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Self-modifying Perl objects

Like the title suggests, each Perl script in this directory modifies itself by overwriting its own contents when it is executed. The mechanism for doing this is described in detail here. While this probably sounds like some useless academic exercise, I now use these scripts when writing new projects in Perl and in other languages.

Each script here is MIT-licensed as usual. The only potential caveat is that they link to the GNU Readline Perl module, which may mean that the scripts themselves need to be GPL-licensed. In either case, the contents of the scripts can be licensed independently of the scripts themselves.


Each script is fully self-contained and maintains a table of name -> value mappings. Each name exists inside a namespace; this is written as namespace::name. The namespace determines how the attribute is interpreted. So, for example, we can ask ./object to enumerate everything in the data:: namespace like this:

$ ./object ls ^data::

By convention, namespaces are defined by attributes prefixed with meta::type::. So, for instance:

$ ./object ls ^meta::type::

All attribute values are strings, but the namespace is informed when an attribute is defined. This allows scripts to reinterpret the string values in other ways. For example, the function:: namespace evals its values into Perl functions. These functions can access the attribute table using the %data hash, or preferably by using associate() and retrieve():

$ ./object cat function::cat
join "\n", retrieve(@_);
$ ./object cat internal_function::retrieve
my @results = map defined $data{$_} ? $data{$_} : retrieve_with_hooks($_), @_;
wantarray ? @results : $results[0];

Getting started

I find these scripts easy to use, but that's probably because I wrote them. I suspect anyone else will find them to be a frustrating nightmare (let me know if you find this to be the case and I'll see what I can do to fix the problem).

First, you'll need the GNU readline library for Perl. On Arch Linux this is a separate package; I think you can install this either using CPAN or the system's package manager. You don't absolutely need readline, but it enables tab-completion and makes for a much better shell experience. Once you've got that, make sure you've set your $EDITOR or $VISUAL environment variable appropriately:

$ echo $EDITOR
/usr/bin/vim        # /usr/bin/emacs will also work, but is an inappropriate value ;)

At this point you should be all set to create an object. The easiest way to do this is to tell object that you want a child of it. This causes your child to stay up-to-date with changes you make to object later on. Here's how to do that:

$ ./object child my-object

If you run my-object you'll enter its shell; at this point you can create and modify its attribute table. Most attributes are inherited from object, so it will warn you before you modify them. This is a good thing; the logic that overwrites the file in-place is implemented as a series of function:: and internal_function:: attributes. If you changed these you could end up with a script that obliterated itself. I know this because I've done it a few times.

The data:: namespace lets you bind data that won't be inherited by child scripts and that won't be evaluated by Perl. This is a good place to stash script-specific data.

$ ./my-object
> create data::foo          # opens an editor; this is the value of data::foo
> cat data::foo             # prints whatever you put into the editor
> cp data::foo data::bar    # sets data::bar to data::foo
> rm data::foo              # nukes data::foo
> ls data::
> exit                      # saves object state

You can find out what is unique to a given script by using ls -u:

$ ./my-object
> ls -u

Everything in data:: is considered unique because it won't be updated if it changes in the parent. It was inherited once when the original script (in this case object) copied itself, but it isn't technically inherited. You can pull updates from all parents (Perl scripts use multiple-inheritance) using the update function:

> update
my-object(info) updating from ./object
[a whole bunch of junk]

You can also add new parents:

> update-from some-other-perl-object
my-object(info) updating from some-other-perl-object

This creates a new entry in the parent:: namespace and will cause future update operations to inherit changes to the given script.

Data integrity and troubleshooting

These scripts are fairly resilient to breaking changes. For instance, if you write something that prevents a script from functioning correctly, it will refuse to overwrite itself and will instead save a temporary file so that you can figure out what went wrong. A Perl object overwrites itself only when the new copy can hash its contents correctly. Since implementing these checks, I have not lost any data doing normal things with these scripts.

That being said, your data is probably unsafe until you understand how Perl objects work. I highly recommend reading Writing Self-Modifying Perl for this purpose.

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