This is the first part in a five-part series that will cover implementing an earley parser for linguistic utterances.
- Part 1 - Why?, and some theory
- Part 2 - The Earley recognizer
- Part 3 - Felling the Earley parse forest
- Part 4 - Feature-structures and unification
- Part 5 - Wrapping up
Why write this series?
There are already many great explanations of the Earley parse algorithm, which is the parsing algorithm that we'll be using. There are also some great books on feature-structure based linguistic grammars, which I highly recommend. However, as far as I know nobody has put the two together into a beginner-friendly, implementation-oriented tutorial that explains what these tools are, how to use them together, and perhaps most importantly why someone would want to. This series will be trying to explain this through an iterative approach, starting with the simplest possible solution, and expanding that solution only when necessary. The goal is that, when we finish, we'll have the simplest possible solution to our goal, with a codebase that has every line justified.
What are we making, exactly?
The exact thing we're going to be building is a symbolic, linguistic grammar model. Those adjectives describe the somewhat awkward space the thing we're going to be building is in. We'll be using symbolic, rule-based techniques to parse language, but we need to parse a wider variety of structures than most rule-based parsers. And we'll be parsing language, but in a rule-based way, not in a learned way like a neural network. This combination has some serious advantages, and is worth examining in detail.
The other spot we're stuck between are the now-ubiquitous linguistic models that are trained on data — usually based on neural networks. These models, such as BERT, take mountains of data — gigabytes of text — and train a model to do some basic linguistic task, such as predicting missing words. This gives the model an understanding of how the language works. Other engineers can then take this pre-trained language model, "fine tune" it on a smaller (but still substantial) amount of data that more closely matches their specific task, and stick another network on the back that spits out whatever they need.
However, symbolic linguistic models are still alive and kicking in this space, both in the emerging field of neural-symbolic computing, and in very mature, wide-coverage linguistic models such as the English Resource Grammar and the grammar matrix. These symbolic grammars are still used because they have some distinct advantages of trained models: they are predictable, they are analyzable, and they don't require large amounts of training data. Our grammar will be built on these principles, and while it will not have as wide of coverage as the ERG (which has >99% coverage of the New York Times corpus!), it will reach levels of linguistic understanding that a trained model would need mountains of data to reach. On the other hand, our grammar will show the downsides of this method as well: while a neural network can make a "best guess" at a grammar construction its never seen before, that's near-impossible for a symbolic model. If we've only implemented simple sentences with one subject and one object (transitive), our model will choke the first time it sees "Mary gave Sue the book", which has one subject and two objects, "Sue" and "the book" (ditransitive). If we then only implement this ditransitive pattern, it will choke on "Mary gave the book to Sue", where "Sue" is marked with the preposition "to" (dative alternation). Symbolic grammar engineering is a cycle of implementing patterns and finding more patterns your model doesn't yet handle. Eventually, you have to stop. For our purposes, a simple, low-coverage model will be enough to illustrate the important concepts.
Where are we going?
For my specific use case, I'm working on a game, based around the player learning an alien language that I've created. (It's called Themengi, and you can check it out here.) I want to parse player input, both imperative commands and dialogue, and respond appropriately. This language does not exist; there is no training corpus, so a trained model is not possible. On the other hand, this is a language, not an obfuscated version of SQL, so I don't want to simply apply programming-language techniques. I want to be able to handle ambiguity, case, and other linguistic features. In my search, I couldn't find a framework that did quite what I needed, so I decided to write my own. This post series will follow my progress working on this framework.
Along the way, I'll be illustrating the concepts with the running example of creating a tiny English grammar. I'm going to start from first principles, assuming only a little knowledge of grade-school English grammar terms (noun, verb, etc.). We'll start with a simple system of rules, write a recognizer, extend that into a parser, and annotate that parser with linguistic information. Then we'll have some fun at the end, maybe we'll write a tiny grammar for Japanese and do some symbolic translation or write a mini rules-based digital assistant demo. The possibilities are endless, we could write a grammar for Klingon and make the first automated Japanese -> Klingon translation! Who knows!
So, with that out of the way, let's get going!