MeTA also provides models that can be used for part-of-speech tagging. These models, at the moment, are designed for tagging English text, but they should be able to be trained for any language desired once appropriate feature extractors are defined.

To use these models, you should download a tagger model file from the releases page on the Github repository. MeTA currently has two different POS-tagger models available:

# Using Taggers

## Using the CRF

First, extract your model files into a directory. You should modify your config.toml to contain a [crf] group like so:

[crf]
prefix = "path/to/crf/model/folder"

where prefix has been set to the folder that contains the model files you extracted.

### Interactive tagging

You can interactively tag sentences using the provided pos-tag tool.

./pos-tag config.toml

This application will load the CRF model, and then proceed to tag sentences typed in at the prompt. You can stop tagging by simply inputting a blank line.

### As an analyzer

The CRF model can also be used as an analyzer during index creation to create features based on n-grams of part-of-speech tags. To do so, you would need to add an analyzer group to your configuration that looks like the following:

[[analyzers]]
method = "ngram-pos"
ngram = 2 # the order n-gram you want to generate
filter = [{type = "icu-tokenizer"}, {type = "ptb-normalizer"}]
crf-prefix = "path/to/crf/model/folder"

You can alter the filter chain if you would like, but we strongly recommend sticking with the above setup as it is designed to match the original Penn Treebank tokenization format that the supplied model is trained on.

### Programmatically

To use the CRF inside your own program, your code might look like this:

// load the model
meta::sequence::crf model{"path/to/crf/model/folder"};

// create a tagger
auto tagger = crf.make_tagger();

// load the sequence analyzer (for feature generation)
auto analyzer = meta::sequence::default_pos_analyzer();

meta::sequence::sequence seq;

// tag a sequence
const auto& ana = analyzer; // access the analyzer via const ref
// so that no new feature ids are generated
ana.analyze(seq);
tagger.tag(seq);

// print the tagged sequence
for (const auto& obs : seq)
std::cout << obs.symbol() << "_" << analyzer.tag(obs.label()) << " ";
std::cout << "\n";

Have a look at the API documentation for the meta::sequence::crf class for more information.

## Using the greedy tagger (Perceptron)

First, extract your model files into a directory. You should modify your config.toml to contain a [sequence] group like so:

[sequence]
prefix = "path/to/perceptron/model/folder/"

where prefix has been set to the folder that contains the model files you extracted.

### Interactive tagging

The pos-tag tool doesn’t currently use this tagger (patches welcome!), but you can still interactively tag sentences using the profile tool. See the profile tutorial for a walkthrough of that demo application.

### Programmatically

To use the greedy Perceptron-based tagger inside your own program, your code might look like this:

// load the model
meta::sequence::perceptron tagger{"path/to/perceptron/model/folder"};

meta::sequence::sequence seq;

// tag a sequence
tagger.tag(seq);

// print the tagged sequence
for (const auto& obs : seq)
std::cout << obs.symbol() << "_" << obs.tag() << " ";
std::cout << "\n";

This API is a bit simpler than that of the CRF. For more information, you can check the API documentation for the meta::sequence::perceptron class.

# Training Taggers

In order to train your own models using our provided training programs, you will need to have a copy of the Penn Treebank (v2) extracted into your data prefix (see the overview tutorial). Your folder structure should look like the following:

prefix
|---- penn-treebank
|---- treebank-2
|---- tagged
|---- wsj
|---- 00
|---- 01
...
|---- 24


## Training a CRF

To train your own CRF model from the Penn Treebank data, you should be able to use the provided crf-train executable. You will first need to adjust your [crf] group in your config.toml to look something like this:

[crf]
prefix = "desired/crf/model/location"
treebank = "penn-treebank" # relative to data prefix
corpus = "wsj"
section-size = 99
# these are the standard training/development/testing splits for POS tagging
train-sections = [0, 18]
dev-sections = [19, 21]
test-sections = [22, 24]

You should now be able to run the training procedure:

./crf-train config.toml

This will train a CRF model using the default training options. For more information on the options available, please see the API documentation for the meta::sequence::crf class (in particular, the parameters struct). If you would like to try different options, you can use the code provided in src/sequence/crf/tools/crf_train.cpp as a starting point. You will need to change the call to crf.train() to use a non-default parameters struct.

The model will take several hours to train. Its termination is based on convergence of the loss function.

## Training a greedy Perceptron-based tagger

To train your own greedy tagger model from the Penn Treebank data, you should be able to use the provided greedy-tagger-train executable. You will need to first adjust your [sequence] group in your config.toml to look something like this (very similar to the above):

[sequence]
prefix = "desired/perceptron/model/location"
treebank = "penn-treebank" # relative to data prefix
corpus = "wsj"
section-size = 99
# these are the standard training/development/testing splits for POS tagging
train-sections = [0, 18]
dev-sections = [19, 21]
test-sections = [22, 24]

You should now be able to run the training procedure:

./greedy-tagger-train config.toml

This will train the averaged Perceptron model using the default training options. The termination criteria is simply a maximum iteration count, which defaults to 5 as of the time of writing. This means that the greedy tagger is signifigantly faster to train than its corresponding CRF model. In practice, the two achieve nearly the same accuracy with our default settings (the CRF being just slightly better).

If you want to adjust the number of training iterations, you can use the code provided in src/sequence/tools/greedy_tagger_train.cpp as a starting point. You will need to change the call to tagger.train() to use a non-default training_options struct.

# Testing Taggers

If you follow the instructions above for the tagger type you wish to test, you should be able to test them with their corresponding testing executables. For the CRF, you would use

./crf-test config.toml

and for the greedy Perceptron-based tagger, you would use

./greedy-tagger-test config.toml

Both will run over the testing section defined in config.toml and report precision, recall, and F1 score for each class, as well as the overall token-level accuracy. The current CRF model achieves 97% accuracy, and the greedy Perceptron model achieves 96.9% accuracy.