From 6cd861f47e44d8e0431ef5eeea194b8996de915f Mon Sep 17 00:00:00 2001
From: Roland Conybeare <rconybeare@gmail.com>
Date: Sat, 17 Jan 2026 01:11:50 -0500
Subject: [PATCH] xo-expression2: scaffold variable eval in VSM

---
 xo-expression2/include/xo/expression2/exprtype.hpp       | 2 ++
 .../include/xo/interpreter2/VirtualSchematikaMachine.hpp | 8 ++++++++
 .../src/interpreter2/VirtualSchematikaMachine.cpp        | 9 +++++++++
 3 files changed, 19 insertions(+)

diff --git a/xo-expression2/include/xo/expression2/exprtype.hpp b/xo-expression2/include/xo/expression2/exprtype.hpp
index cf66d39e..0d5e8299 100644
--- a/xo-expression2/include/xo/expression2/exprtype.hpp
+++ b/xo-expression2/include/xo/expression2/exprtype.hpp
@@ -31,8 +31,10 @@ namespace xo {
             apply,
             /** function definition **/
             lambda,
+#endif
             /** variable reference **/
             variable,
+#ifdef NOT_YET
             /** if-then-else **/
             ifexpr,
             /** sequence **/
diff --git a/xo-interpreter2/include/xo/interpreter2/VirtualSchematikaMachine.hpp b/xo-interpreter2/include/xo/interpreter2/VirtualSchematikaMachine.hpp
index 6facb3fb..8bbe6bb4 100644
--- a/xo-interpreter2/include/xo/interpreter2/VirtualSchematikaMachine.hpp
+++ b/xo-interpreter2/include/xo/interpreter2/VirtualSchematikaMachine.hpp
@@ -45,6 +45,12 @@ namespace xo {
              **/
             void _do_eval_constant_op();
 
+            /** evaluate a variable expression
+             *  Require:
+             *  - expression in @ref expr_
+             **/
+            void _do_eval_variable_op();
+
         private:
             /*
              * Some registers are preserved by evaluation:
@@ -60,8 +66,10 @@ namespace xo {
             /** program counter **/
             VsmInstr pc_ = VsmInstr::halt();
 
+#ifdef NOT_YET
             /** stack pointer **/
             Stack stack_;
+#endif
 
             /** expression register **/
             obj<AExpression> expr_;
diff --git a/xo-interpreter2/src/interpreter2/VirtualSchematikaMachine.cpp b/xo-interpreter2/src/interpreter2/VirtualSchematikaMachine.cpp
index 012b16f6..bd29e968 100644
--- a/xo-interpreter2/src/interpreter2/VirtualSchematikaMachine.cpp
+++ b/xo-interpreter2/src/interpreter2/VirtualSchematikaMachine.cpp
@@ -45,6 +45,9 @@ namespace xo {
             case exprtype::constant:
                 _do_eval_constant_op();
                 break;
+            case exprtype::variable:
+                _do_eval_variable_op();
+                break;
             }
         }
 
@@ -58,6 +61,12 @@ namespace xo {
             this->pc_ = this->cont_;
         }
 
+        void
+        VirtualSchematikaMachine::_do_eval_variable_op()
+        {
+            // not implemented
+            assert(false);
+        }
     } /*namespace scm*/
 } /*namespace xo*/