diff --git a/cmake/xo_expression2Config.cmake.in b/cmake/xo_expression2Config.cmake.in
index ee1852c8..d48c29a4 100644
--- a/cmake/xo_expression2Config.cmake.in
+++ b/cmake/xo_expression2Config.cmake.in
@@ -8,6 +8,7 @@ include(CMakeFindDependencyMacro)
 #
 find_dependency(xo_gc)
 find_dependency(reflect)
+find_dependency(xo_object2)
 find_dependency(xo_printable2)
 find_dependency(xo_flatstring)
 find_dependency(indentlog)
diff --git a/idl/Expression.json5 b/idl/Expression.json5
index 9daca006..f156dffe 100644
--- a/idl/Expression.json5
+++ b/idl/Expression.json5
@@ -1,6 +1,9 @@
 {
     mode: "facet",
-    includes: ["\"TypeRef.hpp\""],
+    includes: [ "\"TypeRef.hpp\"",
+                "\"exprtype.hpp\"",
+                "<xo/reflect/TypeDescr.hpp>"],
+
     namespace1: "xo",
     namespace2: "scm",
     facet: "Expression",
@@ -10,8 +13,24 @@
     doc: [
         "Representation for executable Schematika expressions"
     ],
-    types: [],
+    types: [
+        // using TypeDescr = xo::reflect::TypeDescr;
+        {
+            name: "TypeDescr",
+            doc: ["struct describing a type"],
+            definition: "xo::reflect::TypeDescr"
+        },
+    ],
     const_methods: [
+        {
+            name: "extype",
+            doc: ["expression type (constant | apply | ..)"],
+            return_type: "exprtype",
+            args: [],
+            const: true,
+            noexcept: true,
+            attributes: [],
+        },
         {
             name: "typeref",
             doc: ["placeholder for type giving possible values for this expression"],
diff --git a/include/xo/expression2/DConstant.hpp b/include/xo/expression2/DConstant.hpp
index b7cc5cce..29776912 100644
--- a/include/xo/expression2/DConstant.hpp
+++ b/include/xo/expression2/DConstant.hpp
@@ -3,7 +3,10 @@
  *  @author Roland Conybeare, Jan 2026
  **/
 
-#include <xo/reflect/TypeDescr.hpp>
+#include "TypeRef.hpp"
+#include "exprtype.hpp"
+#include <xo/reflect/TaggedPtr.hpp>
+#include <xo/gc/GCObject.hpp>
 
 namespace xo {
     namespace scm {
@@ -12,24 +15,36 @@ namespace xo {
          **/
         struct DConstant {
         public:
+            using TaggedPtr = xo::reflect::TaggedPtr;
             using TypeDescr = xo::reflect::TypeDescr;
+            using AGCObject = xo::mm::AGCObject;
+            using typeseq = xo::reflect::typeseq;
 
         public:
-            DConstant(TypeDescr td, void * value);
+            explicit DConstant(obj<AGCObject> value) noexcept;
 
-            TypeDescr value_td() const { return value_td; }
+            bool is_resolved() const noexcept { return typeref_.is_resolved(); }
 
-            TypeRef typeref() const noexcept;
-            TypeDescr valuetype() const noexcept;
-            void assign_valuetype(TypeDescr td) noexcept;
+            exprtype extype() const noexcept { return exprtype::constant; }
+            TypeDescr value_td() const noexcept { return typeref_.td(); }
+            TaggedPtr value_tp() const noexcept { return TaggedPtr(typeref_.td(), value_.data()); }
+
+            TypeRef typeref() const noexcept { return typeref_; }
+            TypeDescr valuetype() const noexcept { return typeref_.td(); }
+            obj<AGCObject> value() const noexcept { return value_; }
+
+            void assign_valuetype(TypeDescr td) noexcept { typeref_.resolve(td); }
 
         private:
-            /** type for value of this expression **/
-            TypeRef type_ref_;
-            /** type description for destination *value_ **/
-            TypeDescr valuetype_;
+            static TypeDescr _lookup_td(typeseq tseq);
+
+        private:
+            /** type for value of this expression
+             *  or unification breadcrumb before unification
+             **/
+            TypeRef typeref_;
             /** literal value **/
-            void * value_;
+            obj<AGCObject> value_;
         };
     } /*namespace scm*/
 } /*namespace xo*/
diff --git a/include/xo/expression2/Expression.hpp b/include/xo/expression2/Expression.hpp
index 311b627d..c6960c07 100644
--- a/include/xo/expression2/Expression.hpp
+++ b/include/xo/expression2/Expression.hpp
@@ -2,7 +2,7 @@
  *
  *  Generated automagically from ingredients:
  *  1. code generator:
- *       [/home/roland/proj/xo-umbrella2-claude1/xo-facet/codegen/genfacet]
+ *       [/Users/roland/proj/xo-umbrella2/xo-facet/codegen/genfacet]
  *     arguments:
  *       --input [idl/Expression.json5]
  *  2. jinja2 template for facet .hpp file:
diff --git a/include/xo/expression2/TypeRef.hpp b/include/xo/expression2/TypeRef.hpp
index 8ee08716..cd789d23 100644
--- a/include/xo/expression2/TypeRef.hpp
+++ b/include/xo/expression2/TypeRef.hpp
@@ -26,6 +26,11 @@ namespace xo {
             TypeRef() = default;
             TypeRef(const type_var & id, TypeDescr td);
 
+            /** trivial typeref, where already resolved.
+             *  Require: @p td non-null
+             **/
+            static TypeRef resolved(TypeDescr td);
+
             /** if @p td is non-null
              *   -> type is already resolved
              *
@@ -43,7 +48,10 @@ namespace xo {
             TypeDescr td() const noexcept { return td_; }
 
             /** true iff type at this location has been resolved **/
-            bool is_concrete() const noexcept;
+            bool is_resolved() const noexcept;
+
+            /** resolve TypeRef by supplying final type-description **/
+            void resolve(TypeDescr td) noexcept { td_ = td; }
 
         private:
             /** unique (probably generated) name for type at this location **/
diff --git a/include/xo/expression2/detail/AExpression.hpp b/include/xo/expression2/detail/AExpression.hpp
index 942c940b..ae53af05 100644
--- a/include/xo/expression2/detail/AExpression.hpp
+++ b/include/xo/expression2/detail/AExpression.hpp
@@ -2,7 +2,7 @@
  *
  *  Generated automagically from ingredients:
  *  1. code generator:
- *       [/home/roland/proj/xo-umbrella2-claude1/xo-facet/codegen/genfacet]
+ *       [/Users/roland/proj/xo-umbrella2/xo-facet/codegen/genfacet]
  *     arguments:
  *       --input [idl/Expression.json5]
  *  2. jinja2 template for abstract facet .hpp file:
@@ -15,6 +15,8 @@
 
 // includes (via {facet_includes})
 #include "TypeRef.hpp"
+#include "exprtype.hpp"
+#include <xo/reflect/TypeDescr.hpp>
 #include <xo/facet/obj.hpp>
 #include <xo/facet/facet_implementation.hpp>
 #include <xo/facet/typeseq.hpp>
@@ -35,6 +37,8 @@ public:
     // types
     /** integer identifying a type **/
     using typeseq = xo::facet::typeseq;
+    /** struct describing a type **/
+    using TypeDescr = xo::reflect::TypeDescr;
     ///@}
 
     /** @defgroup scm-expression-methods **/
@@ -42,6 +46,8 @@ public:
     // const methods
     /** RTTI: unique id# for actual runtime data representation **/
     virtual typeseq _typeseq() const noexcept = 0;
+    /** expression type (constant | apply | ..) **/
+    virtual exprtype extype(Copaque data)  const  noexcept = 0;
     /** placeholder for type giving possible values for this expression **/
     virtual TypeRef typeref(Copaque data)  const  noexcept = 0;
     /** type giving possible values for this expression. Maybe null before typecheck **/
diff --git a/include/xo/expression2/detail/IExpression_Any.hpp b/include/xo/expression2/detail/IExpression_Any.hpp
index 0fac3289..e76cc3a6 100644
--- a/include/xo/expression2/detail/IExpression_Any.hpp
+++ b/include/xo/expression2/detail/IExpression_Any.hpp
@@ -2,7 +2,7 @@
  *
  *  Generated automagically from ingredients:
  *  1. code generator:
- *       [/home/roland/proj/xo-umbrella2-claude1/xo-facet/codegen/genfacet]
+ *       [/Users/roland/proj/xo-umbrella2/xo-facet/codegen/genfacet]
  *     arguments:
  *       --input [idl/Expression.json5]
  *  2. jinja2 template for abstract facet .hpp file:
@@ -44,6 +44,7 @@ namespace scm {
 
         /** integer identifying a type **/
         using typeseq = xo::facet::typeseq;
+        using TypeDescr = AExpression::TypeDescr;
 
         ///@}
         /** @defgroup scm-expression-any-methods **/
@@ -55,6 +56,7 @@ namespace scm {
 
         // const methods
         typeseq _typeseq() const noexcept override { return s_typeseq; }
+        [[noreturn]] exprtype extype(Copaque)  const  noexcept override { _fatal(); }
         [[noreturn]] TypeRef typeref(Copaque)  const  noexcept override { _fatal(); }
         [[noreturn]] TypeDescr valuetype(Copaque)  const  noexcept override { _fatal(); }
 
diff --git a/include/xo/expression2/detail/IExpression_Xfer.hpp b/include/xo/expression2/detail/IExpression_Xfer.hpp
index 55abfd05..9f7356bc 100644
--- a/include/xo/expression2/detail/IExpression_Xfer.hpp
+++ b/include/xo/expression2/detail/IExpression_Xfer.hpp
@@ -2,7 +2,7 @@
  *
  *  Generated automagically from ingredients:
  *  1. code generator:
- *       [/home/roland/proj/xo-umbrella2-claude1/xo-facet/codegen/genfacet]
+ *       [/Users/roland/proj/xo-umbrella2/xo-facet/codegen/genfacet]
  *     arguments:
  *       --input [idl/Expression.json5]
  *  2. jinja2 template for abstract facet .hpp file:
@@ -14,6 +14,8 @@
 #pragma once
 
 #include "TypeRef.hpp"
+#include "exprtype.hpp"
+#include <xo/reflect/TypeDescr.hpp>
 
 namespace xo {
 namespace scm {
@@ -28,6 +30,7 @@ namespace scm {
         using Impl = IExpression_DRepr;
         /** integer identifying a type **/
         using typeseq = AExpression::typeseq;
+        using TypeDescr = AExpression::TypeDescr;
         ///@}
 
         /** @defgroup scm-expression-xfer-methods **/
@@ -40,6 +43,9 @@ namespace scm {
 
         // const methods
         typeseq _typeseq() const noexcept override { return s_typeseq; }
+        exprtype extype(Copaque data)  const  noexcept override {
+            return I::extype(_dcast(data));
+        }
         TypeRef typeref(Copaque data)  const  noexcept override {
             return I::typeref(_dcast(data));
         }
diff --git a/include/xo/expression2/detail/RExpression.hpp b/include/xo/expression2/detail/RExpression.hpp
index e9b431e3..9afd711f 100644
--- a/include/xo/expression2/detail/RExpression.hpp
+++ b/include/xo/expression2/detail/RExpression.hpp
@@ -2,7 +2,7 @@
  *
  *  Generated automagically from ingredients:
  *  1. code generator:
- *       [/home/roland/proj/xo-umbrella2-claude1/xo-facet/codegen/genfacet]
+ *       [/Users/roland/proj/xo-umbrella2/xo-facet/codegen/genfacet]
  *     arguments:
  *       --input [idl/Expression.json5]
  *  2. jinja2 template for abstract facet .hpp file:
@@ -31,6 +31,7 @@ public:
     using ObjectType = Object;
     using DataPtr = Object::DataPtr;
     using typeseq = xo::reflect::typeseq;
+    using TypeDescr = AExpression::TypeDescr;
     ///@}
 
     /** @defgroup scm-expression-router-ctors **/
@@ -44,10 +45,13 @@ public:
 
     // const methods
     typeseq _typeseq() const noexcept { return O::iface()->_typeseq(); }
-    TypeRef typeref()  const  noexcept override {
+    exprtype extype()  const  noexcept {
+        return O::iface()->extype(O::data());
+    }
+    TypeRef typeref()  const  noexcept {
         return O::iface()->typeref(O::data());
     }
-    TypeDescr valuetype()  const  noexcept override {
+    TypeDescr valuetype()  const  noexcept {
         return O::iface()->valuetype(O::data());
     }
 
diff --git a/include/xo/expression2/exprtype.hpp b/include/xo/expression2/exprtype.hpp
new file mode 100644
index 00000000..cf66d39e
--- /dev/null
+++ b/include/xo/expression2/exprtype.hpp
@@ -0,0 +1,84 @@
+/** @file exprtype.hpp
+ *
+ *  Author: Roland Conybeare
+ **/
+
+#pragma once
+
+#include <ostream>
+//#include <cstdint>
+
+namespace xo {
+    namespace scm {
+        /** @enum exprtype
+         *  @brief enum to identify subclasses of xo::scm::Expression.
+         *
+         **/
+        enum class exprtype {
+            /** sentinel value **/
+            invalid = -1,
+
+            /** literal constant.  must satisfy both standard_layout_type + trivial **/
+            constant,
+#ifdef NOT_YET
+            /** a literal constant that refers to a linkable named function **/
+            primitive,
+            /** variable/function definition **/
+            define,
+            /** variable assignment **/
+            assign,
+            /** function call **/
+            apply,
+            /** function definition **/
+            lambda,
+            /** variable reference **/
+            variable,
+            /** if-then-else **/
+            ifexpr,
+            /** sequence **/
+            sequence,
+            /** type conversion **/
+            convert,
+#endif
+
+            /** not an expression.  comes last, counts entries **/
+            N
+        };
+
+        inline const char *
+        expr2str(exprtype x)
+        {
+            switch(x) {
+            case exprtype::invalid: return "?exprtype";
+            case exprtype::constant: return "constant";
+#ifdef NOT_YET
+            case exprtype::primitive: return "primitive";
+            case exprtype::define: return "define";
+            case exprtype::assign: return "assign";
+            case exprtype::apply: return "apply";
+            case exprtype::lambda: return "lambda";
+            case exprtype::variable: return "variable";
+            case exprtype::ifexpr: return "if_expr";
+            case exprtype::sequence: return "sequence";
+            case exprtype::convert: return "convert";
+#endif
+            default: break;
+            }
+
+            return "???exprtype???";
+        }
+
+        /** @brief number of built-in expression types,  repr convenient for array sizing **/
+        static constexpr std::size_t n_exprtype = static_cast<std::size_t>(exprtype::N);
+
+        inline std::ostream &
+        operator<<(std::ostream & os,
+                   exprtype x)
+        {
+            os << expr2str(x);
+            return os;
+        }
+    } /*namespace scm*/
+} /*namespace xo*/
+
+/* end exprtype.hpp */
diff --git a/src/expression2/CMakeLists.txt b/src/expression2/CMakeLists.txt
index c0c794ff..b91033c7 100644
--- a/src/expression2/CMakeLists.txt
+++ b/src/expression2/CMakeLists.txt
@@ -2,6 +2,7 @@
 
 set(SELF_LIB xo_expression2)
 set(SELF_SRCS
+    DConstant.cpp
     TypeRef.cpp
     #IExpression_Any.cpp
     expression2_register_facets.cpp
@@ -11,6 +12,7 @@ xo_add_shared_library4(${SELF_LIB} ${PROJECT_NAME}Targets ${PROJECT_VERSION} 1 $
 # note: deps here must also appear in cmake/xo_expression2Config.cmake.in
 xo_dependency(${SELF_LIB} xo_gc)
 xo_dependency(${SELF_LIB} reflect)
+xo_dependency(${SELF_LIB} xo_object2)
 xo_dependency(${SELF_LIB} xo_printable2)
 xo_dependency(${SELF_LIB} xo_flatstring)
 xo_dependency(${SELF_LIB} indentlog)
diff --git a/src/expression2/DConstant.cpp b/src/expression2/DConstant.cpp
new file mode 100644
index 00000000..fcb38f54
--- /dev/null
+++ b/src/expression2/DConstant.cpp
@@ -0,0 +1,53 @@
+/** @file DConstant.cpp
+ *
+ *  @author Roland Conybeare, Jan 2026
+ **/
+
+#include "DConstant.hpp"
+#include "TypeDescr.hpp"
+#include <xo/object2/DFloat.hpp>
+#include <xo/object2/DInteger.hpp>
+#include <xo/reflect/Reflect.hpp>
+#include <xo/reflectutil/typeseq.hpp>
+
+namespace xo {
+    using xo::scm::DFloat;
+    using xo::scm::DInteger;
+    using xo::reflect::Reflect;
+    using xo::reflect::TypeDescr;
+    using xo::reflect::typeseq;
+
+    namespace scm {
+        DConstant::DConstant(obj<AGCObject> value) noexcept
+            :
+            //typeref_{TypeRef::resolved(td)},
+              value_{value}
+        {
+            // todo: use ObjectConverter here
+
+            auto tseq = value_._typeseq();
+
+            TypeDescr td = this->_lookup_td(tseq);
+
+            if (td) {
+                typeref_ = TypeRef::resolved(td);
+            }
+        }
+
+        TypeDescr
+        DConstant::_lookup_td(typeseq tseq)
+        {
+            if (tseq == typeseq::id<DFloat>()) {
+                /* double */
+                return Reflect::require<DFloat::value_type>();
+            } else if (tseq == typeseq::id<DInteger>()) {
+                /* long */
+                return Reflect::require<DInteger::value_type>();
+            }
+
+            return nullptr;
+        }
+    } /*namespace scm*/
+} /*namespace xo*/
+
+/* end DConstant.cpp */
diff --git a/src/expression2/TypeRef.cpp b/src/expression2/TypeRef.cpp
index 2ceb6132..7b187ecd 100644
--- a/src/expression2/TypeRef.cpp
+++ b/src/expression2/TypeRef.cpp
@@ -11,6 +11,15 @@ namespace xo {
                 : id_{id}, td_{td}
         {}
 
+        TypeRef
+        TypeRef::resolved(TypeDescr td)
+        {
+            assert(td);
+
+            type_var null;
+            return TypeRef(null, td);
+        }
+
         TypeRef
         TypeRef::dwim(prefix_type prefix, TypeDescr td)
         {
@@ -18,8 +27,7 @@ namespace xo {
                 /* type already resolved
                  *  -> we don't need a type variable name
                  */
-                type_var null;
-                return TypeRef(null, td);
+                return TypeRef::resolved(td);
             } else {
                 /* type is not resolved yet.
                  *  -> give it a unique name,
@@ -49,7 +57,7 @@ namespace xo {
         }
 
         bool
-        TypeRef::is_concrete() const noexcept
+        TypeRef::is_resolved() const noexcept
         {
             return (td_ != nullptr);
         }