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Perl Module Monday: Object::Tiny (and friends)

September 26th, 2011 Posted in CPAN, Perl

Seriously, I’m going to have to create a new tag with “module” in the plural form, at this rate. This week’s post started out just looking at one module, but as it happens there are three variant forms of it that are just as interesting and useful as the original. So it would be highly unfair to leave any of them out.

Let’s start with the module at the core of it all, Object::Tiny. Object::Tiny is exactly as the name implies: tiny. With the POD it’s just under 9000 bytes, and not counting the POD the current version (1.08) is 553 bytes. I like the *::Tiny modules, I like seeing people getting the most functionality in a truly modular way with a minimum of code and no dependencies. Object::Tiny gives you a truly minimal way to create objects with simple (read-only) accessors. It also gives you a dead-simple constructor if you don’t already have one (well, most of the time— I’ll come back to this in a bit). And it does it all with an extremely small footprint and a minimum of intrusiveness.

Some might be wondering why you would need this— after all, the object you create from Object::Tiny is little more than a hash-reference with delusions of grandeur: the storage is just a basic hash reference (no extra keys or meta-data), the accessors are read-only by design (but see the bit on related modules further down) no additional methods besides an inheritable new() are provided, etc. But it’s a hash-reference that can call methods, for one thing. And your users don’t need to know that it is just a lowly hash-ref under the hood. Plus, there are plenty of applications for read-only data structures. Indeed, at its core, functional programming calls for immutable data. Writing methods to effect changes by returning new objects with the updated member values would be pretty trivial. You could, for example, define a clone() operation that allows updated values to be passed in as:

sub clone {
    my $self = shift;

    return __PACKAGE__->new(%{$self}, @_);

(For the less-experienced Perl users, this calls the new() of the package that the code is in, with the contents of the existing object flattened from a hash to an array. In addition to that, it also passes any arguments to clone() itself. Because of the way array/hash flattening/conversion works, anything in the arguments will override similar-named keys in the original hash, effectively “updating” those keys.)

Of course, this being CPAN, you don’t need to do that. You just need to consider one of the alternatives:

  • Object::Tiny::RW – This variant creates accessors that are read/write, by slightly altering the code that is generated. Accessors will then be able to accept an argument that, if present, becomes the new value for the key: $obj->foo(2) sets the “foo” key to 2. Of course, no type-checking is done.
  • Object::Tiny::Lvalue – This variant also creates read/write accessors, but rather than using the “$obj->foo(2)” syntax it creates the accessors as lvalue methods, allowing you to do the same thing with “$obj->foo = 2“.
  • Object::Tiny::XS – This variant breaks the “tiny” rule slightly by depending on Class::XSAccessor. It generates read-only accessors and the inheritable new() method using Class::XSAccessor. Interestingly, it does not generate read/write accessors like the other two variants do, even though Class::XSAccessor provides a simple alternative that does this. That might be worthy of a feature-request, if someone feels strongly about it.

Since the usage syntax of these is all identical (well, except for the lvalue variant), one could even prototype with one module and switch to another later on if so needed. Particularly with Object::Tiny vs. Object::Tiny::XS, since both adhere to the read-only model of the generated accessors.

One thing I have noticed in the logic of Object::Tiny that seems to be reflected in all three of the variants, is: the class is only added to your (calling) class’ inheritance hierarchy if you do not already have something in your @ISA variable. If you do, it will not add itself to your inheritance path. This means that you won’t get an inherited constructor in cases where you might be expecting to. For example, your class might utilize some form of an exporter and have that in the @ISA array. But it doesn’t provide you a constructor. If you were expecting Object::Tiny to provide the constructor, you may be surprised when it doesn’t. Mind you, the constructor it provides is dead-simple, but just be sure to know this behavior before it catches you off-guard. This behavior isn’t a bug, either, because it makes sense that if your class inherits from another class, it is probably either inheriting a constructor or providing its own constructor as an override. So I don’t consider this a problem with Object::Tiny, it’s just something to be aware of when using it.

Object::Tiny (and friends): for your minimalist class-construction needs!

3 Responses to “Perl Module Monday: Object::Tiny (and friends)”

  1. Steffen MuellerNo Gravatar Says:

    Just a bit of additional info: Object::Tiny::XS doesn’t support rw or w accessors because it was written to be an exact drop-in replacement for Object::Tiny as it worked back then. A quick look shows it still works that way, so I’d be inclined to reject a request to add RW variants, though I’d be open for debate.

    Either way, once you have O::T::XS, you can add rw accessors as follows:

    use Class::XSAccessor accessors => [qw(foo bar baz)];

    It also supports write-only accessors using the ‘setters’ parameter and lvalue accessors using ‘lvalue_accessors’:

    use Class::XSAccessor
    setters => {set_foo => ‘foo’, set_bar => ‘bar’}
    lvalue_accessors => [qw(baz)];

    Indeed, this requires understanding and using yet another API, but I believe the small featureset and simplicity of Object::Tiny are worth preserving.

    I hope this code won’t be mangled. Won’t get my hopes up, though. :)

  2. Adam KennedyNo Gravatar Says:

    Actually, that might be a bug after all.

    If you inherit from something that doesn’t have a new method, odds are it should be treated as a role and so we would…

    @ISA = ( @ISA, ‘Object::Tiny’ );

  3. AdamNo Gravatar Says:

    I just wanted to let you know there is a new member of the Object::Tiny family! Object::Tiny::RW::XS


    Disclosure: I’m the author.

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