oalgebras.cs

//Algebras case
//Suppose we have few expressions with Eval operation and parser in library.dll
interface IExpAgl<T>
{
    T lit(int i);
    T add(T x, T y);    
}
public interface IEval { string Eval(); }
public class EvalAlg : IExpAgl<IEval>{}
class Parser { T Parse<T>(IExpAgl<T> a, String s); }
public static class ParserProvider { public static Parser GetParser() => new Parser(); }

//There is client.dll
public static class Client 
{
    public static void Main(string[] args)
    {
        var e = ParserProvider.GetParser().Parse(new EvalAlg(), s[0]);
        Console.WriteLine(e.Eval());
    }
}

//======= Suppose we want to add BoolExp with Algebras =======
//(1) add bool exp and eval operation on it
interface IBoolAlg<T> : IExpAgl<T> { T bool(bool i); }
public class BoolEvalAlg : EvalAlg<IEval>, IBoolAgl<IEval> {}  
//(2) modify parser to support bool. ParserProvider RETURN TYPE CHANGE.
class Parser { T Parse<T>(IBoolAgl<T> a, String s); }
public static class ParserProvider { public static BoolParser GetParser() => new BoolParser(); }
//(3) CLIENT HAS TO CHANGE !!!
public static class Main 
{
    public static void Main(string[] args)
    {
        var e = ParserProvider.GetParser().Parse(new BoolEvalAlg(), s[0]);
        Console.WriteLine(e.Eval());
    }
}