Matxin 1.0 New Language Pair HOWTO

This page intends to give a step-by-step walk-through of how to create a new translator in the Matxin platform.

Prerequisites

Main article: Matxin

This page does not give instructions on installing Matxin, but presumes that the following packages are correctly installed.

• lttoolbox (from SVN)
• Freeling (from SVN)
• Matxin (from SVN)
• a text editor
• The fl-* tools for Freeling (for the moment these can be found in apertium-tools/freeling in apertium SVN)

Overview

As mentioned in the lead, this page intends to give a step-by-step guide to creating a new language pair with Matxin from scratch. No programming knowledge is required, all that needs to be defined are some dictionaries and grammars. The Matxin platform is described in detail in Documentation of Matxin and on the Matxin homepage. This page will only focus on the creation of a new language pair, and will avoid theoretical and methodological issues.

The language pair for the tutorial will be Breton to English. This has been chosen as the two languages have fairy divergent word order (Breton is fairly free, allowing VSO, OVS and SVO, where English is fairly uniformly SVO) which can show some of the advantage which Matxin has over Apertium. The sentence that we're going to use for this purpose is,

Ur yezh indezeuropek eo ar brezoneg.

[_compl A language indo-european] [_verb is] [_subj the breton].

Breton is an Indo-European language.

There are two main issues with transfer in this case, the first is re-ordering, from OVS → SVO, the second is removing the definite article preceding a language name ar brezhoneg → Breton.

Getting started

Analysis

The analysis process in Matxin is done by Freeling, an free / open-source suite of language analysers. The analysis is done in four stages, requiring four (or more) sets of separate files. The first is the morphological dictionary, which is basically a full-form list (e.g. Speling format) compiled into a BerkeleyDB format. There are then files for word-category disambiguation and for specifying chunking and dependency rules. There are two more stages that come before morphological analysis, tokenisation and sentence splitting, but for the purposes of this tutorial they will be considered along with morphological analysis.

Normally a single program is used to do all the different stages of analysis, taking as input plain or deformatted text, and outputting a dependency analysis, the behaviour of this program is controlled by a file called config.cfg. In Matxin this program is called Analyzer, however in the following stages, we'll be using separate tools and will leave creating the config file until the last minute as it can get complicated.

As companion reading to this section the Freeling documentation is highly recommended. This tutorial skips over features of Freeling which are not necessary for making a basic MT system with Matxin, and note that it is also possible to use other analysers as input into the chunking / dependency parsing parts of Freeling, for more information see here.

Morphological

In order to create your morphological analyser in Freeling you basically need to make a full-form list. If there is already an Apertium dictionary for the language, you can use the scripts in apertium SVN (module apertium-tools/freeling) to generate a dictionary from scratch, if not, then either build it from scratch, or build a dictionary in lttoolbox and then generate the list.

For the purposes of this exercise, you can just key in a small dictionary manually. We'll call the dictionary matxin-br-en.br.dicc, and it will contain

ul un DI0CN0 
un un DI0CN0 
ur un DI0CN0 
yezhoù yezh NCFPV0 
yezh yezh NCFSV0 yezh AQ0CN0
indezeuropek indezeuropek AQ0CN0 
eo bezañ VMIP3S0 
al an DA0CN0 
ar an DA0CN0 
an an DA0CN0
brezhoneg brezhoneg NCMSV0 
prezhoneg brezhoneg NCMSV0 
vrezhoneg brezhoneg NCMSV0 
. . Fp

The file is space separated with three or more columns. The first is for the surface form of the word, further columns are for a list of lemmas and Parole-style analyses.

After we've keyed this in, we can compile it to BerkleyDB format using the tool indexdict from the Freeling utilities. It is worth noting that Freeling currently only supports the latin1 character encoding, so if you're working in UTF-8, convert the dictionary to latin1 first.

$ cat matxin-br-en.br.dicc | iconv -f utf-8 -t latin1 | indexdict br-en.br.db

Now you should have two files, matxin-br-en.br.dicc, which is the dictionary source, and br-en.br.db which is the dictionary in BerkleyDB format. We cannot however use this analyser without specifying a tokeniser and splitter. These files define how words and sentences will be tokenised. For now we'll use a minimal configuration file for the splitter, so put the following in the file matxin-br-en.spt.dat

<SentenceEnd>
. 0
</SentenceEnd>

Of course, other end of sentence punctuation such as '?' and '!' could also be put in there. And now for the word tokeniser, which we'll put in matxin-br-en.tok.dat

<Macros>
ALPHANUM   [^\]<>[(\.,";:?!'`)^@~|}{_/\\+=&$#*+%\s\-]
OTHERS     [\]<>[(\.,";:?!'`)^@~|}{_/\\+=&$#*+%\-]
</Macros>
<RegExps>
WORD             0  {ALPHANUM}+
OTHERS_C         0  {OTHERS}+
</RegExps>

The macros define regular expressions which are used to tokenise the input into words and punctuation. The regular expression WORD is defined as a sequence of one or more ALPHANUM which in turn is defined as anything except a punctuation character.

So now if we want to morphologically analyse a sentence, we just do:

$ echo "Ur yezh eo ar brezhoneg." | fl-morph matxin-br-en.tok.dat matxin-br-en.spt.dat br-en.br.db  | iconv -f latin1
Ur un DI0CN0 -1   -1
yezh yezh NCFSV0 -1 yezh AQ0CN0 -1
eo bezañ VMIP3S0 -1   -1
ar an DA0CN0 -1   -1
brezhoneg brezhoneg NCMSV0 -1   -1
. . Fp -1

The -1 after each analysis is the a priori probability of the analysis and is calculated from a previously tagged corpus. As we don't have a previously tagged corpus, this is unset.

Category disambiguation

After we have working morphological analysis, the next stage is to create a part-of-speech tagger. Freeling offers various ways to do this, both HMM-based and Relax Constraint Grammar (RelaxCG) based are supported. We're going to demonstrate how to create a RelaxCG tagger as it is easier and does not require tagger training.

Our tagger will be very simple as we only have one ambiguity, yezh 'language' can be a noun or an adjective. As adjectives come after the noun in Breton, we'll weight adjectives after determiners very low,

SETS

CONSTRAINTS

%% after a determiner down-weight adjective
-8.0 AQ*  (-1 D*);

The file (which we will call matxin-br-en.br.relax is made up of two sections, the first SETS defines any sets of tags or lemmas, much like the LIST and SET in VISL Constraint Grammar taggers. The second section defines a series of weighted constraints, in the format of 'weight', followed by space, followed by the tag followed by another space and then the context. The context is defined as a series of positions relative to the tag in question.

So, using this file we should be able to get disambiguated output:

$ echo "Ur yezh eo ar brezhoneg." | fl-morph matxin-br-en.tok.dat matxin-br-en.spt.dat br-en.br.db  | \
    fl-tagger matxin-br-en.br.relax | iconv -f latin1
Ur un DI0CN0 -1
yezh yezh NCFSV0 -1
eo bezañ VMIP3S0 -1
ar an DA0CN0 -1
brezhoneg brezhoneg NCMSV0 -1
. . Fp -1

Chunking

So, after tagging the next stage is chunk parsing. This is somewhat like the chunking available in Apertium (see Chunking), however no transfer takes place, it just groups words into chunks. The grammar is quite familiar, the left side shows the non-terminal, and the right side can be either terminal (in the case of a tag, e.g. NCM*) or non-terminal (in the case of n-m). This extremely simple grammar will chunk the tagged input into the constituents (noun phrases sn and verb-eo) for later use by the dependency parser. It should be fairly straight forward, | is an or statement, and + marks the governor, or head of the chunk. The end of each rule is marked with a full stop . and comments are placed with %.

n-m ==> NCM* .
n-f ==> NCF* .

adj ==> AQ* .

def ==> DA0CN0 .
indef ==> DI0CN0 .

verb-eo ==> VMIP3S0(eo).  %% A specific chunk type for the 'eo' form of bezañ
verb ==> VL* .

punt ==> Fp .

sn ==> def, +n-f, adj | def, +n-f | +n-f, adj | +n-f .
sn ==> def, +n-m, adj | def, +n-m | +n-m, adj | +n-m .
sn ==> indef, +n-m, adj | indef, +n-m | +n-m, adj | +n-m .
sn ==> indef, +n-f, adj | indef, +n-f | +n-f, adj | +n-f .

@START S.

The @START directive states that the start node of the sentence should be labelled S. So, the output of this grammar will be,

$ echo "Ur yezh indezeuropek eo ar brezhoneg." | fl-morph matxin-br-en.tok.dat matxin-br-en.spt.dat br-en.br.db  | \
 fl-tagger matxin-br-en.br.relax  | fl-chunker matxin-br-en.br.gram | iconv -f latin1
S_[
  sn_[
    indef_[
      +(Ur un DI0CN0)
    ]
    +n-f_[
      +(yezh yezh NCFSV0)
    ]
    adj_[
      +(indezeuropek indezeuropek AQ0CN0)
    ]
  ]
  verb-eo_[
    +(eo bezañ VMIP3S0)
  ]
  sn_[
    def_[
      +(ar an DA0CN0)
    ]
    +n-m_[
      +(brezhoneg brezhoneg NCMSV0)
    ]
  ]
  punt_[
    +(. . Fp)
  ]
]

Note the sentence is chunked into sn verb-eo sn. It might be worth playing around a bit with the grammar to get a better feel for it.

Dependency parsing

The next stage is to create a dependency grammar. The dependency grammar describes and labels dependencies between constituents. It is made up of two main sections, <GRPAR> which fixes up the parse provided by the chunker. In the example, the verb is moved to the top of the sentence, above the complement, and <GRLAB> which labels parts of the parse.

Note that in Breton, sentences with eo (a form of bezañ 'to be') always have the structure Object—Verb—Subject, so require special attention. We thus label the left side as the predicate complement and the right side as the subject.

<GRPAR>

1 - - (sn,verb-eo) top_right RELABEL -  % (Ur yezh keltiek (eo))

</GRPAR>

<GRLAB>

verb-eo attr-pred d.label=sn d.side=left p.label=verb-eo %% Label dependent on the left of the verb-eo as attr-pred
verb-eo ncsubj d.label=sn d.side=right p.label=verb-eo %% Label dependent on the right of verb-eo as ncsubj

</GRLAB>

In the above grammar, d.side stands for the side of the dependent, and p.label stands for the label of the parent. This file is comprehensively documented in the section Dependency parser rule file in the Freeling documentation.

We'll put the file in matxin-br-en.br.dep, and the resulting output of the parse is,

$ echo "Ur yezh indezeuropek eo ar brezhoneg." | fl-morph matxin-br-en.tok.dat matxin-br-en.spt.dat br-en.br.db  | \
    fl-tagger matxin-br-en.br.relax | fl-parser matxin-br-en.br.gram matxin-br-en.br.dep | iconv -f latin1
verb-eo/top/(eo bezañ VMIP3S0) [
  sn/attr-pred/(yezh yezh NCFSV0) [
    indef/modnorule/(Ur un DI0CN0)
    adj/modnorule/(indezeuropek indezeuropek AQ0CN0)
  ]
  sn/ncsubj/(brezhoneg brezhoneg NCMSV0) [
    def/modnorule/(ar an DA0CN0)
  ]
  punt/modnomatch/(. . Fp)
]

Configuration file

So now we have a more or less working analysis stage, we need to get this analysis into a form that can be used by Matxin. This will involve writing a configuration file that specifies all of the modules that we've used above in one place. Below is a minimal configuration file for the modules we've used above. All of the options are mandatory, if one is left out, cryptic error messages may occur, so it is best to just copy and paste this, change the paths and then add to it as we go along.

#### Language
Lang=br

## Valid input/output formats are: plain, token, splitted, morfo, tagged, parsed
InputFormat=plain
OutputFormat=dep

# Consider each newline as a sentence end
AlwaysFlush=no

# Tokenizer options
TokenizerFile="/home/fran/MATXIN/source/matxin-br-en/matxin-br-en.tok.dat"

# Splitter options
SplitterFile="/home/fran/MATXIN/source/matxin-br-en/matxin-br-en.spt.dat"

# Morphological analysis options
SuffixFile=""
SuffixAnalysis=no
MultiwordsDetection=no
LocutionsFile=""
DecimalPoint="."
ThousandPoint=","
QuantitiesFile=""
NumbersDetection=no
PunctuationDetection=no
PunctuationFile=""
DatesDetection=no
QuantitiesDetection=no
DictionarySearch=yes
DictionaryFile=/home/fran/MATXIN/source/matxin-br-en/br-en.br.db
ProbabilityAssignment=no
ProbabilityFile=""

# NER options
NERecognition=none
NPDataFile=""

# Tagger options
Tagger=relax
TaggerRelaxFile=/home/fran/MATXIN/source/matxin-br-en/matxin-br-en.br.relax
TaggerRelaxMaxIter=500
TaggerRelaxScaleFactor=670.0
TaggerRelaxEpsilon=0.001
TaggerRetokenize=no
TaggerForceSelect=tagger

# Parser options
GrammarFile=/home/fran/MATXIN/source/matxin-br-en/matxin-br-en.br.gram

# Dependency Parser options
DepParser=txala
DepTxalaFile=/home/fran/MATXIN/source/matxin-br-en/matxin-br-en.br.dep

When we've set up the config file we can use the Analyzer program from the Matxin package. The final output from the Analyzer will be:

$ echo "Ur yezh indezeuropek eo ar brezhoneg." | Analyzer -f matxin-br-en.br-en.cfg  | iconv -f latin1

<?xml version='1.0' encoding='UTF-8' ?>
<corpus>
<SENTENCE ord='1' alloc='0'>
<CHUNK ord='2' alloc='21' type='verb-eo' si='top'>
  <NODE ord='4' alloc='21' form='eo' lem='bezañ' mi='VMIP3S0'>
  </NODE>
  <CHUNK ord='1' alloc='0' type='sn' si='attr-pred'>
    <NODE ord='2' alloc='3' form='yezh' lem='yezh' mi='NCFSV0'>
      <NODE ord='1' alloc='0' form='Ur' lem='un' mi='DI0CN0'>
      </NODE>
      <NODE ord='3' alloc='8' form='indezeuropek' lem='indezeuropek' mi='AQ0CN0'>
      </NODE>
    </NODE>
  </CHUNK>
  <CHUNK ord='3' alloc='24' type='sn' si='ncsubj'>
    <NODE ord='6' alloc='27' form='brezhoneg' lem='brezhoneg' mi='NCMSV0'>
      <NODE ord='5' alloc='24' form='ar' lem='an' mi='DA0CN0'>
      </NODE>
    </NODE>
  </CHUNK>
  <CHUNK ord='4' alloc='36' type='punt' si='modnomatch'>
    <NODE ord='7' alloc='36' form='.' lem='.' mi='Fp'>
    </NODE>
  </CHUNK>
</CHUNK>
</SENTENCE>
</corpus>

Which is an XML representation (see Documentation of Matxin) of the dependency analysis we saw earlier.