Category Archives: M

Lexing and Parsing

I want to talk in more detail about how the MGrammar parser works, but before I delve too deeply in to that I wanted to talk a little bit about some basic parsing concepts so we can be sure we’re on the same page. This may be a bit basic for some, so you can just skip ahead if you already know all this.

A language grammar is actually made up of two different grammars: a lexical grammar and a syntatic grammar. Although a grammar may not distinguish between the two in its specification (as we’ll see later), every grammar has these two things contained within it. The lexical grammar translates characters (the raw material of language) into lexical units called tokens. The syntatic grammar then translates the tokens into syntatic structures called syntax trees.

Let’s take a look at this in practical terms. Let’s say you’ve got the following sentence:

Alice kissed Bob.

Fundamentally, this sentence is just a collection of characters: the upper-case letter A, the lower-case letter l, the lower-case letter i, and so on. Lexically analyzing this string takes these characters and shape them into the basic tokens of English: punctuation and words. So a lexical analysis might return the following tokens: the word Alice, the word kissed, the word Bob, and a period.

One thing to notice here is what’s missing in the list of tokens-every character in the string “Alice kissed Bob.” is represented in the token list except for two characters. We’ve dropped the spaces between the words out of the token list, and we’ve done this because spaces aren’t significant in English syntax. (As anyone who was taught to type two spaces after a period can attest.) In other words, you can have as many spaces as you want between tokens and they don’t mean anything except as delimiters between tokens. In other words, “AlicekissedBob.” is not the same as “Alice kissed Bob.” but “Alice kissed Bob.” is the same as “Alice kissed Bob.” (Although it looks a little funny.) Since spaces don’t matter once we’ve formed the tokens, we toss them out.

Once we’ve got the tokens, we can then proceed to do the syntatic analysis. In the case of English, a common syntax pattern is “<subject> <verb> <direct object>.” Looking at the sentence, we can pretty easily discern this pattern at work: the subject of the sentence is “Alice,” the verb is “kissed,” the direct object is “Bob,” and the sentence ends with a period. Since the verb links the subject and the direct object, we’d probably come up with a syntax tree that looks something like this:

image

As I said before, some languages make a bigger deal of lexical vs. syntatic grammars than others. For example, if you go and read the XML spec, you’ll see that they don’t really call out what’s a token and what’s syntax. On the other hand, the Visual Basic language specification is extremely explicit about the distinction between tokens and syntax, even going so far as to have two separate grammars. How you think about tokens vs. syntax has big implications for the implementation of your parser, which I’ll talk about in my next entry.

DSLs: Definitely a bad idea!

Last Friday Rocky posted an entry on his weblog entitled “DSLs – fun, cool, but maybe a bad idea?” and my reaction was:

Maybe a bad idea? Maybe a bad idea? Of course they’re a bad idea!

This may seem strange coming from someone who’s working on the heart of Oslo/M’s DSL capabilities, but stick with me here.

To begin with, writing a new language, domain-specific or not, is a lot like opening a restaurant: everyone thinks they can do it because they’ve eaten at one and it looks like fun. But the truth is that something like 50% of restaurants fail in the first year, and probably 90% fail over the long haul. It’s incredibly difficult, time consuming, and hard to keep a restaurant as a going concern-but doing that while actually coming up with something people want to eat is nearly impossible. I’m amazed new restaurants open up at all, given the odds.

Languages are the same way. It’s mind-bendingly, stupendously difficult to build a new language, even one that’s largely based on an existing one. Yet many programmers think, “hey, I use languages, how hard can this be?” and go at it. Like restaurants, probably over 98% of them fail to ever gain any traction at all, but god bless the optimists because without them we’d never get the 2% of languages that succeed. I’m personally willing to sacrifice the millions of dollars and hours wasted on languages that never make it just so that we can get languages like C# and Java and Ruby and Python and so on. (One language omitted to preserve at least the appearance of humility.)

So the fact that coming up with a new language is a bad idea shouldn’t dissuade people from developing new DSLs, it should just give them pause and hopefully a little humility. The key, I think, is to start small and to stay small. A domain-specific language needs to stay, well, domain-specific and not try and branch out to become a general purpose programming language. The problem is that it’s really, really hard to resist the temptation to do that. You just have to keep in mind that all the really successful DSLs have remained little languages, and repeat that to yourself every day. If you do that, you’ll be fine. Or, put another way:

Remember kids, DSLs don’t kill people, people kill people.

I am not a theorist…

As I’m sure regular followers of my blog are aware, I have a weakness for disclaimers. As I’ve starting thinking about how I want to talk in more detail about how the MGrammar parser generator works, I felt compelled to make yet another disclaimer: I am not a computer science theorist.

The field of parser generation is an old and storied one (at least, relative to a lot of other things having to do with computers) and there’s a lot of theory that’s built up around it. I simply ask my readers to keep in mind that I am but a humble practitioner, same as they are, and I make no great claims to deep insight or great knowledge of the full theory of parsers. I can not hope to even touch the hem of the garment of those who have gone before me, particularly these guys:

Book Cover

So if you really want to understand the difference between a LL(k) and a LALR(1) parser, go buy the book. Otherwise, you’re stuck with my simplistic explanations.

MGrammar and Rescuing the Princess

One of the major reasons that I decided to come to work on the Oslo team was the experience I had with what was going to become MGrammar. I was interested in prototyping some language and compiler design ideas, and I knew that the Oslo team had some technologies that might help me out, specifically a parser generator. They helpfully pointed me to their source code, I enlisted, built and started to play around. I’d been building some parser technology by hand, but I quickly discarded it once I started playing around with MGrammar in Intellipad. In addition to giving me a bunch of things that I needed, it was also just really. fun. I mean, it was cool to bring up a buffer, start hammering on a syntax and then put in an example and see the result pop up. Or, as was the case most of the time, not pop up. Tweak, tweak, tweak and then. boom! the result shows up.

This storyline is one that I’ve heard a number of times internally and even seen externally a time or two. People just have a great time playing around with grammars and languages and seeing what they can get to come out. There’s frustration, sure, but then there’s also a big payoff when you finally see it working-look what I created!

I hadn’t really thought a lot about it until I came across a presentation that Daniel Cook (author of the Lost Garden blog) gave at an OfficeLabs meeting entitled “Mixing Games and Applications.” He says it a lot better than I can, but I intuitively think that I (and others) enjoy fooling around with MGrammar in Intellipad precisely because it taps in to game playing part of our brain. I’m not sure how to capitalize on that further, but it’s interesting food for thought.

Answering some questions about MGrammar….

As some readers have noticed, I’ve been conspicuously silent since I moved over to the Oslo team. Some of this had to do with getting onto a new blogging engine, some of it had to do with various distractions like jury duty, but a lot of it had to do with the fact that Visual Basic 10.0 has gotten to a pretty stable state (so there isn’t so much for me to say any more) and I am just getting off the ground with Oslo. Not having much useful to say, I figured I’d be better off saying nothing at all rather than continuing to blather on and on about nothing in particular (that’s what Twitter is for).

Part of the process of moving over to Oslo was figuring out exactly what my role was going to be. There’s lots of cool stuff going on and a lot of smart people with strong ideas about things, so I knew it would take a while for things to settle out. Things will still probably evolve over time, but the area I’ve spent the most time on, and the thing I’ll have the most influence on for the time being, is MGrammar. It’s been an interesting experience because I’ve had to relearn a bunch of the theory around parser generation that I had forgotten in my time in VB (since we had a hand-built recursive descent parser). But it’s been enjoyable, and I think there are a lot of neat things we can do with it. So stay tuned on that.

Someone forwarded me Kevin Moore’s 20 questions/thoughts around MGrammar, and I thought answering some of those questions might be a good entree into blogging about my new job. I won’t address all of his questions, but the interesting ones, in my opinion, were:

Could C#/VB implement their compilers using MGrammar? Should they? Would they?

I believe that, as the languages stand today, C# could implement their parser using MGrammar but VB couldn’t. The reason why VB couldn’t is a pretty technical issue relating to the embedding of XML into VB which we may end up solving as well, but given the hacks that we (VB) had to do to the hand generated parser to get it to work I don’t think that reflects badly on MGrammar (or VB for that matter). The bottom line is that the parser generator is certainly powerful enough to handle an industrial-strength language. Whether the C# or VB teams should or will do anything with MGrammar is really a question for them. Both languages have hand-built parsers which allows an extraordinary degree of control over the behavior of a parse and, in particular, error messages and error recovery. They may be loath to give that up, but we’ll see.

Where is the MGrammar for C#? .for VB? .for CSV? .for VS solution files? .for XML?

In general, the Oslo team is just one little team and can only do so much. Although we endeavor to provide you with the maximum number of samples, in truth we are limited in what we can do at any one time. You should expect to see more samples as time goes on. I’ll also note, though, that some of those languages do present interesting challenges to a parser generator. I’ve already covered VB, but XML also has challenges, which I think is really a subject for another entry.

There should be standardized or defacto escape mechanisms for existing grammars.

If I understand the point here correctly, what’s really being asked for is composability of grammars. This is a difficult, but interesting, area of work. To use an example I’ve already referenced, there were a lot of challenges integrating XML into VB because the languages were very different, even down to the lexical level. Having languages compose in some kind of simple way (even to do things like add or override parts of an existing grammar) is an open question that we’re very interested in but we’re still working on.

Will there be tools to go back from a logical model (MSchema) to the input format?

This is on our radar, but we don’t have an answer at the moment. It’s a very natural and logical thing to do, but (of course) running a grammar in reverse is not a trivial matter. I’m not sure we can get to a point where we can reverse a parse automatically, but we do want to provide people with a way to do this in as simple a manner as possible.

Where is the community site for MGrammar?

Well, we’ve got the Oslo Developer Center and the Oslo forums. If you want to start a community site, by all means go ahead!

Any news with Mono? Is Miguel interested?

I think you’d have to ask Miguel that.

Are you making a play to make MGrammar a standard? I’ve heard yes, but just wanted to confirm.

What we’ve committed to is releasing the MGrammar specification (well, actually, the whole M language specification) under the Open Specification Promise. That’s the only commitment we’ve made so far regarding MGrammar’s specification, but it’s a pretty big one.

How is the computer science behind MGrammar?

Pretty good, and I expect it to get even better. Expect to hear more here and elsewhere about internal specifics as time goes on (it’s a little more than can be covered in one blog post).

I hope that answers some questions, and feel free to ask more questions here or on the forums. Thanks!