Language technology for low-resource languages

Jack Rueter and Sjur Moshagen

Rule-based language technology

Our rule-based approach:

two-level morphology (finite state transducers)
disambiguation using Constraint Grammar

Both technologies developed here at HU :-)

Actual implementations:

two-level morphology:
- Xerox XFST family (free, closed source)
- Hfst (free, open source)
- Foma (free, open source, no twolc compiler)
disambiguation and syntax:
- VISLCG3 (free, open source)

Open source is important, to avoid being locked in or spend much more money to redo things.

Word-level technology

Morphophonology:
- twolc (a “variant” of SPE phonology)
- also rewrite rules (“real” SPE phonology)
Morphology: lexc

Both formalisms should be easily recognisable by linguists.

Example — twolc

Alphabet m p ;

Rules

"N:m rule"
N:m <=>  _ p: ;

"p:m rule"
p:m <=> :m _  ;

Target changes:

kaNpat
kammat

Example — rewrite rules

The same changes written using rewrite rules:

[ N -> m ]( _ p )
.o.
[ p -> m ]( m _ );

Rewrite rules are ordered, twolc rules are not.

Example — lexc

A very simplistic English lexicon:

Multichar_Symbols
+V +Pres +3Sg +PresPtc +Past

###  Lexicon containing lexical stems:
LEXICON Root
 walk V ;
 talk V ;
 pack V ;

###  Lexicon containing POS tag only:
LEXICON V
 +V: V-suff ;

###  Lexicon containing inflectional suffixes and corresponding tags:
LEXICON V-suff
 +Pres+3Sg:s   # ;
     +Past:ed  # ;
  +PresPtc:ing # ;
     +Pres:    # ;

Summary

End result: computer model of morphology and morphophonology. This model can analyse and generate word forms.

Sentence level technology

VISLCG3, an extended version of the original Constraint Grammar syntax developed by Karlsson et al. VISLCG3 is maintained and developed in Odense, Denmark. It is open source.

Constraint grammars work “backwards”: instead of imposing a structure on a sentence, it selects or removes invalid readings given a certain contexts. By eliminating readings over and over again, one should be left with only the correct readings. From the CG-3 tutorial:

(a) REMOVE VFIN IF (-1 ART) ;
(b) REMOVE (N) IF (-1 (PERS NOM)) ;

(a) will remove finite verb readings (the target) from a cohort, if the one immediately to the left (-1) contains an article tag, while (b) will remove noun readings in the presence of an immediately preceding personal pronoun in the nominative, thus disambiguating nominal-verbal ambiguities like hit in ”the hit/they hit”.

Note that the target VFIN is a defined set (e.g. consisting of tense or mode tags), while the target (N) is a simple tag, declared as a set on-the-fly by using parentheses.

Constraint grammars require a lot of manual work, but will generally achieve substantially higher scores than similar tools using other techniques (given enough work).

Tools for linguistic research

explicit grammars
analysers
generators
analysed text
try out different phonological models of a language
use model to process (analyse) text

Explicit Grammars

Writing for a computer forces you to be explicit about every aspect of the grammar.
This can bring forth new areas of the grammar to research. Examples from the work on the Sámi languages:
- compounding
- derivations
- phonological alternations in Inari Sami
- North Sámi syntax

Analysers

morphological analysers can themselves be used as a tool for resarch (see “Analysing text”, next)
a full-size lexicon can be used as source material for lexical research

Generators

morphological generators will ruthlessly display all overgenerations, and thus give valuable feedback about the grammar model, and the assumptions built in.

Use Model To Process (Analyse) Text

The most important use of a morphological analyser is to analyse text. Together with disambiguation and syntactic analysis, this can be used to build an analysed corpus that can be searched for different linguistic patterns.

=> Korp (searchable corpus of analysed texts)

[images/Korp.png]

Tools for speakers of minority languages

The listed tools are all supported by the Giella infrastructure.

keyboards (desktop and mobile, soon with spellers)
spell checkers
morphologically aware hyphenators
morphologically aware dictionaries
grammar checkers
Machine translation
iCALL
text-to-speech

Keyboards

Desktop (Windows, macOS, Linux/X11)
Mobile, soon with spellers

Without keyboards, writing texts in any language can become almost impossible. The first step of building language technology for a language is thus to make a keyboard.

Using the keyboard part of the Giella infrastructure, the amount of work needed to produce installable keyboards is minimal. But effort should be put into the desing of the keyboard such that it will work optimally for the language community. Actually building the installation packages is a matter of minutes.

Layout definition

The layout definition is done using yaml syntax, and is just the following:

modes:
  mobile-default: |
    á š e r t y u i o p â
    a s d f g h j k l ä đ
      ž z č c v b n m ŋ
  mobile-shift: |
    Á Š E R T Y U I O P Â
    A S D F G H J K L Ä Đ
      Ž Z Č C V B N M Ŋ

longpress:
  Á: Q
  A: Å   Á À Â Ã Ạ
  á: q
  a: å   á à â ã ạ

Skolt Sami desktop keyboard

(Demo: Skolt Sámi macOS keyboard)

Spell Checkers

The next tool to develop is typically a spell checker.
This is especially important for minority languages, because the minority position often causes language users to be less or very little exposed to their own language, and they are thus often unsure of the correct spelling.
The speller is essentially the lexical fst pluss an error model to create suggestions for misspelled words
presently all spellers built using the Giella infrastructure is using Hfst
every analyser can be turned into a speller in the Giella infrastructure, using a single configure option
spellers are presently built for LibreOffice, and through an external service also for MS Office for Windows (for languages known to Windows)
work is ongoing to build system-wide spellers for Windows, macOS and Linux

(Demo: gaelic)

Morphologically Aware Hyphenators

especially important for compounding languages
built just like spellers using the lexical fst, but such that word boundaries
- hyphenation rules converts input strings to the hyphenated counterparts

Morphologically Aware Dictionaries

if not making spellers first, then morphologically aware dictionaries is typically the alternative first tool to build
morphological analysis is used on the input to arrive at a list of lemma candidates for a given input
this is very important for languages with complex morphophonology or a lot of prefixation, but is generally helpful for all languages with a decent amount of morphology

Grammar Checkers

the first grammar checker in the Giella infrastructure is being built now
it uses a modified version of the morphological analyser together with targeted CG rules for error detection and correction
at least the following error types are targeted in this first version:
- wrongly split compounds (“pika juna” vs “pikajuna”)
- congruency errors
- real word errors

To handle wrongly split compounds, we convert word boundaries in compounds into spaces, and try to analyse the result. If we get an analysis, it is given as an error candidate, subject to further disambiguation and error detection rules.

(Demo: grammar checker)

Machine translation

morphological analysis -> disambiguation -> transfer dictionary -> lexical selection -> transfer rules -> output generation
we use Apertium as our MT infrastructure
analysers, generators and disambiguators are built in the Giella infrastructure
present target is:
- mainly related languages
- mainly understanding
further ahead:
- better quality and coverage
- possibly near-production quality between near-related languages

(Demo using Ávvir)

Icall

language technology is used in several places to
- analyse student input
- analyse text to generate new excercises

Text-To-Speech

so far only one TTS system: North Sámi
built using commercial and closed-source technology
we want to replace as much as possible using open-source technology:
- text processing using Hfst and Vislcg3
- synthesis probably using the Festival system