namespace Eva {
  public interface List : Term {
    private static Empty _empty;
    
    public static List empty {
      get {
        if(_empty == null)
          _empty = new Empty();

        return _empty;
      }
    }
    
    public abstract bool is_empty { get; }
    
    public static List from_list(Gee.List<Term> list) {
      if(list.is_empty)
        return empty;
      else
        return new Cons(list.first(), List.from_list(list[1:list.size]));
    }
    
    private class Empty : Object, Term, List {
      internal Empty() {}
      
      public bool is_empty {
        get {
          return true;
        }
      }
      
      public string to_string() {
        return "[]";
      }
      
      protected bool do_match(Term o, Gee.Map<string, Term> vars, Gee.Map<string, string> aliases) {
        if(o is Var) {
          return o.do_match(this, vars, aliases);
        }
        else {
          return (o == this);
        }
      }
      
      internal void encode(Buffer buffer) {
        buffer.buffer.encode_empty_list();
      }
    }
  }
  
  public class Cons : Object, Term, List {
    public Term head { get; construct;}
    public Term tail { get; construct;}
    
    public bool is_empty {
      get {
        return false;
      }
    }
    
    public Cons(Term head0, Term tail0 = empty) {
      Object(head: head0, tail: tail0);
    }
    
    protected bool do_match(Term o, Gee.Map<string, Term> vars, Gee.Map<string, string> aliases) {
      if(o is Var) {
        return o.do_match(this, vars, aliases);
      }
      
      if(o is Cons) {
        Cons c = o as Cons;
        return (head.do_match(c.head, vars, aliases) && tail.do_match(c.tail, vars, aliases));
      }
      else if(o is String) {
        return do_match(string_to_list((o as String).value), vars, aliases);
      }
      else {
        return false;
      }
    }
    
    public string to_string() {
      string ret = "[" + head.to_string();
      
      unowned Term rest;
      for(rest = tail; rest is Cons; rest = (rest as Cons).tail) {
        ret += "," + (rest as Cons).head.to_string();
      }
      
      if(rest != List.empty) {
        ret += "|" + rest.to_string();
      }
      
      return ret + "]";
    }
    
    internal void encode(Buffer buffer) {
      buffer.buffer.encode_list_header(1);
      head.encode(buffer);
      tail.encode(buffer);
    }
  }
}