xo-expression: + Lambda::captured_var_set; assigned in ctor

include/xo/expression/Lambda.hpp

@@ -125,6 +125,9 @@ namespace xo {
             /** free variables for this lambda **/
             std::set<std::string> free_var_set_;
 
+            /** variables that appear free in some nested lambda **/
+            std::set<std::string> captured_var_set_;
+
             /** map giving unique identity to each variable appearing in this layer.
              *  includes:
              *  - formal parameters

src/expression/Lambda.cpp

@@ -149,24 +149,50 @@ namespace xo {
             this->free_var_set_ = this->calc_free_variables();
 
             std::map<std::string, ref::brw<Lambda>> nested_lambda_map;
+            {
+                this->body_->visit_layer
+                    ([&nested_lambda_map]
+                     (ref::brw<Expression> expr)
+                        {
+                            if (expr->extype() == exprtype::lambda) {
+                                ref::brw<Lambda> lm = Lambda::from(expr);
 
-            this->body_->visit_layer
+                                nested_lambda_map[lm->name()] = lm.get();
-                ([&nested_lambda_map]
+                            }
-                 (ref::brw<Expression> expr)
+                        });
-                    {
+            }
-                        if (expr->extype() == exprtype::lambda) {
-                            ref::brw<Lambda> lm = Lambda::from(expr);
-
-                            nested_lambda_map[lm->name()] = lm.get();
-                        }
-                    });
-
             this->nested_lambda_map_ = std::move(nested_lambda_map);
 
+            /* establish the set of captured local vars.
+             * These are any formal parameters that appear free in
+             * any layer of a nested lambda.
+             */
+            std::set<std::string> captured_var_set;
+            {
+                for (const auto & ix : nested_lambda_map_) {
+                    std::set<std::string> nested_free_var_set
+                        = ix.second->get_free_variables();
+
+                    for (const auto & jx : nested_free_var_set) {
+                        /* check whether variable *jx is one of this lambda's formals */
+                        auto bind = this->local_env_->lookup_local_binding(jx);
+
+                        if (bind.i_link_ == 0) {
+                            /* yup,  it's a formal parameter of this lambda */
+                            captured_var_set.insert(jx);
+                        }
+                    }
+                }
+            }
+
+            this->captured_var_set_ = std::move(captured_var_set);
+
             /* in particular:
-             * - establish binding path for each variable
+             * - establish binding path (intrusively) for each variable
+             *   assigns Variable::path_
              */
             this->body_->attach_envs(local_env_);
+
         } /*ctor*/
 
         void