Recursive Tree Grammar Autoencoders

Machine learning on tree data has been mostly focused on trees as input. Much less research has investigates trees as output, like in molecule optimization for drug discovery or hint generation for intelligent tutoring systems. In this work, we propose a novel autoencoder approach, called recursive tree grammar autoencoder (RTG-AE), which encodes trees via a bottom-up parser and decodes trees via a tree grammar, both controlled by neural networks that minimize the variational autoencoder loss. The resulting encoding and decoding functions can then be employed in subsequent tasks, such as optimization and time series prediction. RTG-AE combines variational autoencoders, grammatical knowledge, and recursive processing. Our key message is that this combination improves performance compared to only combining two of these three components. In particular, we show experimentally that our proposed method improves the autoencoding error, training time, and optimization score on four benchmark datasets compared to baselines from the literature.