Resource grammar writing HOWTO
Author: Aarne Ranta <aarne (at) cs.chalmers.se>
Last update: %%date(%c)

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The purpose of this document is to tell how to implement the GF
resource grammar API for a new language. We will //not// cover how
to use the resource grammar, nor how to change the API. But we
will give some hints how to extend the API.

A manual for using the resource grammar is found in

[``http://www.cs.chalmers.se/~aarne/GF/doc/resource.pdf`` http://www.cs.chalmers.se/~aarne/GF/doc/resource.pdf].

A tutorial on GF, also introducing the idea of resource grammars, is found in

[``http://www.cs.chalmers.se/~aarne/GF/doc/tutorial/gf-tutorial2.html`` ../../../doc/tutorial/gf-tutorial2.html].

This document concerns the API v. 1.0. You can find the current code in 

[``http://www.cs.chalmers.se/~aarne/GF/lib/resource-1.0/`` ..]

See the [``README`` ../README] for
details on how this differs from previous versions.



==The resource grammar API==

The API is divided into a bunch of ``abstract`` modules.
The following figure gives the dependencies of these modules.

[Grammar.png] 

Thus the API consists of a grammar and a lexicon, which is
provided for test purposes.

The module structure is rather flat: most modules are direct
parents of ``Grammar``. The idea
is that you can concentrate on one linguistic aspect at a time, or
also distribute the work among several authors. The module ``Cat``
defines the "glue" that ties the aspects together - a type system
to which all the other modules conform, so that e.g. ``NP`` means
the same thing in those modules that use ``NP``s and those that
constructs them.



===Phrase category modules===

The direct parents of the top will be called **phrase category modules**,
since each of them concentrates on a particular phrase category (nouns, verbs,
adjectives, sentences,...). A phrase category module tells 
//how to construct phrases in that category//. You will find out that
all functions in any of these modules have the same value type (or maybe
one of a small number of different types). Thus we have


- ``Noun``: construction of nouns and noun phrases
- ``Adjective``: construction of adjectival phrases
- ``Verb``: construction of verb phrases
- ``Adverb``: construction of adverbial phrases
- ``Numeral``: construction of cardinal and ordinal numerals
- ``Sentence``: construction of sentences and imperatives
- ``Question``: construction of questions
- ``Relative``: construction of relative clauses
- ``Conjunction``: coordination of phrases
- ``Phrase``: construction of the major units of text and speech
- ``Text``: construction of texts as sequences of phrases
- ``Idiom``: idiomatic phrases such as existentials




===Infrastructure modules===

Expressions of each phrase category are constructed in the corresponding
phrase category module. But their //use// takes mostly place in other modules.
For instance, noun phrases, which are constructed in ``Noun``, are
used as arguments of functions of almost all other phrase category modules. 
How can we build all these modules independently of each other?

As usual in typeful programming, the //only// thing you need to know
about an object you use is its type. When writing a linearization rule
for a GF abstract syntax function, the only thing you need to know is
the linearization types of its value and argument categories. To achieve
the division of the resource grammar to several parallel phrase category modules,
what we need is an underlying definition of the linearization types. This
definition is given as the implementation of

- ``Cat``: syntactic categories of the resource grammar


Any resource grammar implementation has first to agree on how to implement
``Cat``. Luckily enough, even this can be done incrementally: you
can skip the ``lincat`` definition of a category and use the default
``{s : Str}`` until you need to change it to something else. In
English, for instance, many categories do have this linearization type.



===Lexical modules===

What is lexical and what is syntactic is not as clearcut in GF as in
some other grammar formalisms. Logically, lexical means atom, i.e. a
``fun`` with no arguments. Linguistically, one may add to this
that the ``lin`` consists of only one token (or of a table whose values
are single tokens). Even in the restricted lexicon included in the resource
API, the latter rule is sometimes violated in some languages. For instance,
``Structural.both7and_DConj`` is an atom, but its linearization is
two words e.g. //both - and//.

Another characterization of lexical is that lexical units can be added
almost //ad libitum//, and they cannot be defined in terms of already
given rules. The lexical modules of the resource API are thus more like
samples than complete lists. There are two such modules:

- ``Structural``: structural words (determiners, conjunctions,...)
- ``Lexicon``: basic everyday content words (nouns, verbs,...)


The module ``Structural`` aims for completeness, and is likely to
be extended in future releases of the resource. The module ``Lexicon``
gives a "random" list of words, which enable interesting testing of syntax,
and also a check list for morphology, since those words are likely to include
most morphological patterns of the language.

In the case of ``Lexicon`` it may come out clearer than anywhere else
in the API that it is impossible to give exact translation equivalents in
different languages on the level of a resource grammar. In other words,
application grammars are likely to use the resource in different ways for
different languages.


==Language-dependent syntax modules==

In addition to the common API, there is room for language-dependent extensions
of the resource. The top level of each languages looks as follows (with English as example):
```
  abstract English = Grammar, ExtraEngAbs, DictEngAbs
```
where ``ExtraEngAbs`` is a collection of syntactic structures specific to English,
and ``DictEngAbs`` is an English dictionary 
(at the moment, it consists of ``IrregEngAbs``,
the irregular verbs of English). Each of these language-specific grammars has 
the potential to grow into a full-scale grammar of the language. These grammar
can also be used as libraries, but the possibility of using functors is lost.

To give a better overview of language-specific structures, 
modules like ``ExtraEngAbs``
are built from a language-independent module ``ExtraAbs`` 
by restricted inheritance:
```
  abstract ExtraEngAbs = Extra [f,g,...]
```
Thus any category and function in ``Extra`` may be shared by a subset of all
languages. One can see this set-up as a matrix, which tells 
what ``Extra`` structures
are implemented in what languages. For the common API in ``Grammar``, the matrix
is filled with 1's (everything is implemented in every language).

In a minimal resource grammar implementation, the language-dependent
extensions are just empty modules, but it is good to provide them for
the sake of uniformity.


==The core of the syntax==

Among all categories and functions, a handful are 
most important and distinct ones, of which the others are can be 
seen as variations. The categories are
```
  Cl ; VP ; V2 ; NP ; CN ; Det ; AP ;
```
The functions are
```
  PredVP  : NP  -> VP -> Cl ;  -- predication
  ComplV2 : V2  -> NP -> VP ;  -- complementization
  DetCN   : Det -> CN -> NP ;  -- determination
  ModCN   : AP  -> CN -> CN ;  -- modification
```
This [toy Latin grammar  latin.gf] shows in a nutshell how these
rules relate the categories to each other. It is intended to be a
first approximation when designing the parameter system of a new
language. 


===Another reduced API===

If you want to experiment with a small subset of the resource API first, 
try out the module 
[Syntax http://www.cs.chalmers.se/~aarne/GF/doc/tutorial/resource/Syntax.gf]
explained in the
[GF Tutorial http://www.cs.chalmers.se/~aarne/GF/doc/tutorial/gf-tutorial2.html].


===The present-tense fragment===

Some lines in the resource library are suffixed with the comment
```--# notpresent
which is used by a preprocessor to exclude those lines from 
a reduced version of the full resource. This present-tense-only
version is useful for applications in most technical text, since
they reduce the grammar size and compilation time. It can also
be useful to exclude those lines in a first version of resource
implementation. To compile a grammar with present-tense-only, use
```
  i -preproc=GF/lib/resource-1.0/mkPresent LangGer.gf
```



==Phases of the work==

===Putting up a directory===

Unless you are writing an instance of a parametrized implementation
(Romance or Scandinavian), which will be covered later, the
simplest way is to follow roughly the following procedure. Assume you
are building a grammar for the German language. Here are the first steps,
which we actually followed ourselves when building the German implementation
of resource v. 1.0.

+ Create a sister directory for ``GF/lib/resource/english``, named
     ``german``.
```
       cd GF/lib/resource/
       mkdir german
       cd german
```

+ Check out the [ISO 639 3-letter language code 
   http://www.w3.org/WAI/ER/IG/ert/iso639.htm] 
   for German: both ``Ger`` and ``Deu`` are given, and we pick ``Ger``.

+ Copy the ``*Eng.gf`` files from ``english`` ``german``,
     and rename them:
```
       cp ../english/*Eng.gf .
       rename 's/Eng/Ger/' *Eng.gf
```

+ Change the ``Eng`` module references to ``Ger`` references
     in all files:
```
       sed -i 's/English/German/g' *Ger.gf
       sed -i 's/Eng/Ger/g' *Ger.gf
```
  The first line prevents changing the word ``English``, which appears
  here and there in comments, to ``Gerlish``.

+ This may of course change unwanted occurrences of the 
     string ``Eng`` - verify this by
```
       grep Ger *.gf
```
     But you will have to make lots of manual changes in all files anyway!

+ Comment out the contents of these files:
``` 
       sed -i 's/^/--/' *Ger.gf
```
     This will give you a set of templates out of which the grammar
     will grow as you uncomment and modify the files rule by rule.

+ In all ``.gf`` files, uncomment the module headers and brackets,
  leaving the module bodies commented. Unfortunately, there is no
  simple way to do this automatically (or to avoid commenting these
  lines in the previous step) - but uncommenting the first
  and the last lines will actually do the job for many of the files.

+ Uncomment the contents of the main grammar file:
``` 
       sed -i 's/^--//' LangGer.gf
```

+ Now you can open the grammar ``LangGer`` in GF:
``` 
       gf LangGer.gf
```
  You will get lots of warnings on missing rules, but the grammar will compile.

+ At all following steps you will now have a valid, but incomplete
     GF grammar. The GF command
``` 
       pg -printer=missing
```
     tells you what exactly is missing.


Here is the module structure of ``LangGer``. It has been simplified by leaving out
the majority of the phrase category modules. Each of them has the same dependencies
as e.g. ``VerbGer``.

[German.png]


===Direction of work===

The real work starts now. There are many ways to proceed, the main ones being
- Top-down: start from the module ``Phrase`` and go down to ``Sentence``, then
  ``Verb``, ``Noun``, and in the end ``Lexicon``. In this way, you are all the time
  building complete phrases, and add them with more content as you proceed.
  **This approach is not recommended**. It is impossible to test the rules if
  you have no words to apply the constructions to.

- Bottom-up: set as your first goal to implement ``Lexicon``. To this end, you
  need to write ``ParadigmsGer``, which in turn needs parts of 
  ``MorphoGer`` and ``ResGer``.
  **This approach is not recommended**. You can get stuck to details of
  morphology such as irregular words, and you don't have enough grasp about
  the type system to decide what forms to cover in morphology.


The practical working direction is thus a saw-like motion between the morphological
and top-level modules. Here is a possible course of the work that gives enough
test data and enough general view at any point:
+ Define ``Cat.N`` and the required parameter types in ``ResGer``. As we define
```
  lincat N  = {s : Number => Case => Str ; g : Gender} ;
```
we need the parameter types ``Number``, ``Case``, and ``Gender``. The definition
of ``Number`` in [``common/ParamX``  ../common/ParamX.gf] works for German, so we
use it and just define ``Case`` and ``Gender`` in ``ResGer``.

+ Define ``regN`` in ``ParadigmsGer``. In this way you can 
already implement a huge amount of nouns correctly in ``LexiconGer``. Actually
just adding ``mkN`` should suffice for every noun - but, 
since it is tedious to use, you
might proceed to the next step before returning to morphology and defining the
real work horse ``reg2N``.

+ While doing this, you may want to test the resource independently. Do this by
```
       i -retain ParadigmsGer
       cc regN "Kirche"
```

+ Proceed to determiners and pronouns in 
``NounGer`` (``DetCN UsePron DetSg SgQuant NoNum NoOrd DefArt IndefArt UseN``)and 
``StructuralGer`` (``i_Pron every_Det``). You also need some categories and
parameter types. At this point, it is maybe not possible to find out the final
linearization types of ``CN``, ``NP``, and ``Det``, but at least you should
be able to correctly inflect noun phrases such as //every airplane//:
```
  i LangGer.gf
  l -table DetCN every_Det (UseN airplane_N)

  Nom: jeder Flugzeug
  Acc: jeden Flugzeug
  Dat: jedem Flugzeug
  Gen: jedes Flugzeugs
```

+ Proceed to verbs: define ``CatGer.V``,  ``ResGer.VForm``, and
``ParadigmsGer.regV``. You may choose to exclude ``notpresent``
cases at this point. But anyway, you will be able to inflect a good
number of verbs in ``Lexicon``, such as
``live_V`` (``regV "leven"``).

+ Now you can soon form your first sentences: define ``VP`` and
``Cl`` in ``CatGer``, ``VerbGer.UseV``, and ``SentenceGer.PredVP``.
Even if you have excluded the tenses, you will be able to produce
```
  i -preproc=mkPresent LangGer.gf
  > l -table PredVP (UsePron i_Pron) (UseV live_V)

  Pres Simul Pos Main: ich lebe
  Pres Simul Pos Inv:  lebe ich
  Pres Simul Pos Sub:  ich lebe
  Pres Simul Neg Main: ich lebe nicht
  Pres Simul Neg Inv:  lebe ich nicht
  Pres Simul Neg Sub:  ich nicht lebe
```

+ Transitive verbs (``CatGer.V2 ParadigmsGer.dirV2 VerbGer.ComplV2``) 
are a natural next step, so that you can
produce ``ich liebe dich``.

+ Adjectives (``CatGer.A ParadigmsGer.regA NounGer.AdjCN AdjectiveGer.PositA``) 
will force you to think about strong and weak declensions, so that you can
correctly inflect //my new car, this new car//. 

+ Once you have implemented the set
(``Noun.DetCN Noun.AdjCN Verb.UseV Verb.ComplV2 Sentence.PredVP),
you have overcome most of difficulties. You know roughly what parameters
and dependences there are in your language, and you can now produce very
much in the order you please. 



===The develop-test cycle===

The following develop-test cycle will
be applied most of the time, both in the first steps described above
and in later steps where you are more on your own.

+ Select a phrase category module, e.g. ``NounGer``, and uncomment some
  linearization rules (for instance, ``DefSg``, which is
  not too complicated).

+ Write down some German examples of this rule, for instance translations
     of "the dog", "the house", "the big house", etc. Write these in all their
     different forms (two numbers and four cases).

+ Think about the categories involved (``CN, NP, N``) and the
     variations they have. Encode this in the lincats of ``CatGer``.
     You may have to define some new parameter types in ``ResGer``.

+ To be able to test the construction, 
     define some words you need to instantiate it
     in ``LexiconGer``. You will also need some regular inflection patterns
     in``ParadigmsGer``.

+ Test by parsing, linearization,
     and random generation. In particular, linearization to a table should
     be used so that you see all forms produced:
```
       gr -cat=NP -number=20 -tr | l -table
```

+ Spare some tree-linearization pairs for later regression testing. Use the
  ``tree_bank`` command,
```
       gr -cat=NP -number=20 | tb -xml | wf NP.tb
```
  You can later compared your modified grammar to this treebank by
```
       rf NP.tb | tb -c
```



You are likely to run this cycle a few times for each linearization rule
you implement, and some hundreds of times altogether. There are 66 ``cat``s and
458 ``funs`` in ``Lang`` at the moment; 149 of the ``funs`` are outside the two
lexicon modules).

Here is a [live log ../german/log.txt] of the actual process of
building the German implementation of resource API v. 1.0.
It is the basis of the more detailed explanations, which will
follow soon. (You will found out that these explanations involve
a rational reconstruction of the live process! Among other things, the
API was changed during the actual process to make it more intuitive.)


===Resource modules used===

These modules will be written by you.

- ``ResGer``: parameter types and auxiliary operations 
(a resource for the resource grammar!)
- ``ParadigmsGer``: complete inflection engine and most important regular paradigms
- ``MorphoGer``: auxiliaries for ``ParadigmsGer`` and ``StructuralGer``. This need
not be separate from ``ResGer``.


These modules are language-independent and provided by the existing resource
package.

- ``ParamX``: parameter types used in many languages
- ``CommonX``: implementation of language-uniform categories 
    such as $Text$ and $Phr$, as well as of
    the logical tense, anteriority, and polarity parameters
- ``Coordination``: operations to deal with lists and coordination
- ``Prelude``: general-purpose operations on strings, records,
      truth values, etc.
- ``Predefined``: general-purpose operations with hard-coded definitions


An important decision is what rules to implement in terms of operations in
``ResGer``. A golden rule of functional programming says that, whenever
you find yourself programming by copy and paste, you should write a function
instead. This indicates that an operation should be created if it is to be
used at least twice. At the same time, a sound principle of vicinity says that
it should not require too much browsing to understand what a rule does.
From these two principles, we have derived the following practice:
- If an operation is needed //in two different modules//, 
it should be created in ``ResGer``. An example is ``mkClause``, 
used in ``Sentence``, ``Question``, and ``Relative``-
- If an operation is needed //twice in the same module//, but never
outside, it should be created in the same module. Many examples are
found in ``Numerals``.
- If an operation is only needed once, it should not be created (but rather
inlined). Most functions in phrase category modules are implemented in this
way.


This discipline is very different from the one followed in earlier
versions of the library (up to 0.9). We then valued the principle of
abstraction more than vicinity, creating layers of abstraction for
almost everything. This led in practice to the duplication of almost
all code on the ``lin`` and ``oper`` levels, and made the code
hard to understand and maintain.



===Morphology and lexicon===

The paradigms needed to implement
``LexiconGer`` are defined in
``ParadigmsGer``.
This module provides high-level ways to define the linearization of
lexical items, of categories ``N, A, V`` and their complement-taking
variants.



For ease of use, the ``Paradigms`` modules follow a certain
naming convention. Thus they for each lexical category, such as ``N``,
the functions

- ``mkN``, for worst-case construction of ``N``. Its type signature
     has the form
```
       mkN : Str -> ... -> Str -> P -> ... -> Q -> N
```
     with as many string and parameter arguments as can ever be needed to
     construct an ``N``.
- ``regN``, for the most common cases, with just one string argument:
```
       regN : Str -> N
```
- A language-dependent (small) set of functions to handle mild irregularities
     and common exceptions.

For the complement-taking variants, such as ``V2``, we provide

- ``mkV2``, which takes a ``V`` and all necessary arguments, such
     as case and preposition:
```
       mkV2 : V -> Case -> Str -> V2 ;
```
- A language-dependent (small) set of functions to handle common special cases,
     such as direct transitive verbs:
```
       dirV2 : V -> V2 ;
       -- dirV2 v = mkV2 v accusative [] 
```


The golden rule for the design of paradigms is that

- The user will only need function applications with constants and strings,
     never any records or tables.


The discipline of data abstraction moreover requires that the user of the resource
is not given access to parameter constructors, but only to constants that denote 
them. This gives the resource grammarian the freedom to change the underlying
data representation if needed. It means that the ``ParadigmsGer`` module has
to define constants for those parameter types and constructors that 
the application grammarian may need to use, e.g.
```
  oper 
    Case : Type ;
    nominative, accusative, genitive, dative : Case ;
```
These constants are defined in terms of parameter types and constructors
in ``ResGer`` and ``MorphoGer``, which modules are not
visible to the application grammarian.


===Lock fields===

An important difference between ``MorphoGer`` and
``ParadigmsGer`` is that the former uses "raw" record types
for word classes, whereas the latter used category symbols defined in
``CatGer``. When these category symbols are used to denote
record types in a resource modules, such as ``ParadigmsGer``,
a **lock field** is added to the record, so that categories
with the same implementation are not confused with each other.
(This is inspired by the ``newtype`` discipline in Haskell.)
For instance, the lincats of adverbs and conjunctions are the same
in ``CommonX`` (and therefore in ``CatGer``, which inherits it):
```
  lincat Adv  = {s : Str} ;
  lincat Conj = {s : Str} ;
```
But when these category symbols are used to denote their linearization 
types in resource module, these definitions are translated to
```
  oper Adv  : Type = {s : Str  ; lock_Adv  : {}} ;
  oper Conj : Type = {s : Str} ; lock_Conj : {}} ;
```
In this way, the user of a resource grammar cannot confuse adverbs with
conjunctions. In other words, the lock fields force the type checker
to function as grammaticality checker.

When the resource grammar is ``open``ed in an application grammar, the
lock fields are never seen (except possibly in type error messages),
and the application grammarian should never write them herself. If she
has to do this, it is a sign that the resource grammar is incomplete, and
the proper way to proceed is to fix the resource grammar.

The resource grammarian has to provide the dummy lock field values
in her hidden definitions of constants in ``Paradigms``. For instance,
```
  mkAdv : Str -> Adv ;
  -- mkAdv s = {s = s ; lock_Adv = <>} ;
```


===Lexicon construction===

The lexicon belonging to ``LangGer`` consists of two modules:

- ``StructuralGer``, structural words, built by directly using
     ``MorphoGer``.
- ``BasicGer``, content words, built by using ``ParadigmsGer``.


The reason why ``MorphoGer`` has to be used in ``StructuralGer``
is that ``ParadigmsGer`` does not contain constructors for closed
word classes such as pronouns and determiners. The reason why we
recommend ``ParadigmsGer`` for building ``LexiconGer`` is that
the coverage of the paradigms gets thereby tested and that the
use of the paradigms in ``LexiconGer`` gives a good set of examples for
those who want to build new lexica.







==Inside grammar modules==

Detailed implementation tricks
are found in the comments of each module.


===The category system===

- [Common gfdoc/Common.html], [CommonX ../common/CommonX.gf]
- [Cat gfdoc/Cat.html], [CatGer gfdoc/CatGer.gf]


===Phrase category modules===

- [Noun gfdoc/Noun.html], [NounGer ../german/NounGer.gf]
- [Adjective gfdoc/Adjective.html], [AdjectiveGer ../german/AdjectiveGer.gf]
- [Verb gfdoc/Verb.html], [VerbGer ../german/VerbGer.gf]
- [Adverb gfdoc/Adverb.html], [AdverbGer ../german/AdverbGer.gf]
- [Numeral gfdoc/Numeral.html], [NumeralGer ../german/NumeralGer.gf]
- [Sentence gfdoc/Sentence.html], [SentenceGer ../german/SentenceGer.gf]
- [Question gfdoc/Question.html], [QuestionGer ../german/QuestionGer.gf]
- [Relative gfdoc/Relative.html], [RelativeGer ../german/RelativeGer.gf]
- [Conjunction gfdoc/Conjunction.html], [ConjunctionGer ../german/ConjunctionGer.gf]
- [Phrase gfdoc/Phrase.html], [PhraseGer ../german/PhraseGer.gf]
- [Text gfdoc/Text.html], [TextX ../common/TextX.gf]
- [Idiom gfdoc/Idiom.html], [IdiomGer ../german/IdiomGer.gf]
- [Lang gfdoc/Lang.html], [LangGer ../german/LangGer.gf]


===Resource modules===

- [ResGer ../german/ResGer.gf]
- [MorphoGer ../german/MorphoGer.gf]
- [ParadigmsGer gfdoc/ParadigmsGer.html], [ParadigmsGer.gf ../german/ParadigmsGer.gf]


===Lexicon===

- [Structural gfdoc/Structural.html], [StructuralGer ../german/StructuralGer.gf]
- [Lexicon gfdoc/Lexicon.html], [LexiconGer ../german/LexiconGer.gf]


==Lexicon extension==

===The irregularity lexicon===

It may be handy to provide a separate module of irregular
verbs and other words which are difficult for a lexicographer
to handle. There are usually a limited number of such words - a
few hundred perhaps. Building such a lexicon separately also
makes it less important to cover //everything// by the
worst-case paradigms (``mkV`` etc).



===Lexicon extraction from a word list===

You can often find resources such as lists of 
irregular verbs on the internet. For instance, the
[Irregular German Verbs http://www.iee.et.tu-dresden.de/~wernerr/grammar/verben_dt.html] 
page gives a list of verbs in the
traditional tabular format, which begins as follows:
```
  backen (du bäckst, er bäckt)	                 backte [buk]              gebacken
  befehlen (du befiehlst, er befiehlt; befiehl!) befahl (beföhle; befähle) befohlen
  beginnen                                       begann (begönne; begänne) begonnen
  beißen                                         biß                       gebissen
```
All you have to do is to write a suitable verb paradigm
```
  irregV : (x1,_,_,_,_,x6 : Str) -> V ;
```
and a Perl or Python or Haskell script that transforms
the table to
```
  backen_V   = irregV "backen" "bäckt" "back" "backte" "backte" "gebacken" ;
  befehlen_V = irregV "befehlen" "befiehlt" "befiehl" "befahl" "beföhle" "befohlen" ;
```

When using ready-made word lists, you should think about
coyright issues. Ideally, all resource grammar material should
be provided under GNU General Public License.



===Lexicon extraction from raw text data===

This is a cheap technique to build a lexicon of thousands
of words, if text data is available in digital format.
See the [Functional Morphology http://www.cs.chalmers.se/~markus/FM/] 
homepage for details.



===Extending the resource grammar API===

Sooner or later it will happen that the resource grammar API
does not suffice for all applications. A common reason is
that it does not include idiomatic expressions in a given language.
The solution then is in the first place to build language-specific
extension modules. This chapter will deal with this issue (to be completed).


==Writing an instance of parametrized resource grammar implementation==

Above we have looked at how a resource implementation is built by
the copy and paste method (from English to German), that is, formally
speaking, from scratch. A more elegant solution available for 
families of languages such as Romance and Scandinavian is to
use parametrized modules. The advantages are

- theoretical: linguistic generalizations and insights
- practical: maintainability improves with fewer components


In this chapter, we will look at an example: adding Italian to
the Romance family (to be completed). Here is a set of
[slides http://www.cs.chalmers.se/~aarne/geocal2006.pdf]
on the topic.


==Parametrizing a resource grammar implementation==

This is the most demanding form of resource grammar writing.
We do //not// recommend the method of parametrizing from the
beginning: it is easier to have one language first implemented 
in the conventional way and then add another language of the
same family by aprametrization. This means that the copy and
paste method is still used, but at this time the differences
are put into an ``interface`` module. 



This chapter will work out an example of how an Estonian grammar
is constructed from the Finnish grammar through parametrization.


