# The GF Resource Grammar Library

Fourth Version, 2 November 2005.
Third Version, 22 May 2005. Completed 1 July. Second Version, 1 March 2005 First Draft, 7 February 2005

Aarne Ranta

`aarne@cs.chalmers.se`

## GF = Grammatical Framework

GF is a grammar formalism based on functional programming and type theory.

GF was designed to be nice for ordinary programmers to use: by this we mean programmers without training in linguistics.

The mission of GF is to make natural-language applications available for ordinary programmers, in tasks like

- software documentation
- domain-specific translation
- human-computer interaction
- dialogue systems

Thus GF is not primarily another theoretical framework for linguists.

## Multilingual grammars

A GF grammar consists of an abstract syntax and a set of concrete syntaxes.

Abstract syntax: language-independent representation

      cat Prop ; Nat ;
      fun Even : Nat -> Prop ;
      fun NInt : Int -> Nat ;

Concrete syntax: mapping from abstract syntax trees to strings in a language (English, French, German, Swedish,...)

      lin Even x = {s = x.s ++ "is" ++ "even"} ;

      lin Even x = {s = x.s ++ "est" ++ "pair"} ;

      lin Even x = {s = x.s ++ "ist" ++ "gerade"} ;

      lin Even x = {s = x.s ++ "är" ++ "jämnt"} ;

We can translate between languages via the abstract syntax:

      4 is even                  4 ist gerade
                 \              /
                   Even (NInt 4)
                 /              \
      4 est pair                  4 är jämnt

But is it really so simple?

## Difficulties with concrete syntax

Most languages have rules of inflection, agreement, and word order, which have to be obeyed when putting together expressions.

The previous multilingual grammar breaks these rules in many situations:

2 and 3 is even
la somme de 3 et de 5 est pair
wenn 2 ist gerade, dann 2+2 ist gerade
om 2 är jämnt, 2+2 är jämnt
All these sentences are grammatically incorrect.

## Solving the difficulties

GF has tools for expressing the linguistic rules that are needed to produce correct translations in different languages.

Instead of just strings, we need

parameters, tables, and record types. For instance, French:

      param Mod = Ind | Subj ;
      param Gen = Masc | Fem ;

      lincat Nat = {s : Str ; g : Gen} ;
      lincat Prop = {s : Mod => Str} ;

      lin Even x = {s =
          table {
            m => x.s ++
                 case m   of {Ind  => "est" ;  Subj => "soit"} ++
                 case x.g of {Masc => "pair" ; Fem  => "paire"}
          }
        } ;

To learn more about these constructs, consult GF documentation, e.g. the New Grammarian's Tutorial. However, in what follows we will show how to avoid learning them and still write linguistically correct grammars.

## Language + Libraries

Writing natural language grammars still requires theoretical knowledge about the language.

Which kind of a programmer is it easier to find?

- one who can write a sorting algorithm
- one who can write a grammar for Swedish determiners

In main-stream programming, sorting algorithms are not written by hand but taken from libraries.

In the same way, we want to create grammar libraries that encapsulate basic linguistic facts.

Cf. the Java success story: the language is just a half of the success - libraries are another half.

## Example of library-based grammar writing

To define a Swedish expression of a mathematical predicate from scratch:

      Even x =
        let jämn = case <x.n,x.g> of {
          <Sg,Utr>   => "jämn" ;
          <Sg,Neutr> => "jämnt" ;
          <Pl,_>     => "jämna"
          }
        in
        {s = table {
          Main => x.s ! Nom ++ "är" ++ jämn ;
          Inv  => "är" ++ x.s ! Nom ++ jämn ;
          Sub  => x.s ! Nom ++ "är" ++ jämn
          }
        }

To use library functions for syntax and morphology:

      Even = predA (regA "jämn") ;

For the French version, we write

      Even = predA (regA "pair") ;

## Questions in grammar library design

What should there be in the library?
- morphology, lexicon, syntax, semantics,...

  How do we organize and present the library?

- division into modules, level of granularity
- "school grammar" vs. sophisticated linguistic concepts

  Where do we get the data from?

- automatic extraction or hand-writing?
- reuse of existing resources?
  Extra constraint: we want open-source free software and hence cannot use existing proprietary resources.

  ## Answers to questions in grammar library design

  The current GF resource grammar library has made the following decisions:

  The library has, for each language

- complete morphology, some lexicon (500 words), representative fragment of syntax, very little semantics,

  Organization and presentation:

- division into top-level (API) modules, and internal modules (only interesting for resource implementors)
- the API is, as much as possible, common in different languages
- we favour "school grammar" concepts rather than innovative linguistic theory

  Where do we get the data from?

- morphology and syntax are hand-written
- the 500-word lexicon is hand-written, but a tool is provided for automatic lexicon extraction
- we have not reused existing resources
  The resource grammar library is entirely open-source free software (under GNU GPL license).

  ## The scope of a resource grammar library for a language

  All morphological paradigms

  Basic lexicon of structural, common, and irregular words

  Basic syntactic structures

  Currently,

- no semantics,
- no language-specific structures if not necessary for expressivity.

  ## Success criteria

  Grammatical correctness

  Semantic coverage: you can express whatever you want.

  Usability as library for non-linguists.

  (Bonus for linguists:) nice generalizations w.r.t. language families, using the module system of GF.

  ## These are not our success criteria

  Language coverage: to be able to parse all expressions.
  Example: the French passé simple tense, although covered by the morphology, is not used in the language-independent API, but only the passé composé is. However, an application accessing the French-specific (or Romance-specific) modules can use the passé simple.

  Semantic correctness: only to produce meaningful expressions.
  Example: the following sentences can be generated

        colourless green ideas sleep furiously

        the time is seventy past forty-two

  However, an applicatio grammar can use a domain-specific semantics to guarantee semantic well-formedness.

  (Warning for linguists:) theoretical innovation in syntax is not among the goals (and it would be hidden from users anyway!).

  ## So where is semantics?

  GF incorporates a Logical Framework and is therefore capable of expressing logical semantics ā la Montague or any other flavour, including anaphora and discourse.

  But we do not try to give semantics once and for all for the whole language.

  Instead, we expect semantics to be given in application grammars built on semantic models of different domains.

  Example application: number theory

        fun Even : Nat -> Prop ;         -- a mathematical predicate

        lin Even = predA (regA "even") ; -- English translation
        lin Even = predA (regA "pair") ; -- French translation
        lin Even = predA (regA "jämn") ; -- Swedish translation

  How could the resource predict that just these translations are correct in this domain?

  Application grammars are built by experts of these domains who - thanks to resource grammars - do no more need to be experts in linguistics.

  ## Languages

  The current GF Resource Project covers ten languages:
  - `Dan`ish
  - `Eng`lish
  - `Fin`nish
  - `Fre`nch
  - `Ger`man
  - `Ita`lian
  - `Nor`wegian
  - `Rus`sian
  - `Spa`nish
  - `Swe`dish
  The first three letters (`Dan` etc) are used in grammar module names

  ## Library structure 1: language-independent API

- `Lang` is the top module collecting all of the following.

- syntactic `Categories` (parts of speech, word classes), e.g.

        V ; NP ; CN ; Det ;  -- verb, noun phrase, common noun, determiner

- `Rules` for combining words and phrases, e.g.

        DetNP : Det -> CN -> NP ; -- combine Det and CN into NP

- the most common `Structural` words (determiners, conjunctions, pronouns) (now 83), e.g.

        and_Conj : Conj ;

  `Numerals`, number words from 1 to 999,999 with their inflections, e.g.

        n8 : Digit ;

  `Basic` lexicon of (now 218) frequent everyday words

        man_N : N ;

  In addition, and not included in `Lang`, there is

- `SwadeshLex`, lexicon of (now 206) words from the Swadesh list, e.g.

        squeeze_V : V ;

  Of course, there is some overlap between `SwadeshLex` and the other modules.

  ## Library structure 2: language-dependent modules

- morphological `Paradigms`, e.g. Swedish

        mkN : Str -> Str -> Str -> Str -> Gender -> N ; -- worst-case nouns
        mkN : Str -> N ;                                -- regular nouns

- (in some languages) irregular `Verbs`, e.g.

        angripa_V = irregV "angripa" "angrep" "angripit" ;

- (not yet available) `Ext`ended syntax with language-specific rules

        PassBli : V2 -> NP -> VP ;  -- bli överkörd av ngn

  ## How much can be language-independent?

  For the ten languages we have considered, it is possible to implement the current API.

  Reservations:

  - this does not necessarily extend to all other languages
  - this does not necessarily cover the most idiomatic expressions of each language
  - this may not be the easiest API to implement (e.g. negation and inversion with do in English suggest that some other structure would be more natural)
  - it is not guaranteed that same structure has the same semantics in all different languages

    ## Library structure: language-independent API

    ## API documentation

    Categories

    Rules

    Two alternative views on sentence formation by predication: Clause, Verbphrase

    Structural

    Time

    Basic

    Lang

    ## Paradigms documentation

    English paradigms
    example use of English oaradigms
    English verbs

    Finnish paradigms
    example use of Finnish oaradigms

    French paradigms
    example use of French paradigms
    French verbs

    Italian paradigms
    example use of Italian paradigms
    Italian verb conjugations

    Norwegian paradigms
    example use of Norwegian paradigms
    Norwegian verbs

    Spanish paradigms
    example use of Spanish paradigms
    Spanish verb conjugations

    Swedish paradigms
    example use of Swedish paradigms
    Swedish verbs

    ## Use as top-level grammar: testing

    Import a set of `LangX` grammars:

          i english/LangEng.gf
          i swedish/LangSwe.gf

    Alternatively, you can `make` a precompiled package of all the languages by using `lib/resource/Makefile`:

          make
          gf langs.gfcm

    Then you can test with translation, random generation, morphological analysis...

          > p -lang=LangEng "I have loved her." | l -lang=LangFre
          Je l' ai aimée.

          > gr -cat=NP | l -multi
          The sock
          Strumpan
          Strømpen
          La media
          La calza
          La chaussette
          Sukka

    ## Use as top-level grammar: language learning quizzes

    Morpho quiz with words (e.g. French verbs):

          i french/VerbsFre.gf
          mq -cat=V

    Morpho quiz with phrases (e.g. Swedish clauses):

          i swedish/LangSwe.gf
          mq -cat=Cl

    Translation quiz with sentences (e.g. sentences from English to Swedish):

          i swedish/LangEng.gf
          i swedish/LangSwe.gf
          tq -cat=S LangEng LangSwe

    ## Use as library

    Import directly by `open`:

          concrete AppNor of App = open LangNor, ParadigmsNor in {...}

    (Note for the users of GF 2.1 and older: the dummy `reuse` modules and their bulky `.gfr` versions are no longer needed!)

    If you need to convert resource records to strings, and don't want to know the concrete type (as you never should), you can use

          Predef.toStr : (L : Type) -> L -> Str ;

    `L` must be a linearization type. For instance,

          toStr LangNor.CN (ModAP (PositADeg old_ADeg) (UseN car_N))
          ---> "gammel bil"

    ## Use as library through parser

    You can use the parser with a `LangX` grammar when developing a resource.

    Using the `-v` option shows if the parser fails because of unknown words.

          > p -cat=S -v -lexer=words "jag ska åka till Chalmers"
          unknown tokens [TS "åka",TS "Chalmers"]

    Then try to select words that `LangX` recognizes:

          > p -cat=S "jag ska springa till Danmark"
          UseCl (PosTP TFuture ASimul)
            (AdvCl (SPredV i_NP run_V)
            (AdvPP (PrepNP to_Prep (UsePN (PNCountry Denmark)))))

    Use these API structures and extend vocabulary to match your need.

          åka_V = lexV "åker" ;
          Chalmers = regPN "Chalmers" neutrum ;

    ## Syntax editor as library browser

    You can run the syntax editor on `LangX` to find resource API functions through context-sensitive menus. For instance, the shell command

          gfeditor LangEng.gf LangFre.gf

    opens the editor with English and French views. The Editor User Manual gives more information on the use of the editor.

    A restriction of the editor is that it does not give access to `ParadigmsX` modules. An IDE environment extending the editor to a grammar programming tool is work in progress.

    ## Example application: a small translation system

    In this system, you can express questions and answers of the following forms:

          Who chases mice ?
          Whom does the lion chase ?
          The dog chases cats.

    We build the abstract syntax in two phases:
    - Questions defines question and answer forms independently of domain
    - Animals defines a lexicon with animals and things that animals do.

    The concrete syntax of English is built in three phases:

    - QuestionsI is a parametrized module using the API module `Resource`.
    - QuestionsEng is an instantiation of the API with `ResourceEng`.
    - AnimalsEng is a concrete syntax of `Animals` using `ParadigmsEng` and `VerbsEng`.

    The concrete syntax of Swedish is built upon `QuestionsI` in a similar way, with the modules QuestionsSwe and. AnimalsSwe.

    The concrete syntax of French consists similarly of the modules QuestionsFre and AnimalsFre.

    ## Compiling the example application

    The resources are bulky, and it takes a therefore a lot of time and memory to load the grammars. However, they can be compiled into the `gfcm` (GF canonical multilingual) format, which is almost one thousand times smaller and faster to load for this set of grammars.

    To produce an end-user multilingual grammar `animals.gfcm`, write the sequence of compilation commands in a `gfs` (GF script) file, say `mkAnimals.gfs`, and then call GF with

          gf <mkAnimals.gfs

    To try out the grammar,

          > i animals.gfcm

          > gr | l -multi
          vem jagar hundar ?
          qui chasse des chiens ?
          who chases dogs ?

    ## Grammar writing by examples

    (New in GF 2.3)

    You can use the resource grammar as a parser on a special file format, `.gfe` ("GF examples"). Here is the real source, QuestionsI.gfe, which generated QuestionsI.gf. when you executed the GF command

          i -ex AnimalsEng.gf

    Since `QuestionsI` is an incomplete module ("functor"), it need only be built once. This is why only the first command in `mkAnimals.gfs` needs the flag `-ex`.

    Of course, the grammar of any language can be created by parsing any language, as long as they have a common resource API. The use of English resource is generally recommended, because it is smaller and faster to parse than the other languages.

    ## Constants and variables in examples

    The file QuestionsI.gfe uses as resource `LangEng`, which contains all resource syntax and a lexicon of ca. 300 words. A linearization rule, such as

          lin Who love_V2 man_N = in Phr "who loves men ?" ;

    uses as argument variables constants for words that can be found in the lexicon. It is due to this that the example can be parsed. When the resulting rule,

          lin Who love_V2 man_N =
            QuestPhrase (UseQCl (PosTP TPresent ASimul)
              (QPredV2 who8one_IP love_V2 (IndefNumNP NoNum (UseN man_N)))) ;

    is read by the GF compiler, the identifiers `love_V2` and `man_N` are not treated as constants, but, following the normal binding rules of functional languages, as bound variables. This is what gives the example method the generality that is needed.

    To write linearization rules by examples one thus has to know at least one abstract syntax constant for each category for which one needs a variable.

    ## Extending the lexicon on the fly

    The greatest limitation of the example method is that the lexicon may lack many of the words that are needed in examples. If parsing fails because of this, the compiler gives a list of unknown words in its error message. An obvious solution is, of course, to extend the resource lexicon and try again. A more light-weight solution is to add a substitution to the example. For instance, if you want the example "the pope" but the lexicon does not have the word "pope", you can write

          lin Pope = in NP "the man" {man_N = regN "pope"} ;

    The resulting linearization rule is initially

          lin Pope = DefOneNP (UseN man_N) ;

    but the substitution changes this to

          lin Pope = DefOneNP (UseN (regN "pope")) ;

    In this way, you do not have to extend the resource lexicon, but you need to open the Paradigms module to compile the resulting term.

    Of course, the substituted expressions may come from another language than the main language of the example:

          lin Pope = in NP "the man" {man_N = regN "pape" masculine} ;

    If many substitutions are needed, semicolons are used as separators:

          {man_N = regN "pope" ; walk_N = regV "pray"} ;

    ## Implementation details: low-level files

    For developers of resource grammars. The modules listed in this section should never be imported in application grammars.

    Each of the API implementations uses the following auxiliary resource modules:

    - `Types`, the morphological paradigms and word classes
    - `Morpho`, inflection machinery
    - `Syntax`, complex categories and their combinations
    In addition, the following language-independent modules from `lib/prelude` are used.
    - `Predef`, operations whose definitions are hard-coded in GF
    - `Prelude`, generic string and boolean operations
    - `Coordination`, coordination structures for arbitrary categories

    ## Implementation details: the structure of low-level files

    ## How to change a resource grammar?

    In many cases, the source of a bug is in one of the low-level modules. Try to trace it back there by starting from the high-level module.

    (Much more to be written...)

    ## How to write a resource grammar?

    Start with a more limited goal, e.g. to implement the `stoneage` grammar (`examples/stoneage`) for your language.

    For this, you need

    - most of `Types`
    - most of `Morpho`
    - some of `Syntax`
    - most of `Paradigms`

    A useful command to test `oper`s:

          i -retain MorphoRot.gf
          cc regNoun "foo"

    ## The use of parametrized modules

    In two language families, a lot of code is shared.
    - Romance: French, Italian, Spanish
    - Scandinavian: Danish, Norwegian, Swedish
    The structure looks like this.

    ## Current status

    Language | v0.6 | v0.9 API | Paradigms | Basic lex | Verbs
    Danish | - | X | - | - | -
    English | X | X | X | X | X
    Finnish | X | + | X | X | 0
    French | X | X | X | X | X
    German | X | - | * | - | -
    Italian | X | X | X | X | X
    Norwegian | - | X | X | X | X
    Russian | X | * | * | - | -
    Spanish | - | X | X | X | X
    Swedish | X | X | X | X | X

    X = implemented (few exceptions may occur)
    + = implemented for a large part
    * = linguistic material ready for implementation
    - = not implemented
    0 = not applicable

    ## Known bugs and limitations

    (The listed limitations are ones that do not follow from the table on the previous page.)

    Danish

    English: missing uncontracted negations.

    Finnish: compiling the heuristic paradigms is slow; possessive and interrogative suffixes have no proper lexer.

    French: no inverted questions; some verbs in Basic should be reflexive

    German

    Italian: no omission of unstressed subject pronouns; some verbs in Basic should be reflexive; bad forms of reflexive infinitives

    Norwegian: possessives of type bilen min not included

    Russian

    Spanish: no omission of unstressed subject pronouns; no switch to dative case for human objects; some verbs in Basic should be reflexive; bad forms of reflexive infinitives; spurious parameter for verb auxiliary inherited from Romance

    Swedish:

    ## Obtaining it

    Get the grammar package from GF Download Page. The current libraries are in `lib/resource`. Version 0.6 is in `lib/resource-0.6`.

    The very very latest version of GF and its libraries is in Snapshots.