Kyle Dawkins


ObjectivePerl - Objective-C style syntax and runtime for perl


  use ObjectivePerl;
  @implementation MyClass
     @private: $privateInstanceVariable;
     @protected: $normalInstanceVariable, $anotherInstanceVariable;

  + new {
        return ~[$super new];

  - setSomeInstanceVariable: $value {
    $someInstanceVariable = $value;

  - someInstanceVariable {
     return $someInstanceVariable;

then, from a calling script or class:

  use ObjectivePerl;
  my $instance = ~[MyClass new];
  ~[$instance setSomeInstanceVariable: "Hey you!"];
  print ~[$instance someInstanceVariable]."\n";


ObjectivePerl adds obj-c style syntax (although it's implemented with ~[] instead of just []) along with an obj-c style runtime that is very lightweight but makes the perl runtime a little more friendly to obj-c programmers.

Why, you ask? Just because. Obj-c has the easiest-to-read syntax of just about any language. It has Smalltalk-style named arguments that are built into the method signature, so when you invoke those methods, you're forced to invoke them neatly, in a very legible fashion:

   ~[$window setTitleTo:"New window" withColor:0xffffff

can never be misunderstood, whereas

   $window->setTitle("New window", 0xffffff, 0x00000);

could be. Perl offers named arguments already in the form of hashes, but these are unwieldy (to an obj-c programmer).

Defining Classes

The standard perl OO format, (declaring a package, then setting that package's @ISA array to its parent/s) is ugly and kludgey. Using the obj-c style, we end up with a much clearer declaration of the class:

   @implementation ClassName [: ParentClass] <[Protocols...]>

Real Instance Variables

Furthermore, you get real instance variables. In your class, you declare instance variables like this:

   @implementation Rectangle : Shape

and you can use those in your methods directly, like

   - increaseWidth {


   - area {
           return $width * $height;

and it will work as expected. This is much cleaner than the perlish style of

   sub area {
      return $self->{width} * $self->{height};

Instance Variable Visibility

Instance variables currently have two possible levels of visibility, private and protected. Protected is the default level of visibility, and means that your class and any of its subclasses have access to that instance variable. Private means that only an instance of that class --and none of its subclasses-- has access to that instance variable. When you declare your instance variables, you specify their visibility like this:

        @implementation MyClass
                @private: $thisVariableIsPrivate;
                @protected: $thisVariableIsProtected;

NOTE: instance variables can ONLY be scalars. For the most part this will not be a problem; you should always use references anyway. But if the need arises, support could be added for other basic types.

Defining Methods

You can now declare methods as being static or instance methods. In obj-c, a "+" indicates a static method, and a "-" indicates an instance method. To conform with normal perl style, instance methods automagically get the variable $self set correctly and static methods get $className. You define methods like this:

        - instanceMethod {

        + staticMethod {

and you define methods with arguments like this:

        - initWithString:$a andAnotherString:$s {

and invoke that like this:

        ~[SomeObject initWithString:$b andAnotherString: substr($y, 0, 1)];

All methods also have a variable set called '$super'. This is a special variable and it lets the parser know that a method is invoking a method in its superclass. This has a few special caveats; most importantly, you can't choose at runtime which method to invoke, so this is legal:

   my $self = ~[$super new];

but this isn't:

   my $self = ~[$super [MyClass chooseWhichConstructor]];

I can't see this being a serious limitation, but it arises from the fact that the SUPER pseudoclass is invisible outside of your methods, so you can't pass the invocation of a SUPER:: method to the ObjectivePerl runtime... it has to be invoked right from within your method. It may be possible with some judicious 'eval()'-ing but right now I can't be bothered.

Class Hierarchy

In general, ObjectivePerl doesn't mess with the class hierarchy. The only slightly underhand thing it does is to add ObjectivePerl::Object to the far end of your @ISA array for classes that you declare using the @implementation syntax. This allows us to give you default constructors and handlers and results in more objc-like behaviour. It also means that you don't need to provide a constructor at all for your class as the default one instantiates the object for you. You can, however, customise your constructor(s) all you want, as only the "new" method is supplied. ObjectivePerl::Object also provides you with a stub "init" method, that simply returns the object itself. In obj-c the most common idiom for object creation is

        id Object = [[SomeClass alloc] init];

and we can emulate this behaviour (optionally) in ObjectivePerl:

        my $object = ~[~[SomeClass new] init];

Ideally, you would create initialisers where appropriate:

        my $objectFromString = ~[~[SomeClass new] initWithString: $string];

Magic Variables

In your instance methods declared using the "-" syntax, the variable $self will automatically be set to represent the instance. $super will be set to the superclass, so you can invoke SUPER::'s methods just by saying

        ~[$super new];

Any instance variables in your class, and any protected instance variables in your parent classes, are also available to you magically; you can just say

        - setValueOfInstanceVariable: $value {
                $instanceVariable = $value;

without all the mess. Keeps things clean.


There are currently two ways to use ObjectivePerl. The easiest way is to just say

   use ObjectivePerl;

and everything after that in your source file will be parsed for ObjectivePerl declarations. It won't touch anything else.

Otherwise, you can instantiate the parser and do it yourself:

   use ObjectivePerl::Parser;

   my $parser = ObjectivePerl->new();

to see what's going on.


Well, this version is a bit of a kludge. It works as a perl source filter, which means it actually rewrites your perl code on the fly. This is not in itself a bad thing; generating perl code from some non-perl source is pretty common. However, it has some disadvantages, which you can read about below in the BUGS section.

The parser sifts through your code first looking for class declarations. If it finds one, it parses it and translates it into perl. If it notices that the class descends from an as-yet-unparsed class, it suspends parsing until that super-class is imported (and parsed), and then resumes parsing where it left off. This is to enable it to import the symbols (well, instance variable declarations chiefly) from the parent class before processing the current class; it has to do this in order to determine which instance variables can be used in methods in the current class. After that, it locates and rewrites method definitions from the ObjectivePerl syntax into regular perl subs, locates any instance variables that are used in methods, and writes out some perl to import those into the method. After that, perl returns control to the newly rewritten program and if all goes well, your code will be executing.

Message Dispatch

If you define methods using the -/+ syntax, ObjectivePerl will translate those method definitions into regular perl subs with names based on the method name and its arguments. You can then invoke those methods using obj-c style messages. However, there are often times when you need to call older perl code that does not list its arguments in its signature, or perhaps you wish to invoke a method across the PerlObjCBridge in OSX. To do this transparently, the ObjectivePerl runtime uses a method lookup cascade that works as follows:

    1. It tries to find the correctly-defined method
       for the invocation using ObjectivePerl syntax
    2. It tries to find a method whose name corresponds to the
       message name and its argument list, so for this invocation:

      ~[$myObject appendToResponse:$response inContext:$context];

       it will try to find a perl sub called "appendToResponseInContext".
    3. It will try to find a method with the same name
       as step 2 but with a single underscore appended for each
       argument (so in the case of the example, it will search
       for "appendToResponseInContext__").
    4. Failing all of these, it checks if the receiver of the
       message has a method called "handleUnknownSelector", and
       if it does, it invokes that method with the message name
       and selector array as arguments.


Dang, there have to be bugs... as of this release, the parser is not too smart about quoted close brackets, so this will cause problems:

   ~[$object printThis:"]"]

will read the quoted closing bracket as the close bracket of the ObjectivePerl message, rather than the actual one. When someone shows me how to use Damian Conway's Text::Balanced to parse this, the problem will go away.

Another thing that is a bug/feature is that the instance variable syntax described above will *only* work with classes that are represented by blessed hashes. I'll enhance it later to work with other types of class when the need arises (or someone sends me a patch).

There are many things that look bug-like to perl programmers. For example, the

        @implementation ClassName : ParentClass <ProtocalName>

syntax; it appropriates the '@' sign for a use other than designating an array. Same goes for the @private/@protected directives in the instance variable declaration section. Well, That's an obj-c thing and makes the resulting code look very familiar to obj-c programmers. We will eventually provide an option to use a different character instead of the '@' sign.

The parser does a lot of regexp matching, and they're pretty complex regexes, so no doubt there will be times when something doesn't match that should; it'll probably be because I forgot about whitespace or something like that. Fix it and send it to me.

If you specify a parent class in the @implementation line, there *must* be a space after your class name and before the colon, so this is ok:

        @implementation MyClass : SuperClass

but this isn't:

        @implementation MyClass: SuperClass

because I don't know enough about regexes...

Saving the best bug for last; it's really hard to get useful feedback on errors in your code, because the rewritten code has wildly different line numbers from your code. I will work on smoothing this difference out as much as possible... and if anyone has ideas on how to improve it, please let me know.


Still to be done, other than fixing the aforementioned bugs:

  * figure out if the concept of "public" applies here; right now
    instance variables just look like regular lexical variables.
    Since you can't really say $object.$instanceVariable like in
    C++/Java/Python, it's kind of moot.  And kinda dumb, too, 
    anyway... you should be using accessors. :)

  * Allow the user to pass in different start/end regexes for
    parsing ObjectivePerl syntax, and to change the look
    of @implementation into something more perl-ish.

  * Protocols are implemented right now in syntax only; it
    should work ok, but the parser does not force your
    class to implement that protocol, so it's an "informal"
    protocol, not a "formal" one (yet).

  * Since instance variables can only be scalars, the possibility
    of enhancing the system to use other types (arrays,
    hashes, filehandles, typeglobs...) is there.  I personally
    don't need it but I'll add it if anyone ever wants it.
    Of course, in order for that to happen, someone might actually
    have to use this...


    * Any documentation on obj-c or its runtime to learn about the
      terminology and syntax.
    * The documentation on Filter::Simple
    * The Apple website on obj-c:
    * The best book, from the source of obj-c:
          Cox, Brad J. and Andrew J. Novobilski "Object-Oriented
                 Programming: An Evolutionary Approach",
                 Addison-Wesley, 1991.


kyle dawkins, <>


Copyright 2004 by kyle dawkins

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.