PDL::Philosophyp - Online in the Cloud

PROGRAM:

NAME

PDL::Philosophy -- Why did we write PDL?

DESCRIPTION

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, 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.

--- Karl Glazebook, the creator of PDL

Major ideas
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 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 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.

Enjoy!

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