1 files changed, 51 insertions, 63 deletions

diff --git a/src/List.vala b/src/List.vala
index 9340fd7..5706792 100644
--- a/src/List.vala
+++ b/src/List.vala
@@ -1,60 +1,65 @@
 namespace Eva {
-  public class List : Object, Term {
-    private static List _empty;
-    
-    private Term? _head;
-    private Term? _tail;
+  public interface List : Term {
+    private static Empty _empty;
     
     public static List empty {
       get {
         if(_empty == null)
-          _empty = new List.create_empty();
+          _empty = new Empty();
 
         return _empty;
       }
     }
     
-    public Term head {
-      get {
-        assert(!is_empty);
-        return _head;
-      }
-    }
-    public Term tail {
-      get {
-        assert(!is_empty);
-        return _tail;
-      }
+    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]));
     }
     
-    public bool is_empty {
-      get {
-        if(this == _empty) {
-          assert(_head == null && _tail == null);
+    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 {
-          assert(_head != null && _tail != null);
-          return false;
+          return (o == this);
         }
       }
+      
+      public void encode(Erl.Buffer buffer) {
+        buffer.encode_empty_list();
+      }
     }
+  }
+  
+  public class Cons : Object, Term, List {
+    public Term head { get; construct;}
+    public Term tail { get; construct;}
     
-    public List(Term head0, Term tail0 = empty) {
-      _head = head0;
-      _tail = tail0;
-    }
-    
-    private List.create_empty() {
-      _head = null;
-      _tail = null;
+    public bool is_empty {
+      get {
+        return false;
+      }
     }
     
-    public static List from_list(Gee.List<Term> list) {
-      if(list.is_empty)
-        return empty;
-      else
-        return new List(list.first(), List.from_list(list[1:list.size]));
+    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) {
@@ -62,18 +67,9 @@ namespace Eva {
         return o.do_match(this, vars, aliases);
       }
       
-      if(o is List) {
-        List l = o as List;
-        if(is_empty && l.is_empty)
-          return true;
-        
-        if(is_empty && !l.is_empty)
-          return false;
-        
-        if(!is_empty && l.is_empty)
-          return false;
-        
-        return (_head.do_match(l.head, vars, aliases) && _tail.do_match(l.tail, 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);
@@ -84,14 +80,11 @@ namespace Eva {
     }
     
     public string to_string() {
-      if(is_empty)
-        return "[]";
+      string ret = "[" + head.to_string();
       
-      string ret = "[" + _head.to_string();
-      
-      Term rest = _tail;
-      for(rest = _tail; rest is List && rest != List.empty; rest = (rest as List)._tail) {
-        ret += "," + (rest as List)._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) {
@@ -102,14 +95,9 @@ namespace Eva {
     }
     
     public void encode(Erl.Buffer buffer) {
-      if(is_empty) {
-        buffer.encode_empty_list();
-      }
-      else {
-        buffer.encode_list_header(1);
-        _head.encode(buffer);
-        _tail.encode(buffer);
-      }
+      buffer.encode_list_header(1);
+      head.encode(buffer);
+      tail.encode(buffer);
     }
   }
 }