Click here to Skip to main content
Click here to Skip to main content
Add your own
alternative version
Go to top

Implementing Programming Languages Using C# 4.0

, 12 Jul 2012
An introduction to creating programming language tools using C# 4.0.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Diggins.Jigsaw
{
    public class Unifier
    {
        public static bool IsConstant(object e)
        {
            return !(e is List) && !(e is string);
        }

        public static bool Occurs(string s, List e)
        {
            if (e.Head is string)
                if (e.Head == s) 
                    return true;
            if (e.Head is List) 
                return Occurs(s, e.Head);
            return e.Tail == null ? false : Occurs(s, e.Tail);
        }

        public static dynamic Substitute(Dictionary<string, dynamic> bindings, dynamic x)
        {
            if (x is String && bindings.ContainsKey(x)) 
                return bindings[x];
            if (x is List && !x.IsEmpty)
                return List.Cons(Substitute(bindings, x.Head), Substitute(bindings, x.Tail));
            return x;
        }

    public static Dictionary<string, object> Unify(dynamic e1, dynamic e2)
    {
        if ((IsConstant(e1) && IsConstant(e2)))
        {
            if (e1 == e2)
                return new Dictionary<string,object>();
            throw new Exception("Unification failed");
        }

        if (e1 is string)
        {
            if (e2 is List && Occurs(e1, e2))
                throw new Exception("Cyclical binding");
            return new Dictionary<string, object>() { { e1, e2 } };
        }

        if (e2 is string)
        {
            if (e1 is List && Occurs(e2, e1))
                throw new Exception("Cyclical binding");
            return new Dictionary<string, object>() { { e2, e1 } };
        }

        if (!(e1 is List) || !(e2 is List))
            throw new Exception("Expected either list, string, or constant arguments");

        if (e1.IsEmpty || e2.IsEmpty)
        {
            if (!e1.IsEmpty || !e2.IsEmpty)
                throw new Exception("Lists are not the same length");

            return new Dictionary<string, object>(); 
        }

        var b1 = Unify(e1.Head, e2.Head);
        var b2 = Unify(Substitute(b1, e1.Tail), Substitute(b1, e2.Tail));

        foreach (var kv in b2)
            b1.Add(kv.Key, kv.Value);
        return b1;
    }
    }
}

By viewing downloads associated with this article you agree to the Terms of Service and the article's licence.

If a file you wish to view isn't highlighted, and is a text file (not binary), please let us know and we'll add colourisation support for it.

License

This article, along with any associated source code and files, is licensed under The MIT License

Share

About the Author

Christopher Diggins
Software Developer Autodesk
Canada Canada
This article was written by Christopher Diggins, a computer science nerd who currently works at Autodesk as an SDK specialist.
Follow on   Twitter   Google+   LinkedIn

| Advertise | Privacy | Mobile
Web03 | 2.8.140916.1 | Last Updated 12 Jul 2012
Article Copyright 2011 by Christopher Diggins
Everything else Copyright © CodeProject, 1999-2014
Terms of Service
Layout: fixed | fluid