==Coverage==

The GF Resource Grammar Library contains grammar rules for
10 languages (in addition, 2 languages are available as incomplete
implementations, and a few more are under construction). Its purpose
is to make these rules available for application programmers,
who can thereby concentrate on the semantic and stylistic
aspects of their grammars, without having to think about 
grammaticality. The targeted level of application grammarians
is that of a skilled programmer with
a practical knowledge of the target languages, but without
theoretical knowledge about their grammars.
Such a combination of
skills is typical of programmers who, for instance, want to localize
software to new languages.

The current resource languages are
- ``Ara``bic (incomplete)
- ``Cat``alan (incomplete)
- ``Dan``ish
- ``Eng``lish
- ``Fin``nish
- ``Fre``nch
- ``Ger``man
- ``Ita``lian
- ``Nor``wegian
- ``Rus``sian
- ``Spa``nish
- ``Swe``dish


The first three letters (``Eng`` etc) are used in grammar module names.
The incomplete Arabic and Catalan implementations are 
enough to be used in many applications; they both contain, amoung other 
things, complete inflectional morphology.



==A first example==

To give an example application, consider a system for steering
music playing devices by voice commands. In the application,
we may have a semantical category ``Kind``, examples
of ``Kind``s being ``Song`` and ``Artist``. In German, for instance, ``Song`` 
is linearized into the noun "Lied", but knowing this is not
enough to make the application work, because the noun must be 
produced in both singular and plural, and in four different
cases. By using the resource grammar library, it is enough to
write
```
  lin Song = mkN "Lied" "Lieder" neuter
```
and the eight forms are correctly generated. The resource grammar
library contains a complete set of inflectional paradigms (such as
``mkN`` here), enabling the definition of any lexical items.

The resource grammar library is not only about inflectional paradigms - it
also has syntax rules. The music player application
might also want to modify songs with properties, such as "American",
"old", "good". The German grammar for adjectival modifications is
particularly complex, because adjectives have to agree in gender,
number, and case, and also depend on what determiner is used
("ein amerikanisches Lied" vs. "das amerikanische Lied"). All this
variation is taken care of by the resource grammar function
```
  mkCN : AP -> CN -> CN
```
(see the table in the end of this document for the list of all resource grammar
functions). The resource grammar implementation of the rule adding properties
to kinds is
```
  lin PropKind kind prop = mkCN prop kind
```
given that 
```
  lincat Prop = AP
  lincat Kind = CN
```
The resource library API is devided into language-specific 
and language-independent parts. To put it roughly,
- the lexicon API is language-specific
- the syntax API is language-independent


Thus, to render the above example in French instead of German, we need to
pick a different linearization of ``Song``,
```
  lin Song = mkN "chanson" feminine
```
But to linearize ``PropKind``, we can use the very same rule as in German.
The resource function ``mkCN`` has different implementations in the two
languages (e.g. a different word order in French), 
but the application programmer need not care about the difference.



==Note on APIs==

From version 1.1 onwards, the resource library is available via two
APIs:
- original ``fun`` and ``oper`` definitions
- overloaded ``oper`` definitions


Introducing overloading in GF version 2.7 has been a success in improving
the accessibility of libraries. It has also created a layer of abstraction
between the writers and users of libraries, and thereby makes the library
easier to modify. We shall therefore use the overloaded API
in this document. The original function names are mainly interesting
for those who want to write or modify libraries.



==A complete example==

To summarize the example, and also give a template for a programmer to work on,
here is the complete implementation of a small system with songs and properties.
The abstract syntax defines a "domain ontology":
```
  abstract Music = {
    
  cat 
    Kind, 
    Property ;
  fun 
    PropKind : Kind -> Property -> Kind ; 
    Song : Kind ;
    American : Property ;
  }
```
The concrete syntax is defined by a functor (parametrized module),
independently of language, by opening
two interfaces: the resource ``Syntax`` and an application lexicon.
```
  incomplete concrete MusicI of Music = 
      open Syntax, MusicLex in {
  lincat 
    Kind = CN ;
    Property = AP ;
  lin
    PropKind k p = mkCN p k ;
    Song = mkCN song_N ;
    American = mkAP american_A ;
  }
```
The application lexicon ``MusicLex`` is an interface
opening the resource category system ``Cat``. 
```
  interface MusicLex = Cat ** { 
  oper
    song_N : N ;
    american_A : A ;
  }
```
It could also be an abstract syntax that extends ``Cat``, but
this would limit the kind of constructions that are possible in
the interface

Each language has its own concrete syntax, which opens the 
inflectional paradigms module for that language:
```
  interface MusicLexGer of MusicLex = 
      CatGer ** open ParadigmsGer in {    
  oper
    song_N = mkN "Lied" "Lieder" neuter ;
    american_A = mkA "amerikanisch" ;
  }

  interface MusicLexFre of MusicLex = 
      CatFre ** open ParadigmsFre in {
  oper
    song_N = mkN "chanson" feminine ;
    american_A = mkA "américain" ;
  }
```
The top-level ``Music`` grammars are obtained by 
instantiating the two interfaces of ``MusicI``:
```
  concrete MusicGer of Music = MusicI with
    (Syntax = SyntaxGer),
    (MusicLex = MusicLexGer) ;

  concrete MusicFre of Music = MusicI with
    (Syntax = SyntaxFre),
    (MusicLex = MusicLexFre) ;
```
Both of these files can use the same ``path``, defined as
```
  --# -path=.:present:prelude
```
The ``present`` category contains the compiled resources, restricted to
present tense; ``alltenses`` has the full resources.

To localize the music player system to a new language, 
all that is needed is two modules,
one implementing ``MusicLex`` and the other 
instantiating ``Music``. The latter is
completely trivial, whereas the former one involves the choice of correct
vocabulary and inflectional paradigms. For instance, Finnish is added as follows:
```
  instance MusicLexFin of MusicLex = 
      CatFin ** open ParadigmsFin in {
  oper
    song_N = mkN "kappale" ;
    american_A = mkA "amerikkalainen" ;
  }

  concrete MusicFin of Music = MusicI with
    (Syntax = SyntaxFin),
    (MusicLex = MusicLexFin) ;
```
More work is of course needed if the language-independent linearizations in
MusicI are not satisfactory for some language. The resource grammar guarantees
that the linearizations are possible in all languages, in the sense of grammatical,
but they might of course be inadequate for stylistic reasons. Assume, 
for the sake of argument, that adjectival modification does not sound good in
English, but that a relative clause would be preferrable. One can then use
restricted inheritance of the functor:
```
  concrete MusicEng of Music = 
    MusicI - [PropKind] 
      with
        (Syntax = SyntaxEng),
        (MusicLex = MusicLexEng) ** 
    open SyntaxEng in {
  lin
    PropKind k p = mkCN k (mkRS (mkRCl which_RP (mkVP p))) ;
  }
```
The lexicon is as expected:
```
  instance MusicLexEng of MusicLex = 
      CatEng ** open ParadigmsEng in {
  oper
    song_N = mkN "song" ;
    american_A = mkA "American" ;
  }
```


==Lock fields==

//This section is only relevant as a guide to error messages that have to do with lock fields, and can be skipped otherwise.//

FIXME: this section may become obsolete.

When the categories of the resource grammar are used
in applications, a **lock field** is added to their linearization types.
The lock field for a category ``C`` is a record field 
```
  lock_C : {}
```
with the only possible value
```
  lock_C = <>
```
The lock field carries no information, but its presence
makes the linearization type of ``C``
unique, so that categories
with the same implementation are not confused with each other.
(This is inspired by the ``newtype`` discipline in Haskell.)

For example, the lincats of adverbs and conjunctions are the same
in ``CatEng`` (and therefore in ``GrammarEng``, which inherits it):
```
  lincat Adv  = {s : Str} ;
  lincat Conj = {s : Str} ;
```
But when these category symbols are used to denote their linearization 
types in an application, 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, 
and only functions from the resource are used in type-correct way, the
lock fields are never seen (except possibly in type error messages).
If an application grammarian has to write lock fields herself,
it is a sign that the guarantees given by the resource grammar 
no longer hold. But since the resource may be incomplete, the
application grammarian may occasionally have to provide the dummy
values of lock fields (always ``<>``, the empty record). 
Here is an example:
```
  mkUtt : Str -> Utt ;
  mkUtt s = {s = s ; lock_Utt = <>} ;
```
Currently, missing lock field produce warnings rather than errors,
but this behaviour of GF may change in future.


==Parsing with resource grammars?==

The intended use of the resource grammar is as a library for writing
application grammars. It is not designed for parsing e.g. newspaper text. There
are several reasons why this is not practical:
- Efficiency: the resource grammar uses complex data structures, in
particular, discontinuous constituents, which make parsing slow and the
parser size huge.
- Completeness: the resource grammar does not necessarily cover all rules
of the language - only enough many to be able to express everything
in one way or another.
- Lexicon: the resource grammar has a very small lexicon, only meant for test
purposes.
- Semantics: the resource grammar has very little semantic control, and may
accept strange input or deliver strange interpretations.
- Ambiguity: parsing in the resource grammar may return lots of results many
of which are implausible.


All of these problems should be solved in application grammars. 
The task of resource grammars is just to take care of low-level linguistic 
details such as inflection, agreement, and word order.

It is for the same reasons that resource grammars are not adequate for translation.
That the syntax API is implemented for different languages of course makes
it possible to translate via it - but there is no guarantee of translation
equivalence. Of course, the use of functor implementations such as ``MusicI``
above only extends to those cases where the syntax API does give translation
equivalence - but this must be seen as a limiting case, and bigger applications
will often use only restricted inheritance of ``MusicI``.



=To find rules in the resource grammar library=

==Inflection paradigms==

Inflection paradigms are defined separately for each language //L//
in the module ``Paradigms``//L//. To test them, the command 
``cc`` (= ``compute_concrete``)
can be used:
```
  > i -retain german/ParadigmsGer.gf

  > cc mkN "Schlange"
  {
    s : Number => Case => Str = table Number {
      Sg => table Case {
        Nom => "Schlange" ;
        Acc => "Schlange" ;
        Dat => "Schlange" ;
        Gen => "Schlange"
        } ;
      Pl => table Case {
        Nom => "Schlangen" ;
        Acc => "Schlangen" ;
        Dat => "Schlangen" ;
        Gen => "Schlangen"
        }
      } ;
    g : Gender = Fem
  }
```
For the sake of convenience, every language implements these five paradigms:
```
  oper
    mkN  : Str -> N ;   -- regular nouns
    mkA  : Str -> A :   -- regular adjectives
    mkV  : Str -> V ;   -- regular verbs
    mkPN : Str -> PN ;  -- regular proper names
    mkV2 : V   -> V2 ;  -- direct transitive verbs
```
It is often possible to initialize a lexicon by just using these functions,
and later revise it by using the more involved paradigms. For instance, in
German we cannot use ``mkN "Lied"`` for ``Song``, because the result would be a
Masculine noun with the plural form ``"Liede"``. 
The individual ``Paradigms`` modules
tell what cases are covered by the regular heuristics.

As a limiting case, one could even initialize the lexicon for a new language
by copying the English (or some other already existing) lexicon. This would
produce language with correct grammar but with content words directly borrowed from
English - maybe not so strange in certain technical domains.



==Syntax rules==

Syntax rules should be looked for in the module ``Constructors``.
Below this top-level module exposing overloaded constructors,
there are around 10 abstract modules, each defining constructors for
a group of one or more related categories. For instance, the module
``Noun`` defines how to construct common nouns, noun phrases, and determiners.
But these special modules are seldom or never needed by the users of the library.

TODO: when are they needed?

Browsing the libraries is helped by the gfdoc-generated HTML pages,
whose LaTeX versions are included in the present document.


==Special-purpose APIs==

To give an analogy with the well-known type setting software, GF can be compared
with TeX and the resource grammar library with LaTeX. 
Just like TeX frees the author
from thinking about low-level problems of page layout, so GF frees the grammarian
from writing parsing and generation algorithms. But quite a lot of knowledge of
//how// to write grammars is still needed, and the resource grammar library helps
GF grammarians in a way similar to how the LaTeX macro package helps TeX authors.

But even LaTeX is often too detailed and low-level, and users are encouraged to
develop their own macro packages. The same applies to GF resource grammars:
the application grammarian might not need all the choices that the resource
provides, but would prefer less writing and higher-level programming.
To this end, application grammarians may want to write their own views on the
resource grammar. 


==Browsing by the parser==

A method alternative to browsing library documentation is
to use the parser.
Even though parsing is not an intended end-user application 
of resource grammars, it is a useful technique for application grammarians
to browse the library. To find out which resource function implements
a particular structure, one can just parse a string that exemplifies this 
structure. For instance, to find out how sentences are built using 
transitive verbs, write
```
  > i english/LangEng.gf
 
  > p -cat=Cl "she loves him"
  PredVP (UsePron she_Pron) (ComplV2 love_V2 (UsePron he_Pron))
```
The parser returns original constructors, not overloaded ones. Overloaded
constructors can be returned, so far with experimental heuristics, by using
the grammar ``api/toplevel/OverLangEng.gf`` and a special flag:
```
  > i api/toplevel/OverLangEng.gf

  > p -cat=Cl -overload "she loves him"
  mkCl (mkNP she_Pron) love_V2 (mkNP he_Pron)
```
Parsing with the English resource grammar has an acceptable speed, but
with most languages it takes just too much resources even to build the
parser. However, examples parsed in one language can always be linearized into
other languages:
```
  > i italian/LangIta.gf

  > l PredVP (UsePron she_Pron) (ComplV2 love_V2 (UsePron he_Pron))
  lo ama
```
Therefore, one can use the English parser to write an Italian grammar, and also
to write a language-independent (incomplete) grammar. One can also parse strings
that are bizarre in English but the intended way of expression in another language.
For instance, the phrase for "I am hungry" in Italian is literally "I have hunger".
This can be built by parsing "I have beer" in ``OverLangEng`` and then writing
```
  lin IamHungry = 
    let beer_N = mkN "fame" feminine 
    in
    mkCl (mkNP i_Pron) have_V2 (mkNP massQuant beer_N)
```
which uses ``ParadigmsIta.mkN``. 



==Example-based grammar writing==

The technique of parsing with the resource grammar can be used in GF source files,
endowed with the suffix ``.gfe`` ("GF examples"). The suffix tells GF to preprocess
the file by replacing all expressions of the form
```
  in Module.Cat "example string"
```
by the syntax trees obtained by parsing "example string" in ``Cat`` in ``Module``.
For instance,
```
  lin IamHungry = 
    let beer_N = mkN "fame" feminine 
    in
    (in LangEng.Cl "I have beer") ;
```
will result in the rule displayed in the previous section. The normal binding rules
of functional programming (and GF) guarantee that local bindings of identifiers
take precedence over constants of the same forms. Thus it is also possible to
linearize functions taking arguments in this way:
```
  lin
    PropKind car_N old_A = in LangEng.CN "old car" ;
```
However, the technique of example-based grammar writing has some limitations:
- Ambiguity. If a string has several parses, the first one is returned, and
it may not be the intended one. The other parses are shown in a comment, from
where they must/can be picked manually.
- Lexicality. The arguments of a function must be atomic identifiers, and are thus
not available for categories that have no lexical items. 
For instance, the ``PropKind`` rule above gives the result
```
  lin
    PropKind car_N old_A = AdjCN (UseN car_N) (PositA old_A) ;  
```
However, it is possible to write a special lexicon that gives atomic rules for
all those categories that can be used as arguments, for instance,
```
  fun
    cat_CN : CN ;
    old_AP : AP ;
```
and then use this lexicon instead of the standard one included in ``Lang``.


