January 2012 Archives

Jonathan Swartz makes a polemic statement:

cpanm and perlbrew should not run tests by default.

His points are reasonable, but his complaints are mostly about side effects and not the real problem. (I should clarify: the real problem I encounter.)

If running tests slow down installs, speed up the tests. (Do you want to get the wrong answer faster? Easy: it's 42. No need for a quantum computer to do the calculation in constant time. This algorithm is O(0).)

If running tests exposes the fragility of the dependency chain, improve the dependency chain.

If dependency test failures prevent the installation of downstream clients... this is a weakness of the CPAN toolchain. A well-written test suite for a downstream client should reveal whether bugs or other sources of test failures in a dependency affect the correctness of the client.

Note the assumptions in that sentence.

Anyone who's experienced the flash of enlightenment that comes from working with well tested code and who's shared that new zeal with co-workers has undoubtedly heard the hoary old truism that testing cannot prove the complete absence of bugs. It's no less true for its age, though it's also true that good testing only improves our confidence in the correctness and efficacy of our code.

For me, a 95% certainty that my code works and continues to work for the things to which I've tested it is more than sufficient. I focus on testing the things I'm most likely to get wrong and the things which need to keep working correctly. (I don't care much about pixel-perfect placement, but I do care that a book's index uses the right escapes for its data and markup.)

Without tests running on the machines themselves in the environments themselves where I expect my code to run, I don't have that confidence.

Put another way, I'm either not smart enough or far too lazy to want to attempt to debug code without good tests. That's why I write tests, and that's why I run them obsessively. That's good for me as a developer, and you're getting the unvarnished developer perspective.

I also care about the perspective of mere users. (Without users, we're amusing ourselves, and I can think of better ways to amuse myself than by writing software no one uses.).

Yes, an excellent test suite can help a user help a developer debug a problem. Many (most?) CPAN authors have had the wonderful experience of receiving a bug report with a failing test case. Sometimes this even includes a code patch.

Not all users are developers of that sort, nor should they be.

The CPAN ecosystem has improved greatly at automated testing and dependency tracking, but we can improve further. What if we could identify the severity of test failures? (We have TODO and SKIP, but they don't convey semantic meaning.) What if we could identify buggy or fragile tests? (My current favorite is XML::Feed tests versus DateTime::Format::Atom because it catches me far too often, it doesn't affect the operation of the code, and it's a stupid fix that's lingered for a few months.) What if the failures are transient (Mechanize relying on your ISP not ruining DNS lookups for you) or specific to your environment (a test suite written without parallelism in mind).

As Jonathan rightly implies, how do you expect an end-user to understand or care about or debug those things?

I'm still reluctant to agree that disabling tests for end-user installations is the right solution. I want to know about failures in the wild wider world. I want that confidence, but I can't bring myself to trade away that confidence for the sake of a little more speed of installation.

Yet his point about lingering points of fragility in the ecosystem are true and important, even if the proposed solution of skipping tests isn't right. Fortunately, improving dependency management and tracking and use and testing can help solve both issues: perhaps to the point where we can run only those tests users most care about and identify and report material failures in dependencies.

Steven Haryanto's Perl First World Problems #1 reminded me of something I've taken for granted lately.

You may have read my Controlling Test Parallelism with prove and Parallelism and Test Suites. I still have Test::Harness parallelism enabled by default on most of the machines where I install my own Perls. While I haven't yet filed tickets and tried to write patches for modules which need a little help to run tests in parallel, I've only found a few lately that need work. That's nice—having a module install through cpanm in five seconds is a lot better than ten seconds or more. (I like cpanm because it's fast and quiet, and part of its speed comes from not printing to the console.)

I like instant feedback.

Like Steven, I noticed quite a while that installing a custom Perl through perlbrew takes a while, but then I remembered that a lot of work went into the Perl 5 test suite to make tests run in parallel. (We did something similar with Parrot several years ago, and it changed the way I work forever.)

To run core tests in parallel, set the environment variable TEST_JOBS=n, where n depends on your computer. I use a value of 9 on a quad-core machine; in practice, that tends to keep the CPU busy while not blocking anything too long on IO. You can set it globally in your shell's configuration file or create an alias or wrapper for perlbrew.

As most of the time spent compiling and installing Perl 5 through perlbrew goes to running the test suite, this has saved me a measurable amount of time.

Here we go again.

It looks like Red Hat is distributing Perl without the core library ExtUtils::MakeMaker. If you're not familiar with the details of the Perl 5 build chain, all you need to know is this: without MakeMaker, you're not installing anything from the CPAN.

Ostensibly Red Hat and other OS distribution vendors split up Perl 5 into separate packages to save room on installation media. Core Perl 5 is large and includes many, many things that not everyone uses all the time... but the obvious reaction to defining a core subset of Perl 5 that a vendor can call "perl" is another of those recurring discussions which never quite goes anywhere.

For example, who needs the documentation just to run code? (Except that the diagnostics pragma relies on the existence of perldiag.pod to run.) Who needs the huge Unicode encoding tables for ideographic languages such as you might find in Japan, China, Korea, and other Asian locals? (Answer: Asia.) Who needs the ability to install code from the CPAN? (Answer: users.)

While there's a lot of stuff in the core that probably doesn't need to be in the core, or at least installed by default (a LaTex formatter for POD, the deprecated Switch module, Perl 5.005 Thread emulation), one thing is both clear and almost never said.

I'll give you a moment to think about it.

Here's a hint: you're usually better off compiling and installing your own Perl 5 under your complete control such that you can compile in options you want (64-bit integers, for example) and out options you don't (threading imposes a 15% performance penalty even in the single-threaded case) and so that you can manage your own library paths without changing the behavior of the system). perlbrew changes the game. Learn it, like it, love it.

The perpetual discussion misses one important point:

The vendor perl—especially on installation media—is not for general purpose Perl programming. It's there only to support basic administrative programs provided with the system as a whole. That's why you don't replace the system Perl. That's why you don't mess with the system CPAN modules. That's why you fence off whatever's in /usr/bin/perl like it's Yucca Mountain and you're stuck with a '50s reactor design instead of something safe and clean.

Vendors can tune and tweak that Perl to their satisfaction to provide just what they need to install and configure a working system. They can keep it as crufty and out of date as they like. When it breaks, they get to keep all of the pieces and sew them back together like some sort of Fedorastein's monster. They just can't let it out of the lab.

This of course means that they need to provide packages of Perl 5 Actual for users and developers such that it's the full core of Perl 5. (It'd be nice if they called not-a-perl as such, but one thing at a time.)

You can't predict what users will and won't do. That's why you code defensively. The moment distributions started carving up Perl to install just the little bits they needed in the hopes that their guesses as to what users wanted were right, they put everyone in a bind.

Certainly Perl 5 could benefit from a thorough review of what's in core and why, but I suspect that even if p5p came up with packaging guidelines for all of the imaginable use cases and combinations of distributor needs and user wants, it still wouldn't solve the real problem.

(Credit Allison Randal for pointing out the real problem years ago. We've discussed several times the idea of a stripped-down VM for a real language—something with better abstraction and reuse than Bash—with easy access to libraries and a very small footprint, but it's a bigger job than either of us could accomplish. It's still a righter approach than bowdlerizing an upstream distribution.)

I promised in Testing Your Templates to explain how to solve the problem of the divergence between testable, debuggable code in your host language and a big wad of logic in a template language.

This problem is an example of the pattern of Why Writing Your Own DSL is More Difficult Than You Think. Certainly Template Toolkit is among the better templating systems (I've written a couple myself), but it exhibits problems endemic to the process. (Then again, so does PHP. Now multiply that by the fact that some people use templating systems written in PHP and if you have to lie down for a while before the feeling passes, please accept my apologies.)

The semantics of Template Toolkit are great, when they work, but then everything's great when it works the way you expect. Robust software handles the cases you don't expect with aplomb, or at least without a boom.

A simple workaround for Template Toolkit is to avoid the fallback from potential method lookup to keyed hash access when dealing with an object. In other words, if $blessed_hash->do_something() fails, try $blessed_hash->{do_something}.

... except that that doesn't work when you want to call virtual methods on unblessed references, such as calling methods on arrays or hashes.

Another option is to change the syntax such that calling a method is visibly different from accessing a member of an aggregate. Perl does this. It works pretty well, in the sense that if you use the right operator (access element versus invoke method), you've expressed your intent in a visually unambiguous fashion).

... except that people complain about the Perl dereferencing arrow quite a bit. (Okay, you don't need an arrow to do this; as the Modern Perl book explains, the postfix indexed access or postfix keyed operators of {} and [] determine the type of operation effectively.)

... and except that one of the design goals of Template Toolkit was to be robust in the face of changing values provided to the template, such that it provides a loosely coupled interface for the data it expects. That's a fine goal, but it isn't free.

Here's the thing, though. The last time I looked, Template Toolkit compiles templates into Perl code as an optimization. (The last template system I wrote did the same thing, but not as well. We should have used TT, but in our defense, TT didn't exist then.) This transliteration/compilation stage must be very, very cautious to allow standard Perl debugging and introspection tools to treat this generated code correctly. That is to say, I don't want to debug a big wad of generated code. I want to debug the code I actually wrote.

As usual, the solution is another layer of abstraction.

Perl exists in two forms. The first is the source code you and I write. The second is the optree which the Perl VM executes. There's nothing in between. You have one or the other. When your code runs, you have the optree, and the optree has references to the relevant location in the source code it came from, but the correspondence is often less useful than you might like.

While the generated code from Template Toolkit could include the correct file and line positions from templates, that's again less useful than you might like. (It's useful, but it doesn't solve every problem.)

If Perl had instead an intermediate form separate from raw code and raw optrees, something more suitable to introspection and manipulation, we could produce tools which worked with this intermediate form to improve debugging, introspection, and better code generation.

We could even inject new code to add features (fall back to attribute access; prevent the fallback to attribute access) to code, even within lexical scopes. That is to say, we could manipulate how libraries behave from the outside in, and ensure that our changes would not leak out from our desired scopes.

It's certainly possible to replace the Perl opcodes yourself, if you're comfortable reading Perl source code, writing XS, relying on black magic, and dealing with strange issues of thread safety and manipulating global or at least interpreter-global values in a lexical fashion (while dealing with the fact that use is recursive in a sense)—but isn't Perl about not making people write C to do interesting things?

Certainly this isn't a technique you'd use every day, and it's not obviously a way to make Perl run faster (though many optimizations become much easier), but the possibility for better abstraction and extension and correctness has much to recommend it.

And, yes, Lisp demonstrated this idea ages ago.

Imagine if Perl 5's caller returned an object which represented the call chain to the point of the object's creation.

I want to inspect the call stack within a helper module, but I don't care about the call stack within the module. I want to use lots of little helper functions, because that's good design, but caller works against that. Looking up the stack means keeping track of the magic number of call frames within my module I currently use, and that's one more thing to update when I change things.

That's structural code highly coupled to the arrangement of other code, and if that doesn't wrinkle your nose with the subtle aroma of fragility and peril, I don't know what will.

Imagine if instead:

my $caller_object = capture_caller_state();

while (my $call_frame = $caller_object->next)
{
    next if $call_frame->is_eval;
    say $call_frame->location_as_string;
    my $next_frame = $call_frame->previous;
    ...
}

Imagine if you could pass this object around.

I know things get complicated if you pass this object up the call chain, but stack unwinding is a solved problem in that anyone capable of recognizing the problem should be able to figure out cheap and easy ways to fix it.

Alas, Perl 5.14 doesn't have this feature, and it's probably too late for 5.16 to get it, so for today I'm stuck imagining what might be.

(If you've never thought about this sort of thing before, you owe it to yourself to learn more about Continuation Passing Style, which is at least an order of magnitude more mind-bending at the start and at least two orders of magnitude more useful.)

Sometimes you find bugs in the most surprising places.

I've long used test-driven design for code I care about. Knowing that it works, works reliably, and will continue to work helps me write the right thing. As I've gained experience with testing, I've come to appreciate the nuances of how and what to test and why. For example, I often don't bother to test exploratory code—it's more important for me to learn how code might work to help me figure out how to design the real thing than it is to hew to a rigid mindset of "You must always test everything."

Similarly, I see little value in rigorously testing basic accessors, and thank goodness for Moose for turning class declarations into declarative code.

In more serious discussions of testability and testing, people often argue that testing UIs is impossible, or merely difficult, or rarely worth the trouble to do so completely. I agree to some respect; the most effective way I've seen to test UIs is to test the backend data model rigorously, write a very thin controller to marshall and unmarshall requests and responses, then to write a handful of tests which explore the edge cases of UI interactions. That requires a deep understanding of what happens for each interaction, and you can only test those interactions which produce measurable side effects.

Let that latter point sink in for a moment.

My most recent work on multiple projects includes a lot of embarrassingly parallel batch processing work, where the user interface is largely static, produced daily from a series of templates. Generally the templates are correct if they pass the eyeball test with live data.

This works except when it doesn't.

Consider a snippet of Template Toolkit code to display a list of stories:

[% FOR e IN entries %]
    <div class="story story[% loop.count %]">
        <h3><a href="[% e.url %]">[% e.title %]</a></h3>
        <p>[% IF e.author %]
            [% e.author | html %]<br />
        [% END %]
        [% f.format( e.posted_time ) %]</p>
        [% IF e.has_image %]
            <img src="[% e.image.full_file %]" alt="[% e.title | html %] image" class="image" />
        [% END %]
        <p>[% e.short_content | html %] <a href="[% e.url %]">Read More</a></p>
    </div>
[% END %]

That code had a bug. Can you find it by eyeballing it? Neither could I, for a long time. I'll give you a hint: it never displayed images associated with stories. Give up?

I wrote backend test after backend test, fixing a couple of other bugs along the way, then traced through the code in a debugging batch until I had convinced myself that the data model was completely correct. The bug had to be in the template because it could be nowhere else.

Rigorous template tests didn't seem to help either, and I'd almost convinced myself that I had a nasty spooky action at a distance bug somewhere in the data model before I saw the problem:

[% IF e.has_image %]
    <img src="[% e.image.full_file %]" alt="[% e.title | html %] image" class="image" />
[% END %]

When I added a test for the has_image() method on story objects to the data model tests, it failed immediately because that method did not exist. It used to exist, but a refactoring somewhere went too far and removed that method and its tests from everything except the template. Because Template Toolkit is not so strict about the distinction between method and property access, the lack of a method is no big deal. It fell back to property access on a hash and, finding no such value in the hash, returned a false value.

There are ways to fix this. I'm sure there's a plugin for Template Toolkit, and probably a debugging/diagnostic mechanism somewhere I haven't yet discovered. Yet the problem remains.

I've heard it said (Rich Hickey? Martin Odersky? I think it was Rich.) that every bug in your program has passed the type checker. That statement contains wisdom. Yet it's also true that every bug in your program has passed the test suite. (We assume you run with type checks and keep your test suite fully passing.)

Neither is a substitute for careful thought and understanding. Both protect you only as far as you can take advantage of them.

My next post will explore some options for making these kinds of errors easier to catch way before the point of weird debugging sessions.

Update: PDFs, ePub, and Mobi files are available from Modern Perl: The Book, and you can read Modern Perl: The Book online now!

The Modern Perl book went to the printer earlier this week; this is the 2011-2012 edition.

You can already preorder Modern Perl: 2011-2012 from Amazon and it'll be available from other bookstores in the next couple of days. I expect copies to ship by early next week.

What's new?

I closed every bug filed against the previous edition—a handful of typos, a few confusing sections, and some suggestions on improvements. I took most of these suggestions.

I also revised almost every paragraph of the book, even at the sentence level. While I'm proud of the first edition, this new edition is clearer, shorter, more concise, and more accurate.

I removed some sections that didn't work. This didn't amount to much, but some of the things I recommended made little sense anymore and some of the experimental modules I mentioned turned out to be less useful than hoped. You won't miss them. This includes a big revision of the smart matching section in anticipation of changes in 5.16 and 5.18 (simplicity is the rule of the day; the simpler your use of the operator, the less risk you face for confusing behavior and edge cases).

I also added explanations for the new features in Perl 5.14, especially the /r modifier on regular expressions, and prepared for further features added in 5.16.

You might notice subtle improvements in formatting; the font size used for code examples is larger and callouts are much more attractive. We're still experimenting with the best way to format books to make readable text while avoiding brick-sized tomes, so feedback is more than welcome.

As with the first edition, we'll make electronic versions available for free from the Onyx Neon site. We hope to have ePub versions ready in the next two weeks. We'll also put raw HTML online on modernperlbooks.com at the same time (this also allows us to link to translations with greater ease). In addition, we're making most of our publishing tools available as CPAN distributions, including the formatting modules (Pod::PseudoPod::DOM) and the rendering pipeline (Pod::PseudoPod::Book). This will allow you to make your own version of the book in other languages if you desire, or to create new and attractive books in many formats. We'll make our own Kindle version, rather than using Amazon's conversion tools (they didn't work as well as we'd hoped), and we hope to get the book in the Nook store as well.

We plan to release a new version of the book late this year or early next year to cover 5.14 and 5.16. We'll almost certainly drop support and mention of 5.10.

We couldn't have done this book without you. Thanks for reading. Please share with your friends.

Buddy Burden explanation of taking over maintenance of CPAN distributions is important. It's empowering. If you've ever thought "I should contribute something to Perl", start there.

You can do it.

Sure, it's easy for me to say that. I've written a few things about Perl a few people have read. I have a few patches in a few projects and a couple of modules on the CPAN myself. (You're reading this, aren't you? So I have at least one reader. Thank you for your time!)

Even with all that in mind, two questions still come up far too often, and I think they prevent or delay us from providing code and documentation for other people to use freely:

• Does anyone care?
• Is it good enough (yet)?

You can see this attitude in the recurring debate over the responsibilities of authors. While some people say it's irresponsible or childish or unprofessional to upload code without full test coverage or complete documentation or an iron-clad promise to respond to every bug within 24 hours with a fix, an apology, a new release, and a sandwich (and yes, I exaggerate for effect), we're doing each other a disservice setting these demands on ourselves and each other.

Yes, we should do our best.

... but yes, we can start from something small and incomplete and refine it in smaller steps. It doesn't have to pass every test on every platform known to man if you can get feedback on where it fails and release a new version tomorrow. It doesn't have to have every feature you plan to add, if it does something useful that other people might want to use today. It doesn't have to have every option documented if it's usable right now.

I'm not suggesting that we allow ourselves to be irresponsible with what we share, but I am suggesting that we can allow ourselves the freedom to share a little earlier and a little more often. We have the ability to upload new code every day (every hour!) if we want.

So what if it's not perfect? Even if you waited until you thought you'd achieved perfect, you probably would miss, even by a little bit.

Start small. Do your research—work to your best quality—but let yourself hit smaller goals and make things a little bit better a little more often. Share earlier. This is a lot better for everyone.

The Modern::Perl CPAN distribution is much more conservative than I think most people thought it would be when I first released it. (Count me in that group.) Where I once intended to collect a bunch of useful CPAN modules in the style of Task::Kensho or perl5i, both of those do what they do far better than I can do.

Instead, I see Modern::Perl as enabling the core features I wish were on out of the box by default in Perl 5. While it'd be nice to pull in Try::Tiny and sometimes I might wish for fatal warnings, the former is not a core module and the second isn't something I use in every program.

Consequently the module hasn't needed much maintenance. Yet I've added a couple of missing features that should keep it useful and usable into the future. I uploaded a new version yesterday and will upload one more today with a little bit more polish.

First, it now requires autodie as a distribution dependency. It doesn't load autodie, but installing Modern::Perl on 5.10.0 will also install autodie. (Anything 5.10.1 and newer includes autodie in the core.) You don't have to use it, but now you can rely on any Perl considered modern to have autodie available.

Second, it now loads IO::File and IO::Handle so that you can call methods on lexical filehandles without having to load either manually. Perl 5.14 fixed that usability niggle, but Modern::Perl fixes this for people using 5.10 or 5.12. (Why both? I can never remember which one superseded the other in 5.12, but better safe than sorry. I welcome a patch to load one over the other with a version check—and please test carefully.)

Third, I added unimporting support so that you can write no Modern::Perl; within a scope to disable strictures, warnings, and language bundle features. It's an all or nothing switch and will remain that way, but I can see this being useful in specific cases, especially when updating older code in stages.

Finally I added date support to importing. If you write use Modern::Perl; you'll get the features of Perl 5.10 (with the caveat that if you're using Perl 5.11.3 or newer, you also get the unicode_strings feature, of which all you're likely to notice is that Unicode strings work better in more places).

Yet for forward compatibility, you should be using:

use Modern::Perl 2012;

... which enables 5.14 features. You can use 2009 and 2010 to get 5.10 features and 2011 to get 5.12 features. If you use the wrong date on the wrong version of Perl 5, you'll get an error, which is as it should be.

Next year I'm likely to drop support for Perl 5.10, in which case you'll probably get an error message that that year isn't modern enough, but I could be convinced to do a version check instead. I haven't decided. The tradeoff is between providing a minimal module suitable for use in programs which helps people write better code from the start and between telling people what they should and shouldn't use. Besides all that, the relevant code is only a couple of dozen lines of very simple Perl. Anyone reading this can reimplement it almost trivially.

Of course, this all comes about because the Modern Perl book goes to the printer today. We're very proud of the new 2011-2012 edition which concentrates on Perl 5.12 and Perl 5.14. It addresses all of the known typos and confusing parts of the previous edition, covers new features in Perl 5.14, and is, from all reports, even better than the first edition.

The book should be in stores in the next week or so, and we'll have electronic editions up this month for free download and redistribution. (We hope you tell lots of people to buy the print edition because it's great and more people need it on their desks, but sharing is caring and we support that.)

The YAPC::NA 2012 call for presentations has opened! As with every YAPC I've attended, this is a great opportunity to meet other programmers, learn things you know better and don't know yet, and to practice your presentation skills.

A few months ago I exchanged emails with JT Smith about my idea for a talk this year. I've mentioned in passing a few times a small side project my business is investing in. It's a side project, deliberately minimal, and—from the development side—definitely the kind of skunkworks, just get it working, maintain it as little as possible and let it run uninterrupted software that you're likely to find.

That doesn't mean it's quick or dirty. That doesn't mean it's not tested well, or that it has a slapdash design. All it means is that the most important criterion for any design or implementation decision is "is this the simplest thing that could possibly work" instead of "is this elegant" or "what's the standard modern Perl orthodoxy for this problem".

So far the results have been enlightening.

I don't want to give away too many of the details of my talk (if it's accepted), but here are two small hints which may or may not help you.

First, just because a good ORM such as DBIx::Class makes searching and manipulating existing data easy doesn't make it the best way to insert big batches of new data.

Second, while LWP and especially WWW::Mechanize are great tools for automating the behavior of a web client, sometimes wget or curl in a shell script is quicker, easier to parallelize, and more robust.

(As a bonus, consider also that if you're parsing semi-structured data out of HTML that removing all of the HTML is sometimes even easier than using a real HTML parser or even CSS selectors. Sure, semantic markup helps when you can rely on it, and sure, using a regex to remove HTML tags is a bad idea, but there are ways to turn HTML into plain text quickly and easily without doing anything on your own.)