If you are working in a functional, stateless language, but can still get away with for loops in a more conventional way thanks to for-each functions – should you still favor recursion over explicit for loops? Discuss.
Now that I am, as the title implies, “getting there,” I want to reflect a little on the learning process that has been XSLT. In my last post I glossed over what makes it (and functional programming languages generally) distinctive and, for people who are used to procedural languages, unintuitive and hard to grasp at first. This will be a post with several simple points, but that’s quite in keeping with the theme.
The major shift in thinking that needs to happen when working with XSLT, in my opinion, is one of trusting the computer more than we are accustomed to. It all stems from letting go of telling the computer how exactly to figure out when to execute sections of code, and letting it make the decisions for us.
I made a comment recently: “I know I’m getting more comfortable with XSLT because suddenly I’m trying to use recursion everywhere I can, and avoiding the for-loop like a crutch.” As others I talked to put it, this is idiomatic XSLT.*; In other words, it’s one of the mental leaps that you (and I) have to make in order to start writing elegant and functional code (no pun intended) using this language.
What is recursion? In this case, to oversimplify, it’s how XSLT loops.** In a procedural language – C++, Java, most languages other than Lisp dialects to be honest – recursion is clunky and wasteful; telling the computer to specifically “do this for the number of times I tell you, or until this thing reaches this state” is how you get things done. This means that the languages have state, too – you can change the value of variables. This is important for having counters that are the backbone of those loops. If there were no variable to increment or change in another way, the loop would either never execute (such as a while), only execute once, or loop endlessly. None of these things are very helpful.
So how do you get away with counter-based loop, at least of the “for each thing in this set” variety, with a stateless language (all variables are permanent, aka constants) that discourages use of for-each loops in the first place?
The first is much simpler: xsl:apply-templates or xsl:call-template. This involves the trust that I introduced above. With a procedural language it’s hard to trust the computer to take care of things without your telling it exactly how to do it (keep doing this thing until a condition is met) because you’ve had to become so used to it. It might have been hard to get used to having to explain the proverbial peanut butter sandwich recipe in excruciating detail for the sandwich to get made. Now, XSLT is forcing you to go back to the higher level of trust, where you can tell the computer “do this for all X” without telling it how it’s going to do that.
xsl:apply-templates simply means, “for all X, do Y.” (The Y is in the template.) It’s unsettling and worrying, at least for me at first, to just leave this up to the computer. There’s no guarantee that templates will ever be executed, or that they will be executed in order. How can I trust that this is going to turn out okay? Yet, with judicious application of xsl:apply-templates (like, where you want the results to be), it will happen.
Second, the recursive aspect. Keep calling the template until there are no more things left – whether that’s a counter, or a set of stuff. But how to get a counter without being able to change the variable? With each xsl:apply-templates (or call-template), do so with xsl:with-param, and adjust the parameter as needed. Call it with the rest of the set but not the thing that is being modified in the current template. When it runs out of stuff, that is when results are returned. Again, it takes the explicit instruction – xsl:for-each is very heavy-handed – and turns it into “if there’s anything left, keep on doing this.” It may seem from my description that there’s no real difference between these two, and in their end result, there isn’t. But this is a big leap, and moving from instinctively reaching for xsl:for-each to xsl:apply-templates is conceptually profound. It is getting XSLT.
Finally, a note on the brevity and simplicity of XSLT. I’ve noticed that once I’ve found a good, relatively elegant solution to what I’m trying to do (they can’t always be!), suddenly my code becomes very short and very simple. It’s not hard to write and I don’t type for a long time. It’s the thinking and planning that takes up the time. Obviously this is true for programming just about anything, but I find myself doing a whole lot less typing this summer than usual (compared to languages I’ve used such as C, C++, Java, Python).
It’s both satisfying and disappointing at the same time: getting a template that recursively creates arbitrary nested menus wants to make me jump up and high five myself; the fact that it’s only about four lines and incredibly simple makes me wonder if any of it was that hard to begin with. But this isn’t limited to XSLT or even programming: the 90-page thesis seems like more work than the 40-page thesis, but if the shorter one is talking about more profound ideas and/or is simply more well-written, the length and time comparison falls apart. The time spent typing and the length of the output doesn’t tell us as much as we’re used to assuming.
That’s what I have to say about what I’ve been doing this summer, as far as learning XSLT goes. I still can’t say I like it. The syntax is maddening. I haven’t been in this long enough to judge whether it’s the best choice for getting something done within a lot of constraints. But at the very least I’ve finally had that brain shift again, the one I had with Lisp so long ago, to a different approach to problem-solving entirely. And that feeling is profoundly gratifying.
Speaking of a good feeling, I’ve been able to have extended chats with multiple people about XSLT on the U of M School of Information mailing list this summer after someone posted asking for help with it. It’s a good thing I replied despite thinking “I’m not an expert, so I probably don’t have much to offer.” Talking with the questioner and the others who replied-all on our emails was really enlightening, both by getting feedback, hearing others’ questions about how the language works (questions that I hadn’t articulated very well), and also giving my own feedback. There’s nothing like teaching to help you learn. I would not have been able to write this post before talking to my fellow students and figuring it out together. (Or, you would have read a very unclear and aimless post.)
(Very last, I’d like to recommend the O’Reilly book XSLT Cookbook for using this language regularly after getting acquainted with it. If I were continuing on with an XSLT project after this internship, or working on adding more to this one, I’d be using this book for suggestions.)
* Thank you all for reminding me that this word exists.
** XSLT now includes not only the for-each loop, but also the xs:for tag. These do have their appropriate uses and I do use them quite a lot, because my application doesn’t give me a huge number of chances for recursion. I’m being dramatic to make a point.
Cross-posted from the iSchools & Digital Humanities intern blog.