TITLE: Fall Semester Seems Far Away Right Now AUTHOR: Eugene Wallingford DATE: June 10, 2008 4:09 PM DESC: ----- BODY: I'm not yet thinking about my Programming Languages course this fall, but I wish I were. The first temptation came via my newsreader and Martin Fowler's recent piece ParserFear. Martin is writing a book on domain-specific languages and blogging occasionally on his ideas. This article is about just what the title says, programmers' often irrational fears of rolling a parser, which deter them from implementing their own DSLs. He speculates:
So why is there an unreasonable fear of writing parsers for DSLs? I think it boils down to two main reasons. The first is easy to understand, people are naturally nervous of things they don't know about. The second reason is the one that's interesting. What this boils down to is how people come across parsing in universities.
CS students tend to learn about parsing for the first time in a compiler class, working on a large language with a full range of constructs. Parsing such languages is hard. In a few compiler courses, including my own, students still learn to build a parser by hand and so don't even have the chance to use parser generators as a labor- and pain-saving device. That's okay in a compiler course, which students take in large part to learn how their tools really work. Martin doesn't suggest that we change the compiler course, and I don't either (though I'm open to possibilities). He does seem to think it's a shame that students are turned off to parsing and language design by first, and perhaps only, seeing them at their most complex. I agree and think that we should do more to introduce these ideas to students earlier. I introduce students to the idea of parsing in my Programming Languages course, and ask students to write a few very small parsers to handle simple languages. I've been thinking for a while that I should do more in this area, and reading Martin's article has me itching to redesign the latter part of my course to allow more work with parsing and parsers. Another possibility is to use parsing as the content of some of our early programming exercises, when students are nominally learning to program in a functional style. Students can certainly apply the programming techniques they are learning to translate simple data formats into more abstract forms. This might help them to begin to see that parsing is an idea broader than just general-purpose programming languages. It might ease their transition a couple of weeks later to the idea of syntax-as-data structure and allow us to do some simple work with DSLs. I have tried the idea of parsing at its simplest with relative novices as early as CS 1. When I taught a media computation CS1, one of the last programming assignments was to write a program to read a file of simple graphics commands and produce the desired graphical image. The idea was to bypass Java's verbose syntax for doing simple AWT/Swing graphics and allow non-programmers (artists) to make images. I asked students to implement a couple of simple commands, such as "draw line", and create at least one command of their own. I expected all of their graphics languages to be "straight-line", with no control or data structures, but that didn't mean that the resulting DSLs were not useful. They were just simple. A couple of students did really interesting work, creating very high-level commands for swirls and splashes. These students wrote methods to interpret those command using control and data structures that their "programmers" didn't have to know about. Any change to my Programming Languages needs to be "footprint-neutral", to use Shriram Krishnamurthi's phrase. Anything I add has to fit in my fifteen-week course and either displace existing material or work in parallel. Shriram used this phrase in the broader context of rejiggering the teaching of teaching of programming languages within the core ACM curriculum, which a recent SIGPLAN-sponsored workshop tried to do. After having just read Martin's article on parsing, I was eager to refresh my memory on where parsing fits into the core curriculum proposal. Parsing falls under knowledge unit PL3, Language Translation, which has two hours allotted to it in the core. (An optional knowledge unit on Language Translation Systems includes more.) Interestingly, the working group Shriram reports on recommends cutting those two hours to zero, on the grounds that the current coverage is too superficial, and using those hours to build up a 10-hour unit on Functional Programming. That's a worthy goal, though I haven't hard a chance to think deeply about the proposal yet. Working with constrained resources sometimes requires making tough choices. I know that Shriram and the people with whom he worked think that parsing is a worthwhile topic for CS students to know, so perhaps they have in mind something like what I suggested above: piggybacking some coverage of parsing on top of the coverage of functional programming. In any case, I think I'll work on finding more ways for my Programming Languages students to engage parsing and domain-specific languages. -----