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PDL::Philosophy -- Why did we write PDL?
Some 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,
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
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.
--- Karl Glazebook, the creator of PDL
The 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
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"
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
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 purposes
We 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.
Added Karl Glazebrook (2001) contributions by Matthew Kenworthy
Copyright(C) 1997 Tuomas J. Lukka (email@example.com).
Redistribution in the same form is allowed but reprinting requires a
permission from the author.
perl v5.20.2 2015-05-24 PHILOSOPHY(1)