Shift-Reduce Constituency Parser

Introduction

Previous versions of the Stanford Parser for constituency parsing used chart-based algorithms (dynamic programming) to find the highest scoring parse under a PCFG; this is accurate but slow. Meanwhile, for dependency parsing, transition-based parsers that use shift and reduce operations to build dependency trees have long been known to get very good performance in a fraction of the time of more complicated algorithms. Recent work has shown that similar shift-reduce algorithms are also effective for building constituency trees.

Based on this work, we built a Shift-Reduce Parser which is far faster than previous versions of the Stanford Parser while being more accurate than any version other than the RNN parser.

Acknowledgements

Stanford's Shift-Reduce Parser was written by John Bauer. It is based on the prior work of several other researchers:

Obtaining the software

You must download the following packages:

Using the Shift-Reduce Parser using Stanford CoreNLP

Read this if you just want to use the Shift-Reduce Parser from the command-line.

The new parser is integrated into Stanford CoreNLP. The simplest way to use it is to give StanfordCoreNLP the flag

-parse.model <location>

Using resources from a release of stanford-srparser-YYYY-MM-DD-models.jar , you might use:

-parse.model edu/stanford/nlp/models/srparser/englishSR.ser.gz

On the Stanford cluster, the location is in /u/nlp/data/srparser , so

-parse.model /u/nlp/data/srparser/englishSR.ser.gz

There are other models as well. For example, there is a model trained to use beam search. By default, this model will use a beam of size 4. If you want to change that, you can use the flag -parse.flags " -beamsize 4" Note that the space after the quote is necessary for our options processing code. The full list of models currently distributed is:

edu/stanford/nlp/models/srparser/arabicSR.ser.gz

edu/stanford/nlp/models/srparser/englishSR.beam.ser.gz

edu/stanford/nlp/models/srparser/englishSR.ser.gz

edu/stanford/nlp/models/srparser/germanSR.ser.gz

edu/stanford/nlp/models/srparser/chineseSR.ser.gz

edu/stanford/nlp/models/srparser/frenchSR.beam.ser.gz

Calling Parsing from Java

Read this if you want to call the Shift-Reduce Parser from your own code. In the parser package, there is a ShiftReduceDemo.java class.

To compile it (the two jar files are provided by Stanford Parser and the Stanford Tagger, respectively):

javac -cp stanford-parser.jar:stanford-postagger-3.5.0.jar ShiftReduceDemo.java

java -cp .:stanford-parser.jar:stanford-postagger-3.5.0.jar:stanford-srparser-2014-10-23-models.jar ShiftReduceDemo -model edu/stanford/nlp/models/srparser/englishSR.ser.gz -tagger english-left3words-distsim.tagger

Reading POS tagger model from stanford-postagger-full-2014-10-23/models/english-left3words-distsim.tagger ... done [1.2 sec].

Loading parser from serialized file edu/stanford/nlp/models/srparser/englishSR.ser.gz ...done [14.8 sec].

(ROOT (S (NP (PRP$ My) (NN dog)) (VP (VBZ likes) (S (VP (TO to) (VP (VB shake) (NP (PRP$ his) (VBN stuffed) (NN chickadee) (NN toy)))))) (. .)))

How Shift-Reduce Parsing Works

To run it:Note that we need to include the jar file where the parser models are stored, as well as specifying the tagger model (which came from the Stanford Tagger package). This should load the tagger, parser, and parse the example sentence, finishing in under 20 seconds. The output:

The Shift-Reduce Parser parses by maintaining a state of the current parsed tree, with the words of the sentence on a queue and partially completed trees on a stack, and applying transitions to the state until the queue is empty and the current stack only contains a finished tree.

The initial state is to have all of the words in order on the queue, with an empty stack. The transitions which can be applied are:

Shift. A word moves from the queue onto the stack.

Unary reduce. The label of the first constituent on the stack changes. There is a different unary transition for every possible unary node in the treebank used for training.

Binary reduce. The first two nodes on the stack are combined with a new label. These are either right sided or left sided, indicating which child is treated as the head. Once again, there is a different binary transition for every possible binary node. This includes temporary nodes, as trees are built as binarized trees and then later debinarized.

Finalize. A tree is not considered finished until the parser chooses the finalize transition.

Idle. In the case of beam searching, Zhu et al. showed that training an idle transition compensates for different candidate trees using different numbers of transitions.

Transitions are determined by featurizing the current state and using a multiclass perceptron to determine the next transition. Various legality constraints are applied to the transitions to make sure the state remains legal and solvable.

Part-of-speech tags are not assigned by this parser, and are in fact used as features. This is accomplished by pretagging the text, meaning the pos annotator needs to be used in StanfordCoreNLP, for example.

Training is conducted by iterating over the trees repeatedly until some measure of convergence is reached. There are various ways to process the trees during each iteration. The one used by default is to start from the basic state and apply the transitions chosen by the parser until it gets a transition wrong, i.e., it can no longer rebuild the gold tree. The features used at the time of the incorrect decision have their weights adjusted, with the correct transition getting the feature weights increased and the incorrect transition decreased.

Beam Search

In general, the parser uses greedy transitions, continuing until the sentence is finalized. It is also possible to use it in beam search mode, though. In this mode, the parser keeps an agenda of the highest scoring candidate states. At each step, each of the states has a transition applied, updating the agenda with the new highest scoring states. This process continues until the highest scoring state on the agenda is finalized.

Models need to be specifically trained to work with beam search. Otherwise, scores actually get worse. While this increases accuracy quite a bit, it also has the drawback of significantly increasing the size of the model. A model not trained for beam search only ever has features which were present in states reached by the gold sequence of transitions on the gold training trees. A model trained to use beam search trains negative features for incorrect states on the beam, resulting in many more features and therefore a much larger model.

Training the Shift-Reduce Parser

Read this section if you want to train your own Shift-Reduce Parser. This requires you have data in the standard Treebank format.

Basic Guidelines

New English WSJ models can be trained as follows:

java -mx10g edu.stanford.nlp.parser.shiftreduce.ShiftReduceParser -trainTreebank <trainpath> -devTreebank <devpath> -serializedPath model.ser.gz

Concretely, on the NLP machines, this would be

java -mx10g edu.stanford.nlp.parser.shiftreduce.ShiftReduceParser -trainTreebank /afs/ir/data/linguistic-data/Treebank/3/parsed/mrg/wsj 200-2199 -devTreebank /afs/ir/data/linguistic-data/Treebank/3/parsed/mrg/wsj 2200-2219 -serializedPath model.ser.gz

More details on how it trains are below. The key summary is that some time later, probably an hour on a decent machine, there will be a new file model.ser.gz which is the newly trained Shift-Reduce Parser. This model can be tested as follows:

java -mx6g edu.stanford.nlp.parser.shiftreduce.ShiftReduceParser -testTreebank <testpath> -serializedPath model.ser.gz



java -mx6g edu.stanford.nlp.parser.shiftreduce.ShiftReduceParser -testTreebank /afs/ir/data/linguistic-data/Treebank/3/parsed/mrg/wsj 2300-2399 -serializedPath model.ser.gz

However, this ignores a key aspect of the Shift-Reduce Parser. This parser does not produce its own tags and instead expects to use automatically produced tags as features when parsing. The commands above will work, but they will train a model using the gold tags in the treebank. It is generally better to train on the tags provided by the tagger which will be used as test time. This can be done with the flags -pretag -taggerSerializedFile <tagger>

java -mx10g edu.stanford.nlp.parser.shiftreduce.ShiftReduceParser -trainTreebank /afs/ir/data/linguistic-data/Treebank/3/parsed/mrg/wsj 200-2199 -devTreebank /afs/ir/data/linguistic-data/Treebank/3/parsed/mrg/wsj 2200-2219 -preTag -taggerSerializedFile /u/nlp/data/pos-tagger/distrib/wsj-0-18-bidirectional-nodistsim.tagger -serializedPath model.ser.gz



java -mx6g edu.stanford.nlp.parser.shiftreduce.ShiftReduceParser -devTreebank /afs/ir/data/linguistic-data/Treebank/3/parsed/mrg/wsj 2300-2399 -preTag -taggerSerializedFile /u/nlp/data/pos-tagger/distrib/wsj-0-18-bidirectional-nodistsim.tagger -serializedPath model.ser.gz

The bidirectional model is significantly slower than the left3words model, but is somewhat more accurate. It is still faster than the parsing itself. Alternatively, one can just use the left3words tagger for better speed and slightly less accuracy.

Other Languages

It is possible to train the Shift-Reduce Parser for languages other than English. An appropriate HeadFinder needs to be provided. This and other options are handled by specifying the -tlpp flag, which lets you choose the class for a TreebankLangParserParams . A language appropriate tagger is also required.

For example, here is a command used to train a Chinese model. The options not already explained are explained in the next section.

java -mx10g edu.stanford.nlp.parser.shiftreduce.ShiftReduceParser -trainTreebank /u/nlp/data/chinese/ctb7/train.mrg -devTreebank /u/nlp/data/chinese/ctb7/dev_small.mrg -preTag -taggerSerializedFile /u/nlp/data/pos-tagger/distrib/chinese-nodistsim.tagger -serializedPath chinese.ser.gz -tlpp edu.stanford.nlp.parser.lexparser.ChineseTreebankParserParams -trainingThreads 4 -batchSize 12 -trainingIterations 100 -stalledIterationLimit 20

The resulting model is both faster and more accurate than any other model we have, including the Chinese RNN model.

FeatureFactory classes

The features for the Perceptron are extracted using a FeatureFactory. By default, the parser uses edu.stanford.nlp.parser.shiftreduce.BasicFeatureFactory . This FeatureFactory includes most of the basic features you would want, such as features for the head word of the current stack node and several previous stack nodes, the word and tag of incoming queue items, and combinations of those strings.

Another included FeatureFactory is the DistsimFeatureFactory. This can be used by setting the -featureFactory argument:

-featureFactory edu.stanford.nlp.parser.shiftreduce.DistsimFeatureFactory(<path_to_distsim>)

Multiple FeatureFactory classes can be combined by using a semicolon separated list. For example:

-featureFactory edu.stanford.nlp.parser.shiftreduce.BasicFeatureFactory;edu.stanford.nlp.parser.shiftreduce.DistsimFeatureFactory(<path_to_distsim>)

Additional Options

‑trainingMethod See below. ‑beamSize Size of the beam to use when running beam search. 4 is already sufficient to greatly increase accuracy without affecting speed too badly. ‑trainBeamSize Size of the beam to use when training. ‑trainingThreads Training can be run in parallel. This is done by training on multiple trees simultaneously. ‑batchSize How many trees to batch together when training. This allows training in parallel to get repeatable results, as each of the trees are scored using the weights at the start of the training batch, and then all updates are applied at once. ‑trainingIterations The maximum number of iterations to train. Defaults to 40. ‑stalledIterationLimit The heuristic for ending training before -trainingIterations iterations is to stop when the current dev set score has not improved for this many iterations. Defaults to 20. ‑averagedModels When the perceptron has finished training, in general, the model with the best score on the dev set is kept. This flag averages together the best K models and uses that as the final model instead. Defaults to 8. This has the potential to greatly increase the amount of memory needed, so can be set to a lower number if memory is a barrier. ‑featureFrequencyCutoff If a feature is not seen this many times when training, it is removed from the final model. This can eliminate rarely seen features without impacting overall accuracy too much. It is especially useful in the case of model training using a beam (or oracle, if that method is ever made to work), as that training method results in many features that were only seen once and don't really have much impact on the final model. ‑saveIntermediateModels By default, training does not save the intermediate models any more, since they basically don't do anything. Use this flag to turn it back on. ‑featureFactory The feature factory class to use.

There are several training methods implemented. The default is EARLY_TERMINATION , in which training on an individual tree is halted as soon as the current model is incorrect. Alternatives are:

-trainingMethod GOLD Force the parser to make the correct transition while training, continuing after errors. Takes longer than EARLY_TERMINATION and does not improve accuracy. EARLY_TERMINATION As soon as the current model gets a transition wrong when parsing a tree, stops training on that tree for this iteration. BEAM An agenda of the highest scoring candidate states is kept. Training continues until the gold state is no longer on the agenda. At each step, the gold transition for the gold state gets its feature weights increased, and transition chosen for the highest scoring state gets its feature weights decreased. ORACLE An experimental training method in which an oracle is used to figure out what transition would result in the best possible parse from the current state. Unfortunately, this results in poor scores, either because of a bug in the oracle training code or incorrect oracle logic.

Performance

The tables below summarize the Shift-Reduce Parser's performance, based on experiments run in 2014.

English Penn WSJ sec 23 (all lengths) Model Loading

(sec.) Parsing

(sec.) LP/LR

F 1 wsjPCFG.ser.gz 1.0 426 85.54 wsjSR.ser.gz 4.5 14 85.99 wsjFactored.ser.gz 7.6 1759 86.53 wsjSR.beam.ser.gz, beam size 4 15.4 34 88.55 wsjRNN.ser.gz 2.6 1038 89.96