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1.2. Issues and Challenges

Morphological parsing tries to eliminate or alleviate the variability of word forms to provide higher-level linguistic units whose lexical and morphological properties are explicit and well defined. It attempts to remove unnecessary irregularity and give limits to ambiguity, both of which are present inherently in human language.

By irregularity, we mean existence of such forms and structures that are not described appropriately by a prototypical linguistic model. Some irregularities can be understood by redesigning the model and improving its rules, but other lexically dependent irregularities often cannot be generalized.

Ambiguity is indeterminacy in interpretation of expressions of language. Next to accidental ambiguity and ambiguity due to lexemes having multiple senses, we note the issue of syncretism, or systematic ambiguity.

Morphological modeling also faces the problem of productivity and creativity in language, by which unconventional but perfectly meaningful new words or new senses are coined. Usually, though, words that are not licensed in some way by the lexicon of a morphological system will remain completely unparsed. This unknown word problem is particularly severe in speech or writing that gets out of the expected domain of the linguistic model, such as when special terms or foreign names are involved in the discourse or when multiple languages or dialects are mixed together.

1.2.1. Irregularity

Morphological parsing is motivated by the quest for generalization and abstraction in the world of words. Immediate descriptions of given linguistic data may not be the ultimate ones, due to either their inadequate accuracy or inappropriate complexity, and better formulations may be needed. The design principles of the morphological model are therefore very important.

In Arabic, the deeper study of the morphological processes that are in effect during inflection and derivation, even for the so-called irregular words, is essential for mastering the whole morphological and phonological system. With the proper abstractions made, irregular morphology can be seen as merely enforcing some extended rules, the nature of which is phonological, over the underlying or prototypical regular word forms [15, 16].

In Example 1–7, 009fig01.jpg is the second-person feminine singular perfective verb in active voice, member of the 009fig02.jpg ‘to see’ lexeme of the 009fig03.jpg root. The prototypical, regularized pattern for this citation form is 009fig04.jpg, as we saw with 009fig05.jpg in Example 1–6. Alternatively, we could assume the pattern of 009fig06.jpg to be 009fig07.jpg, thereby asserting in a compact way that the final root consonant and its vocalic context are subject to the particular phonological change, resulting in 009fig08.jpg like 009fig09.jpg instead of 009fig10.jpg like 009fig11.jpg. The occurrence of this change in the citation form may have possible implications for the morphological behavior of the whole lexeme.

Table 1–1 illustrates differences between a naive model of word structure in Arabic and the model proposed in Smrž [12] and Smrž and Bielický [17] where morphophonemic merge rules and templates are involved. Morphophonemic templates capture morphological processes by just organizing stem patterns and generic affixes without any context-dependent variation of the affixes or ad hoc modification of the stems. The merge rules, indeed very terse, then ensure that such structured representations can be converted into exactly the surface forms, both orthographic and phonological, used in the natural language. Applying the merge rules is independent of and irrespective of any grammatical parameters or information other than that contained in a template. Most morphological irregularities are thus successfully removed.

Table 1–1. Discovering the regularity of Arabic morphology using morphophonemic templates, where uniform structural operations apply to different kinds of stems. In rows, surface forms S of 009fig05.jpg ‘to read’ and 009fig06.jpg ‘to see’ and their inflections are analyzed into immediate I and morphophonemic M templates, in which dashes mark the structural boundaries where merge rules are enforced. The outer columns of the table correspond to P perfective and I imperfective stems declared in the lexicon; the inner columns treat active verb forms of the following morphosyntactic properties: I indicative, S subjunctive, J jussive mood; 1 first, 2 second, 3 third person; M masculine, F feminine gender; S singular, P plural number

In contrast, some irregularities are bound to particular lexemes or contexts, and cannot be accounted for by general rules. Korean irregular verbs provide examples of such irregularities.

Korean shows exceptional constraints on the selection of grammatical morphemes. It is hard to find irregular inflection in other agglutinative languages: two irregular verbs in Japanese [18], one in Finnish [19]. These languages are abundant with morphological alternations that are formalized by precise phonological rules. Korean additionally features lexically dependent stem alternation. As in many other languages, i- ‘be’ and ha- ‘do’ have unique irregular endings. Other irregular verbs are classified by the stem final phoneme. Table 1–2 compares major irregular verb classes with regular verbs in the same phonological condition.

Table 1–2. Examples of major Korean irregular verb classes compared with regular verbs

1.2.2. Ambiguity

Morphological ambiguity is the possibility that word forms be understood in multiple ways out of the context of their discourse. Words forms that look the same but have distinct functions or meaning are called homonyms.

Ambiguity is present in all aspects of morphological processing and language processing at large. Morphological parsing is not concerned with complete disambiguation of words in their context, however; it can effectively restrict the set of valid interpretations of a given word form [20, 21].

In Korean, homonyms are one of the most problematic objects in morphological analysis because they prevail all around frequent lexical items. Table 1–3 arranges homonyms on the basis of their behavior with different endings. Example 1–8 is an example of homonyms through nouns and verbs.

Table 1–3. Systematic homonyms arise as verbs combined with endings in Korean

We could also consider ambiguity in the senses of the noun nan, according to the Standard Korean Language Dictionary: nan1 ‘egg’, nan2 ‘revolt’, nan5 ‘section (in newspaper)’, nan6 ‘orchid’, plus several infrequent readings.

Arabic is a language of rich morphology, both derivational and inflectional. Because Arabic script usually does not encode short vowels and omits yet some other diacritical marks that would record the phonological form exactly, the degree of its morphological ambiguity is considerably increased. In addition, Arabic orthography collapses certain word forms together. The problem of morphological disambiguation of Arabic encompasses not only the resolution of the structural components of words and their actual morphosyntactic properties (i.e., morphological tagging [22, 23, 24]) but also tokenization and normalization [25], lemmatization, stemming, and diacritization [26, 27, 28].

When inflected syntactic words are combined in an utterance, additional phonological and orthographic changes can take place, as shown in Figure 1–1. In Sanskrit, one such euphony rule is known as external sandhi [29, 30]. Inverting sandhi during tokenization is usually nondeterministic in the sense that it can provide multiple solutions. In any language, tokenization decisions may impose constraints on the morphosyntactic properties of the tokens being reconstructed, which then have to be respected in further processing. The tight coupling between morphology and syntax has inspired proposals for disambiguating them jointly rather than sequentially [4].


Figure 1–1. Complex tokenization and normalization of euphony in Arabic. Three nominal cases are expressed by the same word form with 012fig01.jpg ‘my study’ and 012fig02.jpg ‘my teachers’, but the original case endings are distinct. In katabtumūhā ‘you-MASC-PL wrote them’, the liaison vowel ū is dropped when tokenized. Special attention is needed to normalize some orthographic conventions, such as the interaction of 012fig05.jpg ‘carrying out’ and the cliticized hu ‘his’ respecting the case ending or the merge of the definite article of 012fig05a.jpg ‘regret’ with the preposition li ‘for’

Czech is a highly inflected fusional language. Unlike agglutinative languages, inflectional morphemes often represent several functions simultaneously, and there is no particular one-to-one correspondence between their forms and functions. Inflectional paradigms (i.e., schemes for finding the form of a lexeme associated with the required properties) in Czech are of numerous kinds, yet they tend to include nonunique forms in them.

Table 1–4 lists the paradigms of several common Czech words. Inflectional paradigms for nouns depend on the grammatical gender and the phonological structure of a lexeme. The individual forms in a paradigm vary with grammatical number and case, which are the free parameters imposed only by the context in which a word is used.

Table 1–4. Morphological paradigms of the Czech words dům ‘house’, budova ‘building’, stavba ‘building’, stavení ‘building’. Despite systematic ambiguities in them, the space of inflectional parameters could not be reduced without losing the ability to capture all distinct forms elsewhere: S singular, P plural number; 1 nominative, 2 genitive, 3 dative, 4 accusative, 5 vocative, 6 locative, 7 instrumental case

Looking at the morphological variation of the word stavení ‘building’, we might wonder why we should distinguish all the cases for it when this lexeme can take only four different forms. Is the detail of the case system appropriate? The answer is yes, because we can find linguistic evidence that leads to this case category abstraction. Just consider other words of the same meaning in place of stavení in various contexts. We conclude that there is indeed a case distinction made by the underlying system, but it need not necessarily be expressed clearly and uniquely in the form of words.

The morphological phenomenon that some words or word classes show instances of systematic homonymy is called syncretism. In particular, homonymy can occur due to neutralization and uninflectedness with respect to some morphosyntactic parameters. These cases of morphological syncretism are distinguished by the ability of the context to demand the morphosyntactic properties in question, as stated by Baerman, Brown, and Corbett [10, p. 32]:

  • Whereas neutralization is about syntactic irrelevance as reflected in morphology, uninflectedness is about morphology being unresponsive to a feature that is syntactically relevant.

For example, it seems fine for syntax in Czech or Arabic to request the personal pronoun of the first-person feminine singular, equivalent to ‘I’, despite it being homonymous with the first-person masculine singular. The reason is that for some other values of the person category, the forms of masculine and feminine gender are different, and there exist syntactic dependencies that do take gender into account. It is not the case that the first-person singular pronoun would have no gender nor that it would have both. We just observe uninflectedness here. On the other hand, we might claim that in English or Korean, the gender category is syntactically neutralized if it ever was present, and the nuances between he and she, him and her, his and hers are only semantic.

With the notion of paradigms and syncretism in mind, we should ask what is the minimal set of combinations of morphosyntactic inflectional parameters that covers the inflectional variability in a language. Morphological models that would like to define a joint system of underlying morphosyntactic properties for multiple languages would have to generalize the parameter space accordingly and neutralize any systematically void configurations.

1.2.3. Productivity

Is the inventory of words in a language finite, or is it unlimited? This question leads directly to discerning two fundamental approaches to language, summarized in the distinction between langue and parole by Ferdinand de Saussure, or in the competence versus performance duality by Noam Chomsky.

In one view, language can be seen as simply a collection of utterances (parole) actually pronounced or written (performance). This ideal data set can in practice be approximated by linguistic corpora, which are finite collections of linguistic data that are studied with empirical methods and can be used for comparison when linguistic models are developed.

Yet, if we consider language as a system (langue), we discover in it structural devices like recursion, iteration, or compounding that allow to produce (competence) an infinite set of concrete linguistic utterances. This general potential holds for morphological processes as well and is called morphological productivity [31, 32].

We denote the set of word forms found in a corpus of a language as its vocabulary. The members of this set are word types, whereas every original instance of a word form is a word token.

The distribution of words [33] or other elements of language follows the “80/20 rule,” also known as the law of the vital few. It says that most of the word tokens in a given corpus can be identified with just a couple of word types in its vocabulary, and words from the rest of the vocabulary occur much less commonly if not rarely in the corpus. Furthermore, new, unexpected words will always appear as the collection of linguistic data is enlarged.

In Czech, negation is a productive morphological operation. Verbs, nouns, adjectives, and adverbs can be prefixed with ne- to define the complementary lexical concept. In Example 1–9, budeš ‘you will be’ is the second-person singular of být ‘to be’, and nebudu ‘I will not be’ is the first-person singular of nebýt, the negated být. We could easily have číst ‘to read’ and nečíst ‘not to read’, or we could create an adverbial phrase like noviny nenoviny that would express ‘indifference to newspapers’ in general:

Example 1–9 has the meaning of Example 1–1 and Example 1–6. The word noviny ‘newspaper’ exists only in plural whether it signifies one piece of newspaper or many of them. We can literally translate noviny as the plural of novina ‘news’ to see the origins of the word as well as the fortunate analogy with English.

It is conceivable to include all negated lexemes into the lexicon and thereby again achieve a finite number of word forms in the vocabulary. Generally, though, the richness of a morphological system of a language can make this approach highly impractical.

Most languages contain words that allow some of their structural components to repeat freely. Consider the prefix pra- related to a notion of ‘generation’ in Czech and how it can or cannot be iterated, as shown in Example 1–10:

In creative language, such as in blogs, chats, and emotive informal communication, iteration is often used to accent intensity of expression. Creativity may, of course, go beyond the rules of productivity itself [32].

Let us give an example where creativity, productivity, and the issue of unknown words meet nicely. According to Wikipedia, the word googol is a made-up word denoting the number “one followed by one hundred zeros,” and the name of the company Google is an inadvertent misspelling thereof. Nonetheless, both of these words successfully entered the lexicon of English where morphological productivity started working, and we now know the verb to google and nouns like googling or even googlish or googleology [34].

The original names have been adopted by other languages, too, and their own morphological processes have been triggered. In Czech, one says googlovat, googlit ‘to google’ or vygooglovat, vygooglit ‘to google out’, googlování ‘googling’, and so on. In Arabic, the names are transcribed as ğūğūl ‘googol’ and ğūğil ‘Google’. The latter one got transformed to the verb ğawğal ‘to google’ through internal inflection, as if there were a genuine root ğ w ğ l, and the corresponding noun ğawğalah ‘googling’ exists as well.

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