return objects in a scalar context in Perl 6.

NOTES ON FREEZE

This is mainly a philosophical document designed to show how the methods described in RFC 159: True Polymorphic Objects can be used to give us a greater amount of core power in Perl 6. One actual proposed implementation of these ideas can be found in RFC 48, which suggests a new date function which produces a polymorphic object in scalar context.

The only fear was performance-related; however, as Dan Sugalski himself pointed out, the -internals group will probably wind up putting vtable stuff in core. In this case, performance should not be a concern because OO will be tightly integrated from ground up.

DESCRIPTION

When several of the mechanisms proposed in other RFC's combine, especially RFC 159, we have the power in Perl 6 to pass many more things around as objects, converting them to strings/numbers/etc as they're needed. This gives us much power, since we present both objects and "true" scalars to beginners and advanced Perl users alike with one common set of functions. As such, objects are embedded in Perl from the ground up.

Others have proposed typing objects, and extracting them that way:

   my $uid   = getpwnam $user;    # "true" scalar uid
   my pw $pw = getpwnam $user;    # object of type "pw"

However, this has a couple problems:

   1. You have to have the correct object class and, at
      the very least, alter your script's syntax.

   2. You can't make $pw look like $uid transparently.

Instead, having objects that walk and talk like scalars on demand is a more powerful approach. Note that this RFC does not necessarily preclude being able to type objects and pull out specific classes. The two approaches could be combined, giving multi-class objects that can be transparently accessed as "true" scalars.

We'll start out with complex examples, where it's obvious how this is a benefit, and go down to simpler functions, where it might be less obvious.

Complex Functions

Let's choose stat, since it's an easy target. Currently, it only returns one of two things:

   1. A massive 13-element list (LIST context)
   2. A boolean success/failure (SCALAR context)

Neither of these is particularly useful, unless you like pain. To get a decent interface, you have to use File::stat or some other module. Instead, let's put an object-oriented interface in the core:

   $stat = stat $file;        # returns an object
   @stat = stat $file;        # legacy list (like current)
   %stat = stat $file;        # hash (see RFC 21)

   print "$stat";             # calls $stat->STRING, which 
                              # could print the filename or
                              # some other useful piece of info

Note that our legacy calling methods are unaffected, but we now have an object too. The boolean truth value is simplistic to determine still, you simply have to say:

   $stat = stat $file or die "Can't stat $file: $!";

The object methods of the $stat object are powerful enough that special cases like _ no longer have to exist. You can stat multiple files out of order and still get the benefits of cached stat calls by using the object interface:

   $f1 = stat $file1 or die;
   $f2 = stat $file2 or die;
   
   if ( $f1->size > 0 and $f2->owner == 0 ) {
      print "root-owned $file1 is mode ", $f1->mode & 07777;
      if ( $f1->mtime > time ) {
          # here, "$f1" is $f1->STRING == $f1->filename
          warn "Warning: bad mojo, $f1 has an mtime in the future!\n";
      }
   }

Now, we have a full object-oriented stat implementation, wrapped in a tidy function that can work just like Perl 5's if you want it to.

As a second example, let's examine getpwnam, whose return options are:

   1. A pretty dang big 9-element list
   2. The numeric user id

getpwnam differs from stat in that you have to actually use what you get in a SCALAR context in many situations. However, this can be easily addressed thanks to RFC 159:

   $pw = getpwnam $username;         # get the object
   die "Not root" unless ($pw == 0); # ->NUMBER, which produces the
                                     # numeric uid, like currently

Here, the current return value (numeric user id) is preserved. No need to redo any of the documentation - but advanced users can be told "By the way, $pw is actually an object..." and so on.

Medium-Level Functions

As a medium-level example, let's take fork. At first, making what's returned from fork into an object might not seem to have much use. However, consider how cool it would be if fork maintained stuff like:

    $fork = fork;    # create a new process

    $fork->pid       # current pid
    $fork->ppid      # parent's process id
    $fork->errno     # EAGAIN, for example
    $fork->is_parent # parent?
    $fork->is_child  # child?

Then, you could fork multiple times, without having to worry about maintaining info on which fork is which - it's all right there in the $fork objects. Furthermore, the default function would be $fork->pid, yielding the ability to treat fork the same as it has always acted. Adapted from Camel-3 p. 715:

   if ( $pid = fork ) {
      # parent here
      print "fork returned with errno ", $fork->errno;
   }

This example is fairly trite, true. However, I'm sure others who do a lot of forking can fill the object in with valuable data they wish they could preserve easily.

As a second example, consider system. Who cares, right? Return success or failure. But consider:

   $sys = system "some command";   # object
  
   $sys->command     # for record-keeping 
   $sys->errno       # system errno || 0
   $sys->stdout      # standard output from command
   $sys->stderr      # standard error from command
  

There's probably more stuff that could go in there too. Why do this? For example:

   $sys = system "command";
   if ( $sys->errno ) {
      warn $sys->command, " failed with error ",
           $sys->errno;
      die "Error output: ", $sys->stderr;
   }

For one simple operation, this seems like overkill. And it is. However, remember that the success value is still in there as the creation of the object:

   system("command") or die "command failed: $?\n";

That certainly looks familiar. :-)

Again, you get the entire power of objects hidden behind a clean, familiar looks-like-Perl-5 frontend.

Simple Functions

Okay, who cares about getting an object back from chomp? Is that really necessary?

>From a functional standpoint, most string-related functions would be overkill. For example, chomp, grep, etc. The main advantage is not with string manipulation functions, but rather with functions that do complicated stuff and return lots of values, like date, stat, etc.

Just for kicks, though, consider chomp:

   $newstr = chomp $string;        # new syntax, RFC 58

   print "new string is $newstr";  # calls $chomp->STRING
   print $newstr->numchars, " were chomped\n";

You get the idea.

MIGRATION

If done right, none. We simply have to use the methods described in RFC 159 to properly overload the objects so they appear to work "just like the same old scalars" they've always been. Again, the key goal is hiding this stuff just under the hood, so all the power is there but it doesn't get in the way of one-liners.

IMPLEMENTATION

Assuming this RFC is accepted, we should take Dan Sugalski's suggestion and create a list of those functions that this would be a real win for, and start with these. The ones that pop out in my mind are functions like stat that return huge lists of complex values. I think this would be a Big Win for these types of functions.

I'll gladly write up this list if the RFC is adopted, but in the meantime in order to meat the deadline I must hurriedly update my other RFC's! :-}

REFERENCES

Many many discussions by lots of people about something like this. Most I've read are available on the perl5-porters archive at: www.xray.mpe.mpg.de

RFC 159: True Polymorphic Objects

RFC 48: Replace localtime() and gmtime() with date() and utcdate()

RFC 37: Positional Return Lists Considered Harmful

RFC 21: Replace wantarray with a generic want function

RFC 58: chomp() changes.

RFC 49: Objects should have builtin stringifying STRING method