chapters – Page 6 – Learning Perl

Updates to Chapter 15, “Smart matching and given-when”

[This post notes differences between the fifth and sixth editions.]

The chapter on smart matching and given-when is the newest chapter, but also the one a bit out of date. Since we were writing the fifth edition right on the cusp of the release of Perl 5.10.0, we weren’t aware of all the problems that the new smart match feature had.

I did start to find problems, and I posted to perl6.language, perl5.porters and perlbug around the time we were turning in the book which hit the shelves in June 2008. It takes about four months to go from the end of writing to delivering the dead trees to the reader.

No one had an answer, and the next version of Perl changed quite a bit of the smart matching mechanism. For instance, see my p5p messages for [PATCH] perlsyn: equivalent code for HASH ~~ ARRAY and [perl #50538] when( @n && %n ) fails to smart match.

Updates to Chapter 5, “Input/Output”

[This post notes differences between the fifth and sixth editions.]

With the added emphasis on Unicode, we had to update the chapter on input and output a bit. If we are going to talk about Unicode, we need to talk about encodings, which expands the material on three argument open and brings in binmode too.

For this update, we also introduced filehandle references, although we did not call them references, really. We still present most of the chapter using bareword filehandles, and once we have covered everything we show how you can do the same things with filehandles in variables. However, we still save the meat of filehandle referencs to Intermediate Perl.

Some people think everyone should be using lexical filehandles all of the time, but even if you want to do that for new code, you still have to understand what people did in old code so we have to cover the legacy syntax (and some people would even object to calling it “legacy”).

Updates to Chapter 9, “Processing Text with Regular Expressions”

[This post notes differences between the fifth and sixth editions.]

I didn’t have to make many changes to this chapter. I wanted to put in at least one Perl 5.14 feature, but the only new thing that the substitution operator gets is the /r modifier.

While working through this chapter though, I started to wonder if our terms in the previous editions were the same in the Perl documentation. We called the modifiers “option modifiers”, and sometimes “flags”, perlre just says “modifier”. Personally, I’m used to saying “flag” all the time and I like that term just fine, but for regular people, there’s nothing to connect the everyday use of “flag” to the thing after the match operators. So, “modifier” it is. I’d much rather use “adverb”, which is popular in Perl 6 land, but it’s a bit late for Perl 5 to change terms. When I made the switch in this chapter, I had to go back to Chapters 7 and 8 and do the same thing.

This chapter is also curious in that it ends with a long example the builds up to a perl one-liner. One of the things the reviewers noted about a new edition was a new chapter devoted to one-liners. That’s still possible, I guess.

Updates to Chapter 8, “Matching with Regular Expressions”

[This post notes differences between the fifth and sixth editions.]

There are a couple of interesting updates for Chapter 8. The small change is the slight modification of a footnote. We mentioned that the performance problem of the match variables $& and friends wouldn’t be solved before Perl 6. However, with Perl 5.10’s introduction of the /p match operator flag, problem solved!

Chapter 8 also has a subtle shift in thinking about anchors. Perl 5 introduced the \A, \Z, and \z regular expression anchors. Somehow, never made the shift from the Perl 4 anchors ^ and $. Even after Perl Best Practices pointed out the problem, we failed to update the Llama

I’d never really bothered to check when Perl introduced \A until today. That’s a task I do quite frequently: when did some feature show up in Perl? I could just go through all the tarballs, unpack them, and look at the documentation, but there’s an easier way. Since I have a clone of the perl repository, I have access to the entire perl development history. Each release has a tag, and I can list all the tags:

$ git tag
perl-1.0
perl-1.0.15
perl-1.0.16
perl-2.0
perl-2.001
perl-3.000
perl-3.044
perl-4.0.00
perl-4.0.36
perl-5.000
perl-5.000o
perl-5.001
perl-5.001n
perl-5.002
perl-5.002_01
perl-5.003
...

If I want to see what was going on in a particular release, I checkout the appropriate tag:

git checkout perl-5.000

Now I can see the state of the repo at the point of that release. Sure enough, C<\A>, C<\Z>, and C<\z> are in the documentation back then.

Updates to Chapter 3, “Lists and Arrays”

[This post notes differences between the fifth and sixth editions.]

I went into this chapter thinking that it would be fairly easy: just fix up any possible typos or grammar problems, then move on. However, I was reading through Appendix B and noticed that in previous editions that we had ignored splice. We mention it all the way at the end of the book, but it almost takes as much space to say that we aren’t going to cover to say that we will. So, I move it out of Appendix B and into Chapter 3.

You would think that this chapter would be a natural to pull in things like List::Utils, but we actually save that for later. We make some Perl-pure versions of max in the “Subroutines” chapter, then later in the “Perl Modules” chapter we can abandon the examples we used to illustrate the Perl concepts so the reader can use List::Utils.