diff --git a/include/xo/expression/Lambda.hpp b/include/xo/expression/Lambda.hpp
index 7f6434f8..7f5ebab4 100644
--- a/include/xo/expression/Lambda.hpp
+++ b/include/xo/expression/Lambda.hpp
@@ -35,6 +35,9 @@ namespace xo {
             const std::vector<std::string> & argv() const { return argv_; }
             const ref::rp<Expression> & body() const { return body_; }
 
+            /** return number of arguments expected by this function **/
+            int n_arg() const { return argv_.size(); }
+
             // ----- Expression -----
 
             virtual void display(std::ostream & os) const override;
diff --git a/include/xo/expression/Primitive.hpp b/include/xo/expression/Primitive.hpp
index afae94a9..f4fc21b7 100644
--- a/include/xo/expression/Primitive.hpp
+++ b/include/xo/expression/Primitive.hpp
@@ -33,17 +33,20 @@ namespace xo {
                       FunctionPointer fnptr)
                 : PrimitiveInterface(),
                   name_{name},
-                  value_td_{Reflect::require<FunctionPointer>()},
+                  value_td_{Reflect::require_function<FunctionPointer>()},
                   value_{fnptr}
-                {}
+                {
+                    if (!value_td_->is_function())
+                        throw std::runtime_error("Primitive: expected function pointer");
+                    if (!value_td_->fn_retval())
+                        throw std::runtime_error("Primitive: expected non-null function return value");
+                }
 
             FunctionPointer value() const { return value_; }
 
             // ----- PrimitiveInterface -----
 
-            virtual std::string const & name() const { return name_; }
-            /** FIXME for now **/
-            virtual int n_arg() const { return 1; }
+            virtual std::string const & name() const override { return name_; }
 
             // ----- ConstantInterface -----
 
diff --git a/include/xo/expression/PrimitiveInterface.hpp b/include/xo/expression/PrimitiveInterface.hpp
index c294a440..3274195f 100644
--- a/include/xo/expression/PrimitiveInterface.hpp
+++ b/include/xo/expression/PrimitiveInterface.hpp
@@ -22,7 +22,9 @@ namespace xo {
             }
 
             virtual const std::string & name() const = 0;
-            virtual int n_arg() const = 0;
+            int n_arg() const { return this->value_td()->n_fn_arg(); }
+            TypeDescr fn_retval() const { return this->value_td()->fn_retval(); }
+            TypeDescr fn_arg(uint32_t i) const { return this->value_td()->fn_arg(i); }
 
             //virtual TypeDescr value_td() const override = 0;
             //virtual TaggedPtr value_tp() const override = 0;