PDL::Philosophy (1) - Linux Manuals
PDL::Philosophy: Why did we write PDL?
PDL::Philosophy -- Why did we write PDL?
DESCRIPTIONSome history from the creator of PDL, leading into the philosophy and motivation behind this data language. This is an attempt to summarize some of the common spirit between pdl developers in order to answer the question ``Why PDL''?
The Start of PDL``Why is it that we entertain the belief that for every purpose odd numbers are the most effectual?'' - Pliny the Elder
The PDL project began in February 1996, when I decided to experiment with writing my own `Data Language'. I am an astronomer. My day job involves a lot of analysis of digital data accumulated on many nights observing on telescopes around the world. Such data might for example be images containing millions of pixels and thousands of images of distant stars and galaxies. Or more abstrusely, many hundreds of digital spectra revealing the secrets of the composition and properties of these distant objects.
Obviously many astronomers have dealt with these problems before, and a large amount of software has been constructed to facilitate their analysis. However, like many of my colleagues, I was constantly frustrated by the lack of generality and flexibility of these programs and the difficulty of doing anything out of the ordinary quickly and easily. What I wanted had a name: ``Data Language'', i.e. a language which allowed the manipulation of large amounts of data with simple arithmetic expressions. In fact some commercial software worked like this, and I was impressed with the capabilities but not with the price tag. And I thought I could do better.
As a fairly computer literate astronomer (read ``nerd'' or ``geek'' according to your local argot) I was very familiar with ``Perl'', a computer language which now seems to fill the shelves of many bookstores around the world. I was impressed by its power and flexibility, and especially its ease of use. I had even explored the depths of its internals and written an interface to allow graphics, the ease with which I could then create charts and graphs, for my papers, was refreshing.
Version 5 of Perl had just been released, and I was fascinated by the new features available. Especially the support of arbitrary data structures (or ``objects'' in modern parlance) and the ability to ``overload'' operators - i.e. make mathematical symbols like "+-*/" do whatever you felt like. It seemed to me it ought to be possible to write an extension to Perl where I could play with my data in a general way: for example using the maths operators manipulate whole images at once.
Well one slow night at an observatory I just thought I would try a little experiment. In a bored moment I fired up a text editor and started to create a file called `PDL.xs' - a Perl extension module to manipulate data vectors. A few hours later I actually had something half decent working, where I could add two images in the Perl language, fast! This was something I could not let rest, and it probably cost me one or two scientific papers worth of productivity. A few weeks later the Perl Data Language version 1.0 was born. It was a pretty bare infant: very little was there apart from the basic arithmetic operators. But encouraged I made it available on the Internet to see what people thought.
Well people were fairly critical - among the most vocal were Tuomas Lukka and Christian Soeller. Unfortunately for them they were both Perl enthusiasts too and soon found themselves improving my code to implement all the features they thought PDL ought to have and I had heinously neglected. PDL is a prime example of that modern phenomenon of authoring large free software packages via the Internet. Large numbers of people, most of whom have never met, have made contributions ranging for core functionality to large modules to the smallest of bug patches. PDL version 2.0 is now here (though it should perhaps have been called version 10 to reflect the amount of growth in size and functionality) and the phenomenon continues. I firmly believe that PDL is a great tool for tackling general problems of data analysis. It is powerful, fast, easy to add too and freely available to anyone. I wish I had had it when I was a graduate student! I hope you too will find it of immense value, I hope it will save you from heaps of time and frustration in solving complex problems. Of course it can't do everything, but it provides the framework, the hammers and the nails for building solutions without having to reinvent wheels or levers.
Major ideasThe first tenet of our philosophy is the ``free software'' idea: software being free has several advantages (less bugs because more people see the code, you can have the source and port it to your own working environment with you, ... and of course, that you don't need to pay anything).
The second idea is a pet peeve of many: many languages like Matlab are pretty well suited for their specific tasks but for a different application, you need to change to an entirely different tool and regear yourself mentally. Not to speak about doing an application that does two things at once... Because we use Perl, we have the power and ease of Perl syntax, regular expressions, hash tables, etc. at our fingertips at all times. By extending an existing language, we start from a much healthier base than languages like Matlab which have grown into existence from a very small functionality at first and expanded little by little, making things look badly planned. We stand by the Perl sayings: ``simple things should be simple but complicated things should be possible'' and ``There is more than one way to do it'' (TIMTOWTDI).
The third idea is interoperability: we want to be able to use PDL to drive as many tools as possible, we can connect to OpenGL or Mesa for graphics or whatever. There isn't anything out there that's really satisfactory as a tool and can do everything we want easily. And be portable.
The fourth idea is related to "PDL::PP" and is Tuomas's personal favorite: code should only specify as little as possible redundant info. If you find yourself writing very similar-looking code much of the time, all that code could probably be generated by a simple Perl script. The PDL C preprocessor takes this to an extreme.
Minor goals and purposesWe want speed. Optimally, it should ultimately (e.g. with the Perl compiler) be possible to compile "PDL::PP" subs to C and obtain the top vectorized speeds on supercomputers. Also, we want to be able to calculate things at near top speed from inside Perl, by using dataflow to avoid memory allocation and deallocation (the overhead should ultimately be only a little over one indirect function call plus couple of ifs per function in the pipe).
Go on, try it!Well, that's the philosophy behind PDL - speed, conciseness, free, expandable, and integrated with the wide base of modules and libraries that Perl provides. Feel free to download it, install it, run through some of the tutorials and introductions and have a play with it.
AUTHORAdded Karl Glazebrook (2001) contributions by Matthew Kenworthy
Copyright(C) 1997 Tuomas J. Lukka (lukka [at] fas.harvard.edu).
Redistribution in the same form is allowed but reprinting requires a permission from the author.