diff --git a/include/xo/expression/Lambda.hpp b/include/xo/expression/Lambda.hpp
index a7f7529d..619913a1 100644
--- a/include/xo/expression/Lambda.hpp
+++ b/include/xo/expression/Lambda.hpp
@@ -39,6 +39,8 @@ namespace xo {
             const std::vector<ref::rp<Variable>> & argv() const { return local_env_->argv(); }
             const ref::rp<Expression> & body() const { return body_; }
 
+            bool needs_closure_flag() const { return !free_var_set_.empty(); }
+
             // ----- FunctionInterface -----
 
             virtual const std::string & name() const override { return name_; }
@@ -50,13 +52,7 @@ namespace xo {
             // ----- Expression -----
 
             virtual std::set<std::string> get_free_variables() const override {
-                std::set<std::string> retval = body_->get_free_variables();
-
-                /* but remove formals. */
-                for (const auto & var : local_env_->argv())
-                    retval.erase(var->name());
-
-                return retval;
+                return this->free_var_set_;
             }
 
             virtual std::size_t visit_preorder(VisitFn visitor_fn) override {
@@ -87,6 +83,9 @@ namespace xo {
                    const ref::rp<LocalEnv> & local_env,
                    const ref::rp<Expression> & body);
 
+            /** compute free-variable set for this lambda **/
+            std::set<std::string> calc_free_variables() const;
+
         private:
             /** lambda name.  Initially supporting only form like
              *    (define (foo x y z)
@@ -103,6 +102,9 @@ namespace xo {
             /** function body **/
             ref::rp<Expression> body_;
 
+            /** free variables for this lambda **/
+            std::set<std::string> free_var_set_;
+
             /** established (once) by @ref attach_envs.
              *
              * @note data dependency on ancestor expressions that don't exist yet
diff --git a/src/expression/Lambda.cpp b/src/expression/Lambda.cpp
index 7ad70ceb..d2fe8222 100644
--- a/src/expression/Lambda.cpp
+++ b/src/expression/Lambda.cpp
@@ -45,6 +45,19 @@ namespace xo {
                               body);
         } /*make*/
 
+        std::set<std::string>
+        Lambda::calc_free_variables() const
+        {
+            std::set<std::string> retval
+                = body_->get_free_variables();
+
+            /* but remove formals. */
+            for (const auto & var : local_env_->argv())
+                retval.erase(var->name());
+
+            return retval;
+        } /*calc_free_variables*/
+
         Lambda::Lambda(const std::string & name,
                        TypeDescr lambda_type,
                        const rp<LocalEnv> & local_env,
@@ -64,7 +77,8 @@ namespace xo {
             }
             ss << ")";
 
-            type_str_ = ss.str();
+            this->type_str_ = ss.str();
+            this->free_var_set_ = this->calc_free_variables();
 
             body_->attach_envs(local_env_);
         } /*ctor*/