Hanstholm is a dependency parser that deals with ambiguity in part-of-speech tags
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README.md

Hanstholm - dependency parser with an open feature model

Building

Building hanstholm requires as working installation of CMAKE and the Boost library.

Once the requirements are installed, checkout the project. In the top-level directory, issue:

mkdir build
cd build
cmake -DCMAKE_BUILD_TYPE=Release ..
make

This builds the hanstholm binary in the build directory.

Running

Example. Train the parser on data/train.txt, passing over the data 50 times. After training, evaluate the final model on data/test.txt, placing the predictions in out/test_pred.tsv. The feature model is specified by nivre.txt.

./hanstholm \
--passes 50 \
--template nivre.txt \
--data data/train.txt \
--eval data/test.txt \
--predictions out/test_pred.tsv

Data format

The input file format borrows the concept of feature namespaces and most of the syntax from Vowpal Wabbit. Here is an example of the input:

1-compmod '1-0|w ms. |p NOUN
2-nsubj '1-1|w haag |p NOUN
-1-ROOT '1-2|w plays |p VERB
2-dobj '1-3|w elianti |p NOUN
2-p '1-4|w . |p .

Breaking down the first line,

1-compmod '1-0|w ms. |p NOUN

The label 1-compmod tells us that the head of the first token (index 0) is 1 with the relation compmod. The part after the label is the identifier, which can be any string. The identifier is deliminated in one end by the ' symbol and in the other end by the bar, |. The rest are feature definitions. A bar indicates the beginning of a new namespace and the string touching the bar is the namespace identifier. Thus we have two features: w^ms. and p^NOUN.

No value is given to the features in the example, so they assume the default of 1.0. To specify a feature value, place it the end of the feature name, separated by a colon. E.g. p^NOUN:2.5. Aside from this use, colons cannot be used as part of feature names.

Feature specification

Transition-based parsing has a classification problem at its core, which is to decide which transition to perform next. During parsing, features are extracted based on the current configuration of the stack S and buffer N, and the resulting feature vector is scored by taking the dot product with a weight vector.

In Hanstholm, the features are specified in a plain-text template file. Each line in the file describes a single feature, which may be a conjunction of several feature primitives. Again, we go by example. The line,

S0:w ++ S0:p

is a conjumction of the primitives S0:w and S0:p, with ++ being the conjunction operator. I.e. each distinct combination of word and part-of-speech tags for the token at the top of the stack (position S0) becomes a separate parameter in the model. The numeric value of the feature conjunction is the product of the feature primitives.

Note that the names w and p have no special meaning; they simply refer to the namespaces of the input file. In general, a namespace may contain more than one feature. For instance, a part-of-speech namespace could include the top-k tags, weighted by their probability. Another use-case is word embeddings, which have multiple dimensions. In those cases, the feature template expands to the Cartesian product of the namespaces. Concretely, assume that the token represented by the input line below is in the S0 position.

|w tail |p NOUN:0.7 VERB:0.3

Then the S0:w ++ S0:p conjunction feature template would generate the following two features:

tail+NOUN:0.7 tail+VERB:0.3