xo-interpreter: + toplevel env in VSM

xo-interpreter/src/interpreter/GlobalEnv.cpp

@@ -21,6 +21,40 @@ namespace xo {
                                                                 symtab_{symtab}
         {}
 
+        bool
+        GlobalEnv::local_contains_var(const std::string & vname) const
+        {
+            return symtab_->lookup_local(vname).get();
+        }
+
+        void
+        GlobalEnv::establish_var(bp<Variable> var)
+        {
+            // Warning: altering declared type for an already-existing variable
+            // invalidates any type checking that relied on that variable.
+            //
+            // Ignoring this problem for now.
+            //
+            // Actual solution might look like:
+            // - keep track of which functions/defs depend on each global variable.
+            // - invalidate any jit / types for such variables.
+            // - maybe use seqno's to track
+            // - re-check / re-complie
+            // - need to admit invalid states.
+            //   suppose have mutually recursive functions f(), g()
+            //   want ability to modify type signatures separately
+            //
+            // Alternatives:
+            // - forbid changing type of an already-established variable
+            //   Actually: can't even change values if we intend supporting dependent types
+            // - quietly number variables so new definitions shadow old ones but don't
+            //   affect previously-encountered expressions
+
+            this->symtab_->require_global(var->name(), var);
+
+            this->slot_map_[var->name()] = gp<Object>();
+        }
+
         TaggedPtr
         GlobalEnv::self_tp() const
         {