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Please use stacknn-core instead!


This project implements differentiable stacks and queues in PyTorch. The data structures are implemented in such a way that it should be easy to integrate them into your own models. For example, to construct a differentiable stack and perform a push:

from StackNN.structs import Stack
read_vectors = stack(value_vectors, pop_strengths, push_strengths)

For examples of more complex use cases of this library, refer to the industrial-stacknns repository.

All the code in this repository is associated with the paper Context-Free Transductions with Neural Stacks, which appeared at the Analyzing and Interpreting Neural Networks for NLP workshop at EMNLP 2018. Refer to our paper for more theoretical background on differentiable data structures.

Running a demo

Check example.ipynb for the most up-to-date demo code.

There are several experiment configurations pre-defined in To train a model on one of these configs, do:


For example, to train a model on the string reversal task:

python final_reverse_config

In addition to the experiment configuration argument, takes several flags:

  • --model: Model type (BufferedModel or VanillaModel)
  • --controller: Controller type (LinearSimpleStructController, LSTMSimpleStructController, etc.)
  • --struct: Struct type (Stack, NullStruct, etc.)
  • --savepath: Path for saving a trained model
  • --loadpath: Path for loading a model


You can find auto-generated documentation here.


This project is managed by Computational Linguistics at Yale. We welcome contributions from outside in the form of pull requests. Please report any bugs in the GitHub issues tracker. If you are a Yale student interested in joining our lab, please contact Bob Frank.


If you use this codebase in your research, please cite the associated paper:

    title = "Context-Free Transductions with Neural Stacks",
    author = "Hao, Yiding  and
      Merrill, William  and
      Angluin, Dana  and
      Frank, Robert  and
      Amsel, Noah  and
      Benz, Andrew  and
      Mendelsohn, Simon",
    booktitle = "Proceedings of the 2018 {EMNLP} Workshop {B}lackbox{NLP}: Analyzing and Interpreting Neural Networks for {NLP}",
    month = nov,
    year = "2018",
    address = "Brussels, Belgium",
    publisher = "Association for Computational Linguistics",
    url = "",
    pages = "306--315",
    abstract = "This paper analyzes the behavior of stack-augmented recurrent neural network (RNN) models. Due to the architectural similarity between stack RNNs and pushdown transducers, we train stack RNN models on a number of tasks, including string reversal, context-free language modelling, and cumulative XOR evaluation. Examining the behavior of our networks, we show that stack-augmented RNNs can discover intuitive stack-based strategies for solving our tasks. However, stack RNNs are more difficult to train than classical architectures such as LSTMs. Rather than employ stack-based strategies, more complex stack-augmented networks often find approximate solutions by using the stack as unstructured memory.",


The core implementation of the data structures is stable in Python 2 and 3. The specific tasks that we have implemented require Python 2.7. We use PyTorch version 0.4.1, with the following additional dependencies:

  • numpy
  • scipy (for data processing)
  • matplotlib (for visualization)
  • nltk

Using pip or conda should suffice for installing most of these dependencies. To get the right command for installing PyTorch, refer to the installation widget on the PyTorch website.


A model is a pairing of a controller network with a neural data structure. There are two kinds of models:

  • models.VanillaModel is a simple controller-data structure network. This means there will be one step of computation per input.
  • models.BufferedModel adds input and output buffers to the vanilla model. This allows the network to run for extra computation steps.

To use a model, call model.forward() on every input and model.init_controller() whenever you want to reset the stack between inputs. You can find example training logic in the tasks package.

Data structures

  • structs.Stack implements the differentiable stack data structure.
  • structs.Queue implements the differentiable queue data structure.

The buffered models use read-only and write-only versions of the differentiable queue for their input and output buffers.


The Task class defines specific tasks that models can be trained on. Below are some formal language tasks that we have explored using stack models.

String reversal

The ReverseTask trains a feed-forward controller network to do string reversal. The code generates 800 random binary strings which the network must reverse in a sequence-to-sequence fashion:

Input:   1 1 0 1 # # # #
Label:   # # # # 1 0 1 1

By 10 epochs, the model tends to achieve 100% accuracy. The config for this task is called final_reverse_config.

Context-free language modelling

CFGTask can be used to train a context-free language model. Many interesting questions probing linguistic structure can be reduced to special cases of this general task. For example, the task can be used to model a language of balanced parentheses. The configuration for the parentheses task is final_dyck_config.

Evaluation tasks

We also have a class for evaluation tasks. These are tasks where output i can be succintly expressed as some function of inputs 0, .., i. Some applications of this are evaluation of parity and reverse polish boolean formulae.

Real datasets

The data folder contains several real datasets that the stack can be trained on. We should implement a task for reading in these datasets.


Experiments with differentiable stacks and queues in PyTorch



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