Friday, September 18, 2009

Lisp Outside the Box

Nick Levine has posted the first batch of sample chapters (Chapters 13–16) from his book-in-progress Lisp Outside the Box, which is being published by O'Reilly. My understanding of the book's purpose is that it's intended to be an introduction to Lisp crafted specifically to show that Lisp is applicable to a wide range of problems and that extensions and external libraries extend its application domain outside of what's provided “inside the box.”

With that understanding in hand and recognizing that we are looking at only a small sliver of the book, largely without context, it is worth while taking a look at the chapters and seeing how well they live up to their stated goals. The weakest chapters, in my opinion, are 13 and 14. These two chapters are devoted to a pretty thorough look at AllegroCache, the proprietary persistent object database that comes with Allegro Common Lisp (ACL) from Franz Inc. The question is: Are these chapters useful to someone learning Lisp? They do, it's true, introduce new Lispers to the notion of persistent objects, but in a way that's not apt to be useful to them. Not useful because someone who is new to Lisp and wants to experiment with it is unlikely to be using ACL. Yes, Franz provides a free (but crippled) version of ACL that includes (an even more crippled) version of AllegroCache, but why would a casual usual bother when there are free, industrial strength implementations such as SBCL and Clozure available? An academic license for ACL Professional starts at $599 and versions that can be used to produce commercial code cost much more. In other words, the most likely user of ACL is a professional Lisp developer who is using a version of ACL provided by his employer, not someone trying to learn about Lisp and its capabilities.

At the end of Chapter 14, the author devotes about a page to Rucksack and Elephant, two open source alternatives to AllegroCache. Rather than devote two chapters out of 32 to a proprietary solution that the intended reader is not apt to be using, it would have made more sense to devote those chapters to Rucksack and Elephant and mention AllegroCache as a commercial (and perhaps better) solution to the same problem.

Chapter 15 deals with concurrency. The author repeats the mistake of the previous two chapters by using ACL as his primary example, but here, at least, the sample code easily translates to other implementations. My main complaint about this chapter is that the author uses the term process to describe what everyone else calls threads. He does this because it's the traditional terminology in the Lisp community, and perhaps he has a point, but I think it's a disservice to the new Lisper because it uses process to mean something different from what it means everywhere else. Indeed, in most contemporary writing about Lisp you see the term thread used when discussing the concept that he calls process. Other than these two nits, the chapter is well written and should be helpful to someone trying to learn to use Lisp in the real world.

The last chapter, on garbage collection and memory management, is the strongest. The main lesson here is to trust the garbage collector. In almost every case the beginning Lisper is likely to encounter, it's better to let Lisp handle memory management rather than trying to hand optimize it by using, for example, the destructive versions of commands. The chapter provides a nice, elementary explanation of how garbage collection works and mentions some things to watch out for, such as creating memory leaks by inadvertently maintaining pointers to old data.

All in all this appears to be an interesting and worthwhile book that can help the new Lisper become familiar with the parts of Lisp that1 aren't mentioned in the Common Lisp Specification. The problem, of course, is that these parts are not standardized and thus are implemented differently in different Common Lisps. By considering several Common Lisps, the author shows the reader the various ways these important but nonstandard parts of Lisp are implemented. I do think, however, that it would improve the book to take the majority of the examples from Lisps that a new user is actually apt to be using.

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