Wednesday, April 06, 2005

Perl 6

This is an overview of Perl 6. To avoid a posting of biblical proportions it lacks any technical detail - that will follow in further postings.

Perl 6 is the next generation of the highly successfully Perl programming language. This posting discusses why Perl 6 is necessary,has had to be developed, and gives an overview of changes.


Perl was originally written to replace the scripting language awk when Larry Wall, the author of Perl, became frustrated with awk’s limitations. Perl’s strength lay in text handling, and was an obvious choice for manipulating HTML. It became known as “the duct tape of the web”, although its design pre-dated the web boom. Perl 5, released in 1994, gave it the flexibility and power to rival conventional 4GLs, and boosted it beyond mere scripting and report generation. Successive releases of Perl added database support, references, OO, imbedded low-level code, and a huge range of add-on modules.

But all is not well, ten years is a long time in IT. Essentially designed to slot between C and Unix shell programs, Perl is showing its age. The syntax of Perl 5 is undeniably situated firmly in Unix-land, yet the product has an important niche in proprietary operating systems, such as Microsoft Windows.

Unix was originally a process-based system, and Perl was designed to work within that. Over recent years the advantages of multi-threading beyond "just" GUI systems have become clear, and a number of threading models are in common use. Perl has had several tries at multi-threading implementations but none have been completely satisfactory. Retrofitting multi-threading is difficult and painful.

In Perl 5 Object Orientation capabilities are tacked on as an after-thought. Frankly this is not a problem for Administrators and for small projects, but it is not scalable and lacks many features expected by today's (mostly OO) programmers. For example: a Perl 5 class has no data layout imposed or inherited (inheritance is really only through methods), no data hiding or protection is available (the whole object is visible), and there is no prototype checking for method calls. In Perl 5, multi-threading and OO do not mix; if an object is passed between threads it looses its class.

Perl 5 language can make it difficult to learn. That Perl has been able to get away with this is a tribute to its power and usefulness. Part of the problem has been a basic rule that, so far as possible, upgrades should be backward compatible. In other words, new features could not replace the old ones, only supplement them. "There's more than one way to do it" can be powerful but is confusing when trying to learn the right way to do it.

Perl has begun to loose its dominance on the web with the growth of ASP and the popularity of alternatives like JavaScript and PHP. On the application side Perl is threatened with competition from well-designed rapid development tools and the almost universal adoption of Object Orientation. On the scripting side we see the growth of modern feature-rich tools such as Bash, and the up-coming Microsoft Shell for Longhorn. Directly competing with Perl, and gaining popularity, are the OO scripting languages Ruby and Python. In the long term these could sideline Perl and destine it to the Great Recycle Bin in the sky.

The source code of the Perl interpreter has been highly hacked over the years to the extent that changes are now difficult to implement without breaking apparently unrelated code. This is a common problem with software that has had to adapt to meet changing needs and expectations.

If the language and culture is to survive a radical change is needed.

Enter Perl 6. Perl 6 is being designed and specified - that in itself is a novelty - by a confederation of programmers from the Perl community, shepherded by Larry Wall.

What's new?

Perl 6 is designed for change, and an expected life of over twenty years. There is no desire for backward compatibility. compatibility, Perl 5 scripts will rarely, if ever, run directly on Perl 6. Fortunately conversion routines and transition aids are being produced.

There is a price to pay for Perl 6 in the investment made in Perl 5 skills. Those who are comfortable with Perl 5 are naturally aggrieved to find they are beginners again. It should be made clear that Perl 6 is still undeniably Perl, after all it was written by Perl people who have probably invested more in Perl skills anyone else. There are a huge number of new features, but many of them are optional. As with any other product, users of Perl 6 will learn and use just those features needed to get the job done.

A technical overview of changes in Perl 6 is given as an Appendix to the QA Perl 5 Programming course.


In Perl 6, the language syntax and implementation internals have been separated, in a similar way to the Common Language Runtime in Microsoft's .Net product, and Sun's Java runtime. The language independent runtime component is called Parrot. Parrot is downloadable now, and you can write Parrot Assembler (PASM) if you really must. A higher level interface is Parrot Intermediate Representation, or PIR. PIR is expected to take the place of Perl 5's XS interface for low-level code.

Although its aim was to drive Perl 6 code, it was realised quite early on that other similar languages could be plugged into Parrot. Implementations of Ruby, Python, Java, PHP, and others, exist or are being developed. More can be added "merely" by generating the writing the PASM or PIR interface. Part of the attraction of this approach is that objects created for Parrot may be shared, regardless of the language. The Database Interface (DBI) module from Perl 5 is being rewritten for Parrot, so it should be usable from any Parrot language. Parrot also enables general functionality such as multi-threading and garbage collection to be abstracted by the language.

While there is no specific aim to compete, there is an interesting overlap between Parrot and Microsoft's .Net, more specifically between Parrot and Mono, which is the portable implementation of .Net, now owned by Novell. For example, both intend to run Python and PHP, and C# on Parrot is a possibility. Will Parrot and .Net compete head-to–head? Possibly, but it is doubtful that Parrot will run C# as fast as .Net, or that .Net will run Perl 6 in a satisfactory way.

Perl 6 OO

Perl 6 is Object Oriented from the ground up and for many Perl programmers that will be enough reason to use it. The syntax to define a class is similar to other languages, and variables are objects. For example, to find the length of a variable we now call a method on it, rather than a library routine. Classes in Perl 6 replace packages in Perl 5, and data layout may be imposed and inherited. Private, public and protected attributes and methods are supported, as well as delegation, roles and so on.

Multi-threading and OO should co-exist happily in Perl 6, but implementation details are still under wraps. The new garbage collector should remove memory leak problems that plague Perl 5 multi-threading.

In Perl 6, methods and subroutines have been considerably improved, allowing all the features normally expected, including named and typed arguments, and overloading.

The inclusion of Object Orientation is not to everyone’s taste, and for many users of the language it will be an irrelevance. Not every job needs OO and knowledge of OO will not be a pre-requisite to enable effective use of Perl 6.

Basic syntax and operator changes

There are a number of changes to the basic Perl syntax, which will be immediately obvious to a Perl 5 programmer, such as accessing an array element or hash value. Many of the syntax elements and operators are different, and some are less than obvious. This is not the place to list them, partly because they have not yet been finalised.

A number of new operators are available in Perl 6, particularly for handling lists. Some allow operations on all elements in a list and others give the ability to join and match lists in a number of different ways.

The reasoning behind the operator changes is to make Perl 6 easier. New Perl programmers will be able grasp its concepts more readily. Experienced Perl 5 programmers will have to learn new ways, but at least they already know the concepts and can concentrate on learning the new syntax.

Regular Expressions – Rules

A Regular Expression (RE) describes text of interest, usually for extraction or editing. It consists of a "meta-language", and the one chosen for Perl 5 is a well known dialect based on POSIX and used by many other languages and tools, including Microsoft .Net.

Perl 6 has modified the language with the intention of clarifying it. Very few of the familiar meta-characters are changed in Perl 6, but the most significant (the one everyone is shouting about) is the use of white space. In Perl 6 white space is used as in any other part of the program - it is not significant unless quoted.

Changes such as new quantifier delimiters are part of assertion syntax. Assertions are an extension of the concept of character classes that either match (true) or do not (false). Assertions return rules that can be stored meta-characters, a single or list of literals, array elements or hash keys, and even code. Many of these features can be achieved in Perl 5, but rules bring them together. The Perl 6 language itself will be expressed in rules.

Traits and Attributes

A feature of most scripting languages is that variables are type-less until a value is assigned to them. Later in its life the variable (and possibly its value) might change type. Perl 5 has always had that feature and, by default, so does Perl 6. For some operations typed variables are required and, although some checking is possible in Perl 5, Perl 6 extends this with optional built-in traits applied at compile-time. Aside from the additional help these supply to program logic, they also help the optimiser.

Attributes enable a value to be overridden depending on its context at run-time. This is a familiar theme in Perl 5, but Perl 6 has introduced many more contexts and ways to force them.


Good question. We were promised a development release in Quarter 3, 2005, but funding problems have caused slippage. At the time of writing a pessimistic guess is Quarter 1 2006 for the development release and Quarter 1 2007 for a production release. What state the development release will be in is anyone's guess, but for sure it's website will get a huge number of downloads from day 1. The community will make it work, and work well.

Is Perl 5 dead?

No, far from it. Partly because of the uncertain Perl 6 timetable, Perl 5 maintenance continues, and releases are planned for several years yet. Unlike commercial organisations, the Perl community has no interest in pressuring people to upgrade, Perl 4 maintenance continued for some time after 5 was released.

Ponie is rather contrived from Perl On New Internal Engine. It is the interface required to run Perl 5 upon Parrot. It should enable some of the advantages of Parrot, like operating system abstraction, to be fitted to Perl 5. Versions of Perl 5 running with Ponie on Parrot should be available at Perl 5.12. It is intended that eventually all Perl 5 development will move to Ponie on Parrot.

Perl 5 modules from CPAN should be able to run on Perl 6 via Ponie. This will reduce the problem of a lack of Perl 6 modules in the early days.


Perl 6 represents a change of emphasis for the Perl language. It should be easier to learn and use for today's generation of programmers. Existing Perl programmers will hate some changes but like others. Some will stay on Perl 5, but will be fighting a rear-guard action as the benefits of Perl 6 become more apparent.

QA ( intends to produce a Perl 6 for Perl 5 Programmers course as soon as a viable version of Perl 6 is available. Pure Perl 6 courses will then follow. It is intended to continue updating the Perl 5 Programming course to keep it in line with Perl 5 maintenance releases. And of course when Perl 7 is released…

From Larry Wall:
"Perl 6 is merely the prototype for Perl 7.:-)"


A technical overview of changes in Perl 6 is given as an Appendix to the QA Perl 5 Programming course.

The Perl 6 development web site is Larry Wall's explanations are given in Apocalypses at, with further information and examples in Exegeses, These are a little out of date, but the Synopses at were updated in December 2004.

A fortnightly digest of events in Perl 6 development is posted onto the O'Reilly Perl website,

No comments: