diff --git a/include/xo/expression/FunctionInterface.hpp b/include/xo/expression/FunctionExprInterface.hpp
similarity index 74%
rename from include/xo/expression/FunctionInterface.hpp
rename to include/xo/expression/FunctionExprInterface.hpp
index a568a71d..62756654 100644
--- a/include/xo/expression/FunctionInterface.hpp
+++ b/include/xo/expression/FunctionExprInterface.hpp
@@ -10,14 +10,14 @@
 
 namespace xo {
     namespace scm {
-        class FunctionInterface : public Expression {
+        class FunctionExprInterface : public Expression {
         public:
-            FunctionInterface(exprtype extype, TypeDescr fn_type)
+            FunctionExprInterface(exprtype extype, TypeDescr fn_type)
                 : Expression(extype, fn_type) {}
 
             /** downcast from Expression **/
-            static bp<FunctionInterface> from(bp<Expression> x) {
-                return bp<FunctionInterface>::from(x);
+            static bp<FunctionExprInterface> from(bp<Expression> x) {
+                return bp<FunctionExprInterface>::from(x);
             }
 
             virtual const std::string & name() const = 0;
diff --git a/include/xo/expression/Lambda.hpp b/include/xo/expression/Lambda.hpp
index 81a872b6..e4d24cd6 100644
--- a/include/xo/expression/Lambda.hpp
+++ b/include/xo/expression/Lambda.hpp
@@ -6,13 +6,12 @@
 #pragma once
 
 #include "Expression.hpp"
-#include "FunctionInterface.hpp"
+#include "FunctionExprInterface.hpp"
 #include "Variable.hpp"
 #include "LocalSymtab.hpp"
 #include <map>
 #include <vector>
 #include <string>
-//#include <cstdint>
 
 namespace xo {
     namespace scm {
@@ -20,7 +19,7 @@ namespace xo {
          *  @brief abstract syntax tree for a function definition
          *
          **/
-        class Lambda : public FunctionInterface {
+        class Lambda : public FunctionExprInterface {
         public:
             /**
              *  @p name.  Name for this lambda -- must be unique
diff --git a/include/xo/expression/Primitive.hpp b/include/xo/expression/Primitive.hpp
index 6c1c12bc..e53bbcb9 100644
--- a/include/xo/expression/Primitive.hpp
+++ b/include/xo/expression/Primitive.hpp
@@ -5,7 +5,7 @@
 
 #pragma once
 
-#include "PrimitiveInterface.hpp"
+#include "PrimitiveExprInterface.hpp"
 #include "pretty_expression.hpp"
 #include "llvmintrinsic.hpp"
 #include "xo/reflect/Reflect.hpp"
@@ -38,7 +38,7 @@ namespace xo {
          *          won't work here.
          **/
         template <typename FunctionPointer>
-        class Primitive: public PrimitiveInterface {
+        class Primitive: public PrimitiveExprInterface {
         public:
             using Reflect = xo::reflect::Reflect;
             using TaggedPtr = xo::reflect::TaggedPtr;
@@ -111,7 +111,7 @@ namespace xo {
                       FunctionPointer fnptr,
                       bool explicit_symbol_def,
                       llvmintrinsic intrinsic)
-                : PrimitiveInterface(fn_type),
+                : PrimitiveExprInterface(fn_type),
                   name_{name},
                   value_td_{Reflect::require_function<FunctionPointer>()},
                   value_{fnptr},
diff --git a/include/xo/expression/PrimitiveInterface.hpp b/include/xo/expression/PrimitiveExprInterface.hpp
similarity index 82%
rename from include/xo/expression/PrimitiveInterface.hpp
rename to include/xo/expression/PrimitiveExprInterface.hpp
index c9e5660e..1011e5dc 100644
--- a/include/xo/expression/PrimitiveInterface.hpp
+++ b/include/xo/expression/PrimitiveExprInterface.hpp
@@ -1,29 +1,27 @@
-/** @file PrimitiveInterface.hpp
+/** @file PrimitiveExprInterface.hpp
  *
  *  Author: Roland Conybeare
  **/
 
 #pragma once
 
-#include "FunctionInterface.hpp"
+#include "FunctionExprInterface.hpp"
 #include "llvmintrinsic.hpp"
-
-//#include <cstdint>
-
 #include <type_traits>
+
 namespace xo {
     namespace scm {
-        class PrimitiveInterface : public FunctionInterface {
+        class PrimitiveExprInterface : public FunctionExprInterface {
         public:
             using void_function_type = void (*)();
 
         public:
-            explicit PrimitiveInterface(TypeDescr fn_type)
-                : FunctionInterface(exprtype::primitive, fn_type) {}
+            explicit PrimitiveExprInterface(TypeDescr fn_type)
+                : FunctionExprInterface(exprtype::primitive, fn_type) {}
 
             /** downcast from Expression **/
-            static bp<PrimitiveInterface> from(bp<Expression> x) {
-                return bp<PrimitiveInterface>::from(x);
+            static bp<PrimitiveExprInterface> from(bp<Expression> x) {
+                return bp<PrimitiveExprInterface>::from(x);
             }
 
             /** if true,  Jit will try to explicitly symbol for this primitive
diff --git a/src/expression/Lambda.cpp b/src/expression/Lambda.cpp
index 7a140f04..d944b690 100644
--- a/src/expression/Lambda.cpp
+++ b/src/expression/Lambda.cpp
@@ -259,7 +259,7 @@ namespace xo {
                        TypeDescr lambda_td,
                        const rp<LocalSymtab> & local_env,
                        const rp<Expression> & body)
-            : FunctionInterface(exprtype::lambda, lambda_td),
+            : FunctionExprInterface(exprtype::lambda, lambda_td),
               name_{name},
               body_{body},
               local_env_{local_env}