Add errors ection to Part 4 of compiler posts

code/compiler/04/type.cpp

@@ -1,6 +1,7 @@
 #include "type.hpp"
 #include <sstream>
 #include <algorithm>
+#include "error.hpp"
 
 void type_var::print(const type_mgr& mgr, std::ostream& to) const {
     auto it = mgr.types.find(name);
@@ -87,7 +88,7 @@ void type_mgr::unify(type_ptr l, type_ptr r) {
         if(lid->name == rid->name) return;
     }
 
-    throw 0;
+    throw unification_error(l, r);
 }
 
 void type_mgr::bind(const std::string& s, type_ptr t) {