This is a Python/DyNet implementation of the neural stack LSTM model presented in the following paper:

Grefenstette, Edward, Karl Moritz Hermann, Mustafa Suleyman, and Phil Blunsom. "Learning to transduce with unbounded memory." In Advances in Neural Information Processing Systems, pp. 1828-1836. 2015.

The purpose of this repository is to serve as a reference implementation for those who might be interested in reproducing the results presented in this paper, as fine-tuning the model to converge on algorithmic behavior can prove to be quite challenging. This repository includes a script for training a stack LSTM on the sequence reversal transduction task.

This code runs in Python 3 and requires a relatively recent (as of October 2017) version of DyNet. As DyNet is under active development, it is recommended to use the script ./setup to install the exact version of DyNet tested with this code into an isolated virtualenv.

./setup

Instructions for installing the virtualenv command can be found here.

In order to run a script inside the virtualenv, first source its activate script, then run the python script as usual.

. virtualenv/bin/activate
python train.py

The script train.py can be used to train and test a stack LSTM on the sequence reversal transduction task as described in the paper. Note that it is rather typical for this architecture to get permanently stuck in a local minimum during training, so you may need to try more than once to get a successful run. Using the default values for train.py, you should be able to successfully train a model about 80% of the time.

Note that the default size of the model is quite large, and the script is best run on GPU in this case. If you want to test out a smaller version of the model first, try running train.py with smaller values for number of hidden units, embedding size, etc. You can control the dimensions of the model trained, number of training samples, learning rate, and more by passing command line arguments to train.py. You can also save the parameters of the trained model to an output file. Run python train.py --help for more details.

For example:

python train.py \
    --hidden-units 20 \
    --source-alphabet-size 2 \
    --embedding-size 5 \
    --stack-embedding-size 3 \
    --training-length-range 10,10 \
    --test-length-range 10,10 \
    --test-data-size 10 \
    --output parameters.txt

The script test.py can be used to run just the test phase on a pre-trained model. Note that the model dimensions passed to test.py need to match those used with train.py.

As noted by the authors, success during training is extremely sensitive to parameter initialization. In my experiments, I have found that initializing the parameters for the push and pop signal layers close to 0 has a marked improvement on success rate, so they are initialized with an Xavier initialization gain of 0.5 in this implementation, as opposed to the recommended value of 4. Success is also very sensitive to the choice of optimizer (RMSProp in this case) and initial learning rate. The set of default values for train.py is one that happens to work well. If you deviate from these default values, you will need to fiddle a lot with the various arguments to train.py to get an experimental setup that succeeds.

The source code for the model is found in stack_lstm.py.

Brian DuSell