Grammatical Framework (GF) is a programming language for writing grammars of natural languages. GF is capable of parsing and generating texts in several languages simultaneously while working from a language-independent representation of meaning. Grammars written in GF can be compiled into different formats including JavaScript and Java and can be reused as software components. A companion to GF is the GF Resource Grammar Library, a reusable library for dealing with the morphology and syntax of a growing number of natural languages.
Both GF itself and the GF Resource Grammar Library are open-source. Typologically, GF is a functional programming language. Formally, it is a type-theoretic formalism based on the Martin-Löf type theory.
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This example is taken from the LREC 2010 tutorial
Goal: write a multilingual grammar for expressing statements about John and Mary loving each other.
In GF, grammars are divided to two module types:
Consider the following:
Abstract syntax
abstract Zero = {
cat
S ; NP ; VP ; V2 ;
fun
Pred : NP -> VP -> S ;
Compl : V2 -> NP -> VP ;
John, Mary : NP ;
Love : V2 ;
}
Concrete syntax: English
concrete ZeroEng of Zero = {
lincat
S, NP, VP, V2 = Str ;
lin
Pred np vp = np ++ vp ;
Compl v2 np = v2 ++ np ;
John = "John" ;
Mary = "Mary" ;
Love = "loves" ;
}
Notice: Str (token list or "string") as the only linearization type.
A single abstract syntax may be applied to many concrete syntaxes, in our case one for each new natural language we wish to add. The same system of trees can be given:
Concrete syntax: French
concrete ZeroFre of Zero = {
lincat
S, NP, VP, V2 = Str ;
lin
Pred np vp = np ++ vp ;
Compl v2 np = v2 ++ np ;
John = "Jean" ;
Mary = "Marie" ;
Love = "aime" ;
}
We can now use our grammar to translate phrases between French and English. The following commands can be executed in the GF interactive shell.
Import many grammars with the same abstract syntax
> import ZeroEng.gf ZeroFre.gf Languages: ZeroEng ZeroFre
Translation: pipe linearization to parsing
> parse -lang=Eng "John loves Mary" | linearize -lang=Fre Jean aime Marie
Multilingual generation: linearize into all languages
> generate_random | linearize -treebank Zero: Pred Mary (Compl Love Mary) ZeroEng: Mary loves Mary ZeroFre: Marie aime Marie
Latin has cases: nominative for subject, accusative for object.
We use a parameter type for case (just 2 of Latin's 6 cases). The linearization type of NP is a table type: from Case to Str. The linearization of John is an inflection table. When using an NP, we select (!) the appropriate case from the table.
Concrete syntax: Latin
concrete ZeroLat of Zero = {
lincat
S, VP, V2 = Str ;
NP = Case => Str ;
lin
Pred np vp = np ! Nom ++ vp ;
Compl v2 np = np ! Acc ++ v2 ;
John = table {Nom => "Ioannes" ; Acc => "Ioannem"} ;
Mary = table {Nom => "Maria" ; Acc => "Mariam"} ;
Love = "amat" ;
param
Case = Nom | Acc ;
}
In Dutch, the verb heeft lief is a discontinuous constituent. The linearization type of V2 is a record type with two fields. The linearization of Love is a record. The values of fields are picked by projection (.)
Concrete syntax: Dutch
concrete ZeroDut of Zero = {
lincat
S, NP, VP = Str ;
V2 = {v : Str ; p : Str} ;
lin
Pred np vp = np ++ vp ;
Compl v2 np = v2.v ++ np ++ v2.p ;
John = "Jan" ;
Mary = "Marie" ;
Love = {v = "heeft" ; p = "lief"} ;
}
For Hebrew, NP has gender as its inherent feature — a field in the record. VP has gender as its variable feature — an argument of a table. In predication, the VP receives the gender of the NP.
Concrete syntax: Hebrew
concrete ZeroHeb of Zero = {
flags coding=utf8 ;
lincat
S = Str ;
NP = {s : Str ; g : Gender} ;
VP, V2 = Gender => Str ;
lin
Pred np vp = np.s ++ vp ! np.g ;
Compl v2 np = table {g => v2 ! g ++ "את" ++ np.s} ;
John = {s = "ג׳ון" ; g = Masc} ;
Mary = {s = "מרי" ; g = Fem} ;
Love = table {Masc => "אוהב" ; Fem => "אוהבת"} ;
param
Gender = Masc | Fem ;
}
GF has inbuilt functions which can be used for visualizing parse trees and word alignments.
The following commands will generate parse trees for the given phrases and open the produced PNG image using the system's eog command.
> parse -lang=Eng "John loves Mary" | visualize_parse -view="eog" > parse -lang=Dut "Jan heeft Marie lief" | visualize_parse -view="eog"
In general, this gives phrase alignment. Links can be crossing, phrases can be discontinuous. The align_words command follows a similar syntax:
> parse -lang=Fre "Marie aime Jean" | align_words -lang=Fre,Dut,Lat -view="eog"
In natural language applications, libraries are a way to cope with thousands of details involved in syntax, lexicon, and inflection. The GF Resource Grammar Library is the standard library for Grammatical Framework. It covers the morphology and basic syntax of 20 languages:
Amharic (partial), Arabic (partial), Bulgarian, Catalan, Danish, Dutch, English, Finnish, French, German, Hindi (fragments), Interlingua, Italian, Latin (fragments), Nepali, Norwegian bokmål, Persian, Polish, Punjabi, Romanian, Russian, Spanish, Swedish, Thai (fragments), Turkish (fragments), Urdu
A full API documentation of the library can be found at the RGL Synopsis page. The RGL status document gives the languages currently available in the GF Resource Grammar Library, including their maturity.
GF was first created in 1998 at Xerox Research Centre Europe, Grenoble, in the project Multilingual Document Authoring. At Xerox, it was used for prototypes including a restaurant phrase book, a database query system, a formalization of an alarm system instructions with translations to 5 languages, and an authoring system for medical drug descriptions.
Later projects using GF and involving third parties include:
Academically, GF has been used in four PhD theses and resulted in around fifty scientific publications (see GF publication list).
There is an active group for developers and users of GF alike, located at http://groups.google.com/group/gf-dev
The second GF Summer school, subtitled Frontiers of Multilingual Technologies was held in 2011 in Barcelona, Spain. It was sponsored by CLT, the Centre for Language Technology of the University of Gothenburg, and by UPC, Universitat Politècnica de Catalunya. The School addressed new languages and also promoted ongoing work in those languages which are already under construction. Missing EU languages were especially encouraged.
The school began with a 2-day GF tutorial, serving those interested in getting an introduction to GF or an overview of on-going work.
All results of the summer school are available as open-source software released under the LGPL license.
The first GF summer school was held in 2009 in Gothenburg, Sweden. It was a collaborative effort to create grammars of new languages in Grammatical Framework, GF. These grammars were added to the Resource Grammar Library, which previously had 12 languages. Around 10 new languages are already under construction, and the School aimed to address 23 new languages. All results of the Summer School were made available as open-source software released under the LGPL license.
The summer school was organized by the Language Technology Group at the Department of Computer Science and Engineering. The group is a part of the Centre of Language Technology, a focus research area of the University of Gothenburg.
The code created by the school participants is made accessible in the GF darcs repository, subdirectory contrib/summerschool.