A Sub-Character Architecture for Korean Language Processing

Proceedings of the 2017 Conference on Empirical Methods in Natural Language Processing, pages 721–726 Copenhagen, Denmark, September 7–11, 2017. c⃝2017 Association for Computational Linguistics A Sub-Character Architecture for Korean Language Processing Karl Stratos Toyota Technological Institute at Chicago stratos@ttic.edu Abstract We introduce a novel sub-character ar- chitecture that exploits a unique com- positional structure of the Korean lan- guage. Our method decomposes each character into a small set of primitive phonetic units called jamo letters from which character- and word-level represen- tations are induced. The jamo letters di- vulge syntactic and semantic information that is difficult to access with conventional character-level units. They greatly alle- viate the data sparsity problem, reducing the observation space to 1.6% of the orig- inal while increasing accuracy in our ex- periments. We apply our architecture to dependency parsing and achieve dramatic improvement over strong lexical baselines. 1 Introduction Korean is generally recognized as a language iso- late: that is, it has no apparent genealogical rela- tionship with other languages (Song, 2006; Camp- bell and Mixco, 2007). A unique feature of the language is that each character is composed of a small, fixed set of basic phonetic units called jamo letters. Despite the important role jamo plays in encoding syntactic and semantic information of words, it has been neglected in existing modern Korean processing algorithms. In this paper, we bridge this gap by introducing a novel composi- tional neural architecture that explicitly leverages the sub-character information. Specifically, we perform Unicode decomposi- tion on each Korean character to recover its un- derlying jamo letters and construct character- and word-level representations from these letters. See ᄉ ᅡ ᆫᄋ ᅳ ᆯ ᄀ ᅡ ᆻᄃ ᅡ ᄀ ᅡ ᆻᄃ ᅡ ᄃ ᅡ ∅ ㅏ ㄷ ᄀ ᅡ ᆻ ㅆ ㅏ ㄱ ᄉ ᅡ ᆫᄋ ᅳ ᆯ ᄋ ᅳ ᆯ ㄹ ㅡ ㅇ ᄉ ᅡ ᆫ ㄴ ㅏ ㅅ Figure 1: Korean sentence “ᄉ ᅡ ᆫᄋ ᅳ ᆯ ᄀ ᅡ ᆻᄃ ᅡ” (I went to the mountain) decomposed to words, characters, and jamos. Figure 1 for an illustration of the decomposi- tion. The decomposition is deterministic; this is a crucial departure from previous work that uses language-specific sub-character information such as radical (a graphical component of a Chinese character). The radical structure of a Chinese character does not follow any systematic process, requiring an incomplete dictionary mapping be- tween characters and radicals to take advantage of this information (Sun et al., 2014; Yin et al., 2016). In contrast, our Unicode decomposition does not need any supervision and can extract correct jamo letters for all possible Korean characters. Our jamo architecture is fully general and can be plugged in any Korean processing network. For a concrete demonstration of its utility, in this work we focus on dependency parsing. McDonald et al. (2013) note that “Korean emerges as a very clear outlier” in their cross-lingual parsing experiments on the universal treebank, implying a need to tai- lor a model for this language isolate. Because of the compositional morphology, Korean suffers ex- treme …

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