Yet Another Format of U niversal D ependencies for K orean (2022)

Abstract: In this study, we propose a morpheme-based scheme for Korean dependency parsing and adopt the proposed scheme to Universal Dependencies. We present the linguistic rationale that illustrates the motivation and the necessity of adopting the morpheme-based format, and develop scripts that convert between the original format used by Universal Dependencies and the proposed morpheme-based format automatically. The effectiveness of the proposed format for Korean dependency parsing is then testified by both statistical and neural models, including UDPipe and Stanza, with our carefully constructed morpheme-based word embedding for Korean. morphUD outperforms parsing results for all Korean UD treebanks, and we also present detailed error analysis.

## introduction dependency parsing is one of the tasks in natural language processing that have been investigated extensively. using the dependency grammar, it finds the relations between the words in a sentence, and forms an acyclic dependency graph that explains the grammatical structure of the sentence. with various machine learning techniques, the current dependency parsers are able to approach human performances given english corpora. 1 previous studies trying to probe or improve korean dependency parsers are lacking, especially for those considering the underlying reason and rationale based on the linguistic properties of korean. the parsers do not perform as well as their english counterparts, partly due to the fact that korean is a language that has more complicated linguistic features that make parsing on the word level difficult. ## representation of morphud in this study, we adopt a morpheme-based format that captures the linguistic properties of the korean language proposed by @xcite . the natural segmentation of korean is based on eojeol, which does not necessarily reflect the actual word or morpheme boundaries of the language. for example, an eojeol of korean may contain both a noun and its postposition, or both a verb and its particles marking tense, aspect, honorifics, etc. while this is typical for korean as an agglutinative language, it creates difficulties and challenges for nlp tasks regarding the korean language, including dependency parsing. it is not ideal that the tokens dependency relations are annotated on are sometimes words, and sometimes phrases as an eojeol may consist of more than a word. furthermore, korean as an agglutinative language has very regular conjugations, which makes it easy and natural to split those words and phrases into morphemes when analyzing the language since nearly every piece of an eojeol can be identified to be of a certain meaning or function. the morpheme-based format aims at decomposing the korean sentences further into morphemes, which means that dependency relations are no longer marked on the eojeol level. instead, they are marked on morphemes such that within each eojeol that is not monomorphemic, a head of that eojeol will be found and all other morphemes will be attached directly to the head. as a result, the head of a non-monomorphemic eojeol carries the dependency relation this eojeol originally has, and all other morphemes will be attached to it. in order to find the head, we develop a script and apply some heuristics which include that the head of an eojeol is usually a noun, a proper noun, or a verb, and while there is no noun or verb in an eojeol, the script we implemented continues to find other morphemes such as pronouns, adjectives, adverbs, numerals, etc. the script also excludes the use of adpositions, conjunctions, and particles as heads in most cases, unless these are the only partof-speeches in an eojeol except for punctuations. while there are multiple morphemes that can be heads in an eojeol, the script will decide which one to take based on the part-of-speeches of the morphemes. for instance, when there are multiple nouns, the last one will carry the dependency relation of the eojeol, whereas when there are multiple verbs, the first verb will carry the dependency relation as korean is a head-final language. once the head of a non-monomorphemic eojeol is found, the other morphemes will be dependent on the head and be assigned with other dependency relations such as compound, case, auxiliary depending on their upos and xpos. ## conclusion we proposed a new annotation scheme for universal dependencies for korean. we have already worked on ner, in which we outperformed the word-level representation dataset by using the morphologically enhanced dataset, and we are planning to extend our idea to the semantic role labeling task. we are also trying to create a consortium to develop the morphologically enhanced universal dependencies for other morphologically rich languages such as basque, finnish, french, german, hungarian, polish, and swedish. all conversion scripts (wordud to morphud, and vice versa), and morphud datasets for ko gsd and ko kaist will be available through author's github at https://github.com/ jungyeul/morphud-korean .