All about Catalyst – interview of Matt S. Trout (Part 3 of 3)

What about the other Perl frameworks, Dancer and Mojolicious? How do they compare to Catalyst?

Dancer’s big strength is making things quick and easy for smaller apps; you don’t have to think in terms of OO unless you want to and plugins generally shove a bunch of extra keywords into your namespace that are connected to global or per-request variables. Where Catalyst doesn’t have an exact opinion about a lot of the structure of your code but very definitely insists that you pick one and implement it, Dancer basically lets you do whatever you like and not really think too much about it.

That really isn’t meant as a criticism: somewhere along the line I picked up a commit bit to Dancer as well and they’ve achieved some really good things – providing something that’s as little conceptual overhead as possible for smaller apps, and something where there’s a very direct mapping between the concepts involved and what’s actually going to happen in terms of request dispatch, whereas Catalyst abstracts things more thoroughly, so there’s a trade-off there. I mean, I was saying before that empty methods with route annotations almost always end up getting some code in them eventually.If you get to 1.0 and most of those methods are still empty, you might’ve been able to write a lot less code or at least do a lot less thinking that turned out not to have been necessary if you’d used Dancer instead. Equally, I’ve seen Dancer codebases that have got complicated enough to turn into a gnarly, tangled mess and the developers are looking and thinking, “You know, maybe I was wrong about Catalyst being overkill…”

I love the accessibility of Dancer though, and the team are great guys. I’ve seen the catalyst community send people who’re clearly lost trying to scale the learning curve to use Dancer instead, and I’ve seen the Dancer community tell people they’re doing enough complicated things at once to go look at Catalyst insteadand hey, we both run on top of PSGI/Plack so you can have /admin served by Dancer and everything else by Catalyst … or the other way around … or whatever.

Meantime, Mojolicious is taking its trade-offs in a different dimension.Sebastian Riedel, the project founder, was also founder of Catalyst; he left the Catalyst project just before the start of, I think, the 5.70 release cycle, because we’d acquired a lot of users who’d bet the business on Catalyst’s stability at that point, and a lot of contributors who thought that was OK, and Sebastian got really, really frustrated at the effort involved in maintaining backwards compatibility.

So he went away and rethought everything, and Mojo has ended up having its own implementations of a lot of things, focused on where they think people’s needs in web development are going over the next few years. A company heavily using Mojo open sourced a real–time IRC web client recently, doing a lot of clever stuff, and Mojolicious helped them with that substantially. But the price they end up paying is that when you step outside the ecosystem it’s quite jarring, because the standards and conventions aren’t quite the same as the rest of modern Perl. Mojolicious has a very well-documented staged backcompat breakage policy which they stick to religiously, and for “move fast and break stuff” style application development, I think they’ve got the policy pretty much spot on and they’re reaping a lot of advantages from it

But for a system where you want to be able to ignore a section that users are happy with and then pick it up down the line when the state of the world changes and it needs extending (for example, for the people doing boring business systems where finding out 5 seconds sooner that somebody edited somethingwould be nice to have) but what really matters is that the end result is correct, then I’m not sure I’m quite so fond of the approach. I think if you were going to talk about a typical backend for each framework, you could say that Dancer’s would be straight SQL talking to MySQL, Catalyst’s would be some sort of ORM talking to PostgreSQL, and Mojolicious’ would be a client library of some sort talking to MongoDB. Everybody’s going to see some criticism of the other two implicit in each of what I said, but take it as a compliment to their favourite: if they don’t, that’s because my metaphor failed rather than anything else

I can’t recall a time when I’ve seen an app that was a reasonable example of its framework where I really thought that either of the other two would’ve worked out nearly as well for them... with the exception of a fairly small Catalyst app that, in spite of being, if anything, a bit small for Catalyst to make sense turned into a crawling horror when ported to Mojolicious – but then again, when I showed that to Sebastian and asked, “am I missing something here?”, the only reply I got was some incoherent screaming, the underlying meaning being, “WHAT HAVE THEY DONE TO MY POOR FRAMEWORK?! CANNOT UNSEE, CANNOT UNSEE!” So I think, as with Dancer, it’s a matter of there being more than one way to do it, and one or other of them is going to be more reasonable depending on the application.

What’s in Catalyst’s wish-list, in which direction is the project moving and do you think that someday Catalyst’s adoption will be so widespread that it will become the reason for restoring the P=Perl back to LAMP?

 The basic goals at the moment are a mixture of adding convenience features for situations that are common enough now to warrant them but weren’t, say, five years ago; continuing to refactor the core to enable easier and cleaner extension; and to figure out a path forwards that lets us clean up the API to push new users onto the best paths while not punishing people that use older approaches.

 As for putting the P back in LAMP? I’ve regarded it as standing for “Perl, PHP or Python” for as long as I can remember. It doesn’t seem to me that treating it as a zero sum game is actually useful: in the world of open source, crushing your enemies might be satisfying but encouraging them and then stealing all their best ideas seems like much more fun to me.

What are your thoughts on Perl 6 and given the opportunity, would you someday re-write Catalyst in it?

 I’ve spent a fair amount of time and energy over the years making sure that the people thinking hard about language design for both the Perl5 and Perl6 languages talk to each other and share ideas and experiences reasonably often, but I’m perfectly comfortable with Perl5 as my primary production language for the moment, and so long as the people who actually know what they’re doing with this stuff are paying attention I don’t feel the need to that much.

 One of the things I’m really hoping works out is the whole MoarVM plan, wherein Rakudo will end up with a solid virtual machine that was designed from the start to be able to embed libperl and thereby call back and forth between the languages. So if that plan comes off, then I don’t think you’d ever write Catalyst in Perl6 so much as you could write parts of Catalyst apps in Perl6 if you wanted to… and maybe one day there’d be something that uses features that are uniquely Perl6-like that turns out to be technologically more awesome. You can still write parts of those apps in Perl5 if it makes sense, but I don’t think looking at the two languages in the Perl family as some sort of competition is that useful. I much prefer a less dogmatic approach, similar to the saner of the people I know who are into various Lisp dialects.

So it’s more about experimenting in similar spaces and learning and sharing things – and while being a language family is often cited as a reason why Lisp never took over the world … Perl taking over the world got us Matt’s Script Archive and a generation of programmers who thought the language was called PERL and fit only for generating write-only line noise whereas being a language family seems to have pretty effectively given Lisp immortality, albeit a not-entirely-mainstream sort of immortality.

 I think, over a long enough timeline, I could pretty much live with that (absent a singularity or something I’ll probably be dead in about the number of years that Lisp has existed), and I think if there is a singularity then programming languages afterwards won’t look anything like they do now… although, admittedly, I still wouldn’t be surprised if my favorite of whatever they do look like was designed by Larry Wall.

Nikos Vaggalis has a BSc in Computer Science and a MSc in Interactive Multimedia. He works as a Database Developer with Linux and Ingres, and programms in both Perl and C#. He is interested in anything related to the RDBMS and loves crafting complex SQL queries for generating reports. As a journalist, he writes articles, conducts interviews and reviews technical IT books