Language Change as a

Constrained Multi-Objective

Optimization

Monojit ChoudhuryMicrosoft Research Lab, India

monojitc@microsoft.com

A tale o

f the la

zy ton

gue

Indo-Australia Workshop on Optimization in Human Language Technology16th Dec 2012, IIT Patna

Language Change

Language Change

• Change in the syntactic/semantic/phonological features of a language

• Perpetual, universal, directional (?)

• Phonological Change: – Affects the sounds– Structured, independent of syntax/semantics– Example: Loss of consonant clusters in Hindi

agni aag, dugdha dUdh, raatri raat

Effects of the “Lazy Tongue”

Assimilation• in+apt = inapt• in+decent = indecent• in+polite = impolite• in+mature = immature• in+legal = illegal• in+regular = irregular

Deletion• cannot can’t• do not don’t• will not won’t• are not ain’t• information info

Explanations for Change

Exogenous causes– Language contact– Socio-political

factors– Communication

medium

Endogenous causes– Functional– Phonetic error-based– Frequency drifts– Evolutionary

Functional Explanation of Language Change

• There are three evolutionary forces on any linguistic system:– Minimization of effort (energy)– Maximization of perceptual distinctiveness

(Minimization of ambiguity)– Maximization of learnability

Language is a perpetually evolving system shaped by these three conflicting

forces

Outline of the Talk

• Morpho-phonological change of Bangla Verb systems and emergence of dialect diversity– Approach: Multi-Objective Constrained Optimization– Technique: Multi-Objective Genetic Algorithm (MOGA)

• Understanding Computer Mediated Communication– Normalization of Texting language – Romanization of Indian Language text

Geography of Bangla

• Standard Colloquial Bengali (SCB)

• Agartala Colloquial Bengali (ACB)

• Sylhetti

History of Bangla

1200 AD 1800 AD

BanglaVerb Morphology

করে�ছি�লামkar-echh-il-aam

Verb root (do)

Aspect (perfect)

Tense (past)

Person (first)

I had done

Cognates in the Dialects

Features Classical SCB ACB

Non-finite kariyA kore kairAPs,2, per. kariyAChila koreChilo korsiloPs,1, cont. kariteChilAm korChilAm kartAslAm

root: kar (to do)

Atomic Phonological Operators

kariteChila

kariChila

kairChila korChila

karitChila

korChilo

Del(e/t_Ch)

Del(t/_Ch)Met(ri/_Ch)

Asm(ao/_i) Mut(a o/_$)

Deletion, MetathesisAssimilation, Mutation

Hypothesis

A sequence of Atomic Phonological Operators, is preferred if the verb forms obtained by application of this sequence on the classical forms have some functional benefit over the classical forms.

Thus, all the modern dialects of Bangla have some functional advantage over the classical dialect.

A Formal Model of Functional Explanation

f1: Effort of articulation

f2: [Acoustic distinctiveness]-1

Unstable languages

Impossible languages

Metastable languages

Genetic Algorithm

Gene (A string of symbols) How the solution actually looks like

GA: search for good solutions mimicking nature [recombination and mutation of genes]

Phenotype

kori

korChi

:

korte

kori

kartAsi

:

kartA

Lexicon consisting of 28 forms for the verb kar

Genotype

A sequence of atomic phonological operators

Del t Met ri NOP Del e Asm a Del i NOP

Dsm e NOP NOP Met ri Asm a Del e NOP

Genotype Phenotype

karikariteChi

karite

Del t Met ri NOP Del e Asm a Del i NOP

karikarieChi

karie

kairkaireChi

kaire

korkorCh

kor

Crossover

Mutation

Multi-Objective GA

Multi-Objective GA: Apply constraints

Multi-Objective GA: Apply constraints

Multi-Objective GA: Finding out good solutions

Multi-Objective GA: But also keep some not-so-good solutions

Multi-Objective GA: But also keep some not-so-good solutions

Multi-Objective GA: After several iterations

Objective functions

• Articulatory effort– fe(Λ): weighted sum of number of syllables,

letters and vowel height differences averaged over all words in the lexicon

• Acoustic Distinctiveness– fd(Λ): Inverse of mean edit distance between

words• Learnability

– fr(Λ): correlation between feature match and edit distance

Experiments

• NSGA – II : a package for fast MOGA• Gene length: 15 APOs• A repertoire of 128 APOs• Population: 1000, Generation: 500• 6 Models with different combinations of

constraints and objectives

Pareto-optimal front

CB

SylhettiACB

SCB

Observations

• vertical and horizontal limb• real dialects on the horizontal limb• Sound changes push the dialects from right

to left (reduce effort)• but never up the limb• why?

Role of Constraints

For more information

Choudhury et al., Evolution optimization and language change: the case of Bengali verb inflections, in Proceedings of ACL SIGMORPHON9, Association for Computational Linguistics, 2007http://research.microsoft.com/people/monojitc/

MOGA and NSGA IIKanpur Genetic Algorithms Laboratoryhttp://www.iitk.ac.in/kangal/index.shtml

Food for Thought

• Evaluation:– Myriads of possible dialects, but only a few

observed in nature• Fixed set of pre-defined APOs – how to

generalize for any change?

• MOGA is an optimization tool, which in no way simulates language change– How do languages optimize themselves?

Outline of the Talk

• Morpho-phonological change of Bangla Verb systems and emergence of dialect diversity– Approach: Multi-Objective Constrained Optimization– Technique: Multi-Objective Genetic Algorithm (MOGA)

• Understanding Computer Mediated Communication– Normalization of Texting language – Romanization of Indian Language text

Computer Mediated Communication

Form

Texting Language• A new genre of English & also other

languages used in chats, sms, emails, blogs, tweets, FB posts, comments etc.

dis is n eg 4 txtin lang

This is an example for Texting language

Texting Language• A new genre of English & also other languages

used in chats, sms, emails, blogs, etc.• Ungrammatical, unconventional spellings

dis is n eg 4 txtin lang

This is an example for Texting language

24 39

The shorter the fasterConstraint: understandability

Analysis of Social Media

• A hot topic in NLP– Normalization– Language identification– Sentiment/Polarity detection– Summarization/trend prediction

Choudhury et al. (2007) Investigation and Modeling of the Structure of Texting Language. In IJCAI Workshop on Analytics of Noisy Data 2007

Tomorrow never dies!!!

• 2moro (9)• tomoz (25) • tomoro (12) • tomrw (5)• tom (2)• tomra (2)• tomorrow (24)• tomora (4)

• tomm (1)• tomo (3)• tomorow (3)• 2mro (2)• morrow (1)• tomor (2)• tmorro (1)• moro (1)

Patterns or Compression Operators

• Phonetic substitution (phoneme)– psycho syco, then den

• Phonetic substitution (syllable)– today 2day , see c

• Deletion of vowels– message mssg, about abt

• Deletion of repeated characters– tomorrow tomorow

Patterns or Compression Operators

• Truncation (deletion of tails)– introduction intro, evaluation eval

• Common Abbreviations– Bangalore blr, text back tb

• Informal pronunciation– going to gonna, better betta

HMMs for SMS Normalization

G1

‘T’

S6

G2

‘O’G3

‘D’G4

‘A’G5

‘Y’

S0P2

/AH/P4

/AY/

S1

“2”

ε T @ ε O @ ε D @ ε A @ ε Y @

Bigram Examples

• TL: would b gd 2 c u some time soon• Op: would be good to see you some time soon

• TL: just wanted 2 say a big thanx 4 my bday card• Op: just wanted to say a big thanks for my today

card

• TL: me wel i fink bein at home makes me feel a lot more stressed den bein away from it

• Op: me well i think being at home makes me feel a lot more stressed deny being away from it

Code mixing

Transliteration

Spelling Change

Indian English

Use of Indian Languages on Online Social Media

Concluding Remarks

• Languages are perpetually evolving and optimizing systems– Computational modeling of language change is

still in its infancy– Lots of scope for research

Thank You!monojitc@microsoft.com

Questions??

Why Computational Models?

FOR AGAINST

FormalizationVirtual experimentation

Exploration

IntractableSimplified assumptions

Toy languages

Can we modelreal world language change?

Objectives and Constraints - 1

• Articulatory effortfe(w) = α1 fe1(w) + α2 fe2(w) + α3 fe3(w)

fe1(w) = |w|fe2(w) = hr(σi)

fe3(w) = |ht(Vi) - ht(Vi+1)|

Objectives and Constraints - 2

• Acoustic distinctivenessfd(Λ) = (1/N) ed(wi,wj)-1

Cd(Λ) = -1 if ed(wi,wj) = 0 for > 2 pairs

• Phonotactic constraints Cp(Λ) = -1 if any of the words violate

the phonotactic constraints of the language

Objectives and Constraints - 3

• Learnability as Regularity– fr: The correlation coefficient between the edit

distance and number of matching morphological attributes for every word pair

– Cr = -1 if fr > 0.8

Emergent dialects

Classical D1 D2 D3

kariteChilAm kartA karChi(korChi)

karteChi(kartAsi)

kariteChila kartAa karCha(korCha)

karteCha(kartAsa)

kariteChilen kartAen karChen(korChen)

karteChen(kartAsen)