That's the approach I'd take. Each instance becomes a "result cache", with DESTROY performing cleanup where necessary.

This approach has a lot in common with functional programming: you're composing a result out of functions. The complication is that a side-effect of each function is a file-based cache that need to be managed.

I used the same technique for an image editor which had a similar need for caching.

Each operation that was applied to an image, produced a new image that was the results of the previous manipulation. The buffers for the images were allocated from a built-in virtual memory manager that used an LRU caching mechanism.

The really nice thing about the technique is it allows you to try two of three different enhancement techniques on the base image and compare them side by side before choosing which one to commit to. Also has the added bonus of making Undo a simple free current and back up to the previous version.

That whole thing was written in a macro assembler, but it was object oriented to the point where the syntax for adding a number from memory to a register was

 
esi.load     Object;
eax.add      Object.Number;
[download]

Thankfully, it was someone elses job to write the virtual memory manager:)

---

Examine what is said, not who speaks.
"Efficiency is intelligent laziness." -David Dunham
"When I'm working on a problem, I never think about beauty. I think only how to solve the problem. But when I have finished, if the solution is not beautiful, I know it is wrong." -Richard Buckminster Fuller

I can't help thinking that if you return an object representing the results of one set of manipulations performed on one set of data, and the user passes this back to you via a call to manipulate a completly different set of data, you have an unresolvable problem?

Some manipulations might require other stages to be performed first and that can be tested for.
If the user passes the wrong object that passes all tests, there isn't much I can do about it - I'd hope the user knows what order they want to do things in.

I can't think of any neat way of taking this away from the user, and honestly don't see any need to do so.
Simplifying the representation of the file(s) to process is probably all I need.

I guess my explaination left something to be desired, so here it is in slightly expanded pseudo-code.

The basic idea is that rather than each operation on an instance of Foo returning an instance of some State class, it returns a new instance of itself. To perform any further processing on that state of the data, he invokes the operation directly on returned object.

That way there are no seperate objects to be passed back to you wrongly.

package Foo;

sub new{
    my ($class, filespec) = @_;
    # check for existance, other init stuff
    return bless {file=>$filespec }, $class;
}

sub sort{ 
    $self = shift;
    system( 'sort', $self->$filespec, '>', "$self->filespec.sorted" );
    return $self->new( "$self->$filespec.sorted" );
}

sub munge{ 
    $self = shift;
    system( 'munge', $self->$filespec, '>', "$self->filespec.munged" )
+;
    return $self->new( "$self->$filespec.munged" );
}

1;
package main;
# Start with a object created  by the user.
my $data = Foo->new( 'datafile' );;

# Munge it two different ways and get two seperate objects back repres
+enting the two new states
my $sorted = $data->sort();
my $munged = $data->munged();

# Further processing on the new state is invoked directly on the new o
+bject
my $sorted'n'munged = $sorted->munge();
my $munged'n'sorted = $munged->sort();

# Or even pipeline them
my $sorted'n'munged'n'sorted_again = $data->sort()->munge()->sort();
[download]

Anyways, if I understood your question correctly, that's how I would do it.

HTH. Sorry, if it doesn't.

---

Examine what is said, not who speaks.
"Efficiency is intelligent laziness." -David Dunham
"When I'm working on a problem, I never think about beauty. I think only how to solve the problem. But when I have finished, if the solution is not beautiful, I know it is wrong." -Richard Buckminster Fuller

I've starting coding up my new and improved module, including feedback from BrowserUk.

The use of statements such as return $self->new( "$self->$filespec.munged" ); doesn't work, as the class fed to the new() method by all other methods is the object, not it's class so I used:

sub new {

    my $class_self = shift;  # Could be a class, or an already blessed
                             # object from another method.

    my $class;

    if (ref $class_self) {
        $class = ref $class_self
    } else {
        $class = $class_self;
    }


    my %arg   = @_;

    my $self = { file => $arg{'filespec'}  };

    bless $self, $class;
}
[download]

---

If the information in this post is inaccurate, or just plain wrong, don't just downvote - please post explaining what's wrong.
That way everyone learns.

Thinking out loud which may or may not have to do w/ OP...

I can't help but think of (C++-like) copy constructor and assignment op overload methods, given one instance of the class, to be used to return a spanking new instance of the same class. (That's what BrowserUk was trying to show, which is illustrated in the last "pipeline" statement.)

Had i been unfamiliar w/ C++, i would myself have been checking for the reference type in the function to create an class instance. Now, writing two separate functions (in Perl) to create class instance does not seem bothersome despite the Perl code that i have seen so far. (BTW, i have yet to write an OO Perl module.)

Getting back to the problem... If using jwest's idea, states could be stored in an array reference, in a hash, w/ file names (or file/IO objects, as appropriate) as the keys. Missing states could be omitted altogether or undef'd. (Needless to say this array ref. value could just as well be a hash ref.)

Personally, i would try BrowerUK's idea first (complete w/ appropriate class cloning/copying method separate from class constructor) as it is simpler in approach. If really necessary, use a hash like above to keep a list of processes applied.

If that works, then i would try the one-big-encapsulating-object idea. ("If the former works, why..." you say? Well, that's up to you, BazB.) Or, if skipping the simpler idea, i would search CPAN and Internet for prior art before embarking upon the latter approach for it seems crude (based on the desrciption alone).