This is a C#/.NET library for using Grammatical Framework, see https://www.grammaticalframework.org/
This project contains:
-
A C# library which wraps the
pgfandgunative libraries The package is listed on nuget (https://www.nuget.org/packages/PGFSharp/). To install, run the following command in the Package Manager ConsolePM> Install-Package PGFSharp -Pre -
A class generator which generates code for representing PGF categories and constructors as C# classes. The classes can be constructed from PGF expressions and converted back into PGF expressions.
Following the example in the Grammatical Framework Tutorial about building a translation application by calling the PGF library from another programming language (Haskell, in the case of the tutorial), we show how the C# PGF code generator can be used to simplify the development of a transfer function used in a query answering application.
The following abstract syntax
abstract Query = {
flags startcat=Question ;
cat
Answer ; Question ; Object ;
fun
Even : Object -> Question ;
Odd : Object -> Question ;
Prime : Object -> Question ;
Number : Int -> Object ;
Yes : Answer ;
No : Answer ;
}To make it easy to define a transfer function, we export the abstract syntax to a system of C# classes:
> pgf2cs.exe Query.pgf
The result is a file named Query.cs containing a namespace named Query which contains abstract classes for each category in the PGF, and concrete classes
for each constructor.
namespace Query {
public abstract class Answer {
public abstract Expression ToExpression();
public static Answer FromExpression(Expression expr) { ... }
public abstract R Accept<R>(IVisitor <R> visitor);
public interface IVisitor<R> {
R VisitYes();
R VisitNo();
}
}
public class Yes : Answer {
public Yes() { }
public override Accept<R>(IVisitor<R> visitor) {
return visitor.VisitYes();
}
public override Expression ToExpression() { ... }
}
...
}Now, we can use these classes to write an application, and the C# type system ensures that the manipulations by the transfer function produce well-formed syntax trees.
namespace QueryApplication
{
class MainClass
{
public static void Main (string[] args)
{
var q = "Is 5 prime?";
Console.WriteLine (q);
using (var grammar = PGF.Grammar.FromFile ("Query.pgf")) {
var language = grammar.Languages.First ().Value;
var input = language.Parse (q).First();
var answer = Application.AnswerQuestion (Query.Question.FromExpression (input)).ToExpression ();
var output = language.Linearize (answer);
Console.WriteLine (output);
}
}
}
public class Application {
private static bool IsOdd (int x) => x % 2 == 1;
private static bool IsEven (int x) => x % 2 == 0;
private static bool IsPrime (int x) => Sieve(Enumerable.Range(2,x)).Contains(x);
private static IEnumerable<int> Sieve(IEnumerable<int> xs) =>
!xs.Any() ? Enumerable.Empty<int>() :
xs.Take(1).Concat(Sieve(xs.Skip(1).Where(n => n % xs.First() > 0)));
private static Query.Answer Test (Func<int, bool> testF, Query.Object obj) =>
testF(((Query.Number)obj).x1) ? (Query.Answer) new Query.Yes() : (Query.Answer) new Query.No();
public static Query.Answer AnswerQuestion(Query.Question q) {
return q.Accept(new Query.Question.Visitor<Query.Answer>(
VisitPrime: obj => Test(IsPrime, obj),
VisitOdd: obj => Test(IsOdd, obj),
VisitEven: obj => Test(IsEven, obj)
));
}
}
}