Syntax-aware Transformer Encoder for Neural Machine Translation

Syntax has been shown a helpful clue in various natural language processing tasks including previous statistical machine translation and recurrent neural network based machine translation. However, since the state-of-the-art neural machine translation (NMT) has to be built on the Transformer based encoder, few attempts are found on such a syntax enhancement. Thus in this paper, we explore effective ways to introduce syntax into Transformer for better machine translation. We empirically compare two ways, positional encoding and input embedding, to exploit syntactic clues from dependency tree over source sentence. Our proposed methods have a merit keeping the architecture of Transformer unchanged, thus the efficiency of Transformer can be kept. The experimental results on IWSLT' 14 German-to-English and WMT14 English-to-German show that our method can yield advanced results over strong Transformer baselines.