Higher Order Perl

by Mark Jason Dominus

Cover image

Publisher: Morgan Kaufmann
Copyright: 2005
ISBN: 1-55860-701-3
Format: Trade paperback
Pages: 563

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Disclaimer: The author and I both used to be (technically still are, but the newsgroup is dead) moderators of comp.lang.perl.moderated, and I knew him electronically from answering questions on the Perl Usenet newsgroups many years ago.

Perl is, in general construction, a typical imperative language in the style of C, with object-oriented capabilities bolted (somewhat awkwardly) on. Most Perl code looks remarkably like C code with more powerful built-in data types and no memory management. A minority of Perl programs embrace object-oriented techniques and remind one of C++ or Java (or Python). But Perl also has powerful capabilities borrowed from functional programming languages and the Lisp tradition, even though many Perl programmers rarely use them and aren't familiar with them. Higher Order Perl focuses on those capabilities and how to use them effectively, starting with callbacks and uses of function pointers and moving into recursion and analysis of recursive functions, iterators, currying, and parsers. It concludes with a fully-worked example of constructing a declarative programming system using the techniques developed earlier in the book.

Higher Order Perl is a programming book for an intermediate to advanced Perl programmer, already a rare topic. This is not a look at how to apply Perl to another area of programming, or a cookbook of techniques. It's an attempt to help a Perl programmer think about and use the language differently, and it follows through on that. It's refreshing and rare to read a programming technique book that's targeted at the practicing expert. (I think the size of the audience is often too small for publishers to target it.) Even the most experienced Perl programmer is probably going to learn something fundamental from this book, not just interesting trivia around the edges.

On the negative side, though, I found the book a bit too focused on computer science and mathematics problems, particularly in the choice of examples and sample scripts. (And I say this as someone who has a master's degree in computer science with a software theory focus.) This is a bit hard to avoid for topics like recursion, where problems like computing Fibonacci numbers are classic, but throughout the book I struggled to focus past feelings of "but why would I ever do that"? The extended discussion of the Newton-Raphson method is the most memorable; I'm not sure that's a problem many Perl programmers would have and need a higher-order technique for. There's also a lot of discussion of recursion analysis and transformations between recursive and iterative expressions of problems, which is ground I remember well from my degree but which I've rarely had any practical use for in day-to-day programming.

This is not a uniform problem, though, just a tendency. There are some great examples that I think are more in the mainstream of Perl problems, including a reinvention of File::Find that shows how to add more flexibility, a web spider, and a great discussion on how to construct a conventional queryable database on top of a flat file (a topic that's near and dear to my heart).

And then there's the chapter on infinite streams. Dominus presents a method of using closures to create a version of an interator: a sort of infinite linked list that can keep generating additional elements. He presents this in several contexts, but one of them is log parsing, and that turned out to be exactly the solution that I needed for a problem I was working on while reading this book. I've written about this elsewhere, but this was a wonderful idea that helped me think about both Perl and a major application area in a completely new way, and I wrote an application using that knowledge that would have taken me much longer using different techniques and would have been much less fun. So, for me, this chapter was more than worth the entire book, and blankets the rest of it in a delighted feeling. Other people may or may not have that experience. I think it will depend on whether one of these techniques hits home for you the way that one did for me.

This is a book with some idiosyncracies, and some sections that may drag. Having lots of fully-worked examples is a major plus, but some of those are so comprehensive that one can get a bit lost in the details. That particularly hit me with the last couple of chapters on parsing and on the example declarative programming application. Quite a bit of that text involved reinventing a recursive descent parser (another very computer science example), when I'm not quite sure why one wouldn't use one of the existing parser generators on CPAN for practical purposes, and inventing a lot of new syntax to try to make parser Perl code more readable. But one certainly can't complain that Dominus omits necessary details, and there is some appeal in watching an experienced programmer work through a problem from analysis to implementation.

But, despite the idiosyncracies, I recommend this book to any experienced Perl programmer who wants to expand their view of the language. The techniques here — closures, higher-order functions, iterators and streams, and formal parsers — are powerful and underused in the Perl community. You may want to pick and choose which sections you pay close attention to, but I think everyone will find something of interest here. In the weeks since I read this book, my opinion of it has only grown. And I can't tell you just how much I loved the infinite stream concept.

Rating: 8 out of 10

Reviewed: 2013-03-13

Last modified and spun 2014-12-21