What about Optimality Theory?


Q: Your book is focused on old-fashioned rewrite rules. It espouses a serialist view of morphology that is rejected by most current phonologists. The prevailing nonsequential approach to phonology, the Optimality Theory, OT, is based not on rewrite rules but on constraints. Constraints are universal but each language may rank them differently. Constraints are violable. A function called Gen maps each lexical form into all possible output candidates. The winning candidate or candidates is selected by checking the number of constraint violations and their severity, determined by the ranking.  The winner(s) may violate some constraints but they come out better than any alternatives in this evaluation. Is there a way to use finite-state techniques to implement OT?

A: There is a lot of literature on this topic. For example, you could start with Karttunen's paper The Proper Treatment of Optimality in Computational Linguistics ( in the Proceedings of FSMNLP'98. International Workshop on Finite-State Methods in Natural Language Processing, June 29-July 1, 1998, pages 1-12, Bilkent University, Ankara, Turkey). Karttunen argues that Gen is most likely a regular relation and that many if not all OT constraints represent regular languages. But the fact that OT in principle requires unlimited counting means that even the classical OT of Smolensky and Prince (1993) cannot be modeled by a finite-state device. The same is true, even without the counting issue, of some other variants of OT such as John McCarthy's Sympathy Theory and Benua's Output-output constraints.

Nevertheless, finite-state tools can be very useful for developing OT descriptions in the classical style. The script Finnish OT Prosody implements Paul Kiparsky's OT account of basic Finnish prosody ("Finnish Noun Inflection". In Generative Approaches to Finnic and Saami Linguistics, Diane Nelson & Satu Manninen (eds.), pp 109-161, CSLI Publications, 2003). The script demonstrates how to build a Gen function that maps an underlying Finnish word into a prosodic description that represents syllabification, primary and secondary stress, and metrical structure. For example, the input opiskelija 'student' generates candidate outputs such as

(ó.pis).(kè.li).ja

where periods represent syllable boundaries, the acute accent marks primary stress, the grave accent secondary stress, and the two trochaic feet are enclosed in parentheses. This happens to be the best output but there are 10450 other candidates to consider. For longer words, the number of output candidates is counted in the millions.

The script also shows how to encode and evaluate prosodic constraints such as Clash, Align-Left, Lapse, Stress-to-Weight, and All-Feet-First as regular expressions and how to evaluate them using an operation called Lenient Composition. Lenient composition is described in Karttunen's 1998 paper and it is implemented in xfst, although the operator is not mentioned in the Book. We explain the idea here briefly.

Let us assume that R is a mapping from the input form or forms to the current set of output candidates and C is the next constraint to be applied. The evaluation of the output candidates with respect to C can be encoded as the regular expression

R .O. C

where .O. is the lenient composition operator (N.B. capital O to distinguish between lenient and ordinary composition). For each input form, if there is at least one output candidate that meets the constraint C, all the ones that violate the constraint are eliminated. Otherwise there is no change in R and the set of output candidates remains the same. Lenient composition guarantees that each input form always has at least one output, no matter how suboptimal.

The example script produces a transducer that maps 25 Finnish words into their prosodic representations, and vice versa. Being able to do computations of this sort can be very useful for a theoretical phonologists because it makes it possible to work with huge sets of output candidates without overlooking anything and because it takes away the drudgery of manual constraint checking. In fact, without such finite-state tools OT is a very difficult art to practice. The implementation of Kiparsky system revealed a bug, not in the implementation but in the constraint system itself. In some cases, the desired winner loses to a candidate that should have been eliminated by. For example, for the input kalasteleminen, the constraints choose

(ká.las).te.(lè.mi).nen

over the desired winner

(ká.las).(tè.le).(mì.nen)

You may find it interesting to compare the OT implementation of Finnish prosody with a non-OT account for the same descriptive generalizations.

Finally, as to charge of serialism, constraint ranking in OT is quite similar to rule ordering in the older rewriting paradigm.

We recommend the list of Computational OT Papers for further study.