use node as lambda return type

README.md

@@ -103,15 +103,17 @@ Output:
 
 C++11 lambda expressions can be used to add logic to your templates. Like a
 `const char*` literal, lambdas can be implicitly converted to `bool`, so they
-must be wrapped in a `mstch::lambda` object when used in a `mstch::node`.
+must be wrapped in a `mstch::lambda` object when used in a `mstch::node`. The 
+lambda expression passed to `mstch::lambda` must itself return a `mstch::node`.
+The returned node will be rendered to a string, then it will be parsed as a
+template.
 
-The lambda expression passed to `mstch::lambda` returns a `std::string` and 
-accepts either no parameters:
+The lambda expression accepts either no parameters:
 
 ```c++
 std::string view{"Hello {{lambda}}!"};
 mstch::map context{
-  {"lambda", mstch::lambda{[]() {
+  {"lambda", mstch::lambda{[]() -> mstch::node {
     return std::string{"World"};
   }}}
 };
@@ -125,14 +127,13 @@ Output:
 Hello World!
 ```
 
-Or it accepts a `const std::string&` that gets the unrendered literal block.
-The returned string will be parsed in both cases:
+Or it accepts a `const std::string&` that gets the unrendered literal block:
 
 ```c++
 std::string view{"{{#bold}}{{yay}} :){{/bold}}"};
 mstch::map context{
   {"yay", std::string{"Yay!"}},
-  {"bold", mstch::lambda{[](const std::string& text) {
+  {"bold", mstch::lambda{[](const std::string& text) -> mstch::node {
     return "<b>" + text + "</b>";
   }}}
 };

include/mstch/mstch.hpp

@@ -35,7 +35,7 @@ class object_t {
   mutable std::map<std::string, N> cache;
 };
 
-template <typename T>
+template<class T, class N>
 class is_fun {
  private:
   using not_fun = char;
@@ -43,9 +43,9 @@ class is_fun {
   using fun_with_args = char[3];
   template <typename U, U> struct really_has;
   template <typename C> static fun_without_args& test(
-      really_has<std::string(C::*)() const, &C::operator()>*);
+      really_has<N(C::*)() const, &C::operator()>*);
   template <typename C> static fun_with_args& test(
-      really_has<std::string(C::*)(const std::string&) const,
+      really_has<N(C::*)(const std::string&) const,
       &C::operator()>*);
   template <typename> static not_fun& test(...);
 
@@ -54,36 +54,48 @@ class is_fun {
   static bool const has_args = sizeof(test<T>(0)) == sizeof(fun_with_args);
 };
 
-}
+template<class N>
+using node_renderer = std::function<std::string(const N& n)>;
 
-class lambda {
+template<class N>
+class lambda_t {
  public:
   template<class F>
-  lambda(F f, typename std::enable_if<internal::is_fun<F>::no_args>::type* =0):
-      fun([f](const std::string&){return f();})
+  lambda_t(F f, typename std::enable_if<is_fun<F, N>::no_args>::type* = 0):
+      fun([f](node_renderer<N> renderer, const std::string&) {
+        return renderer(f());
+      })
   {
   }
 
   template<class F>
-  lambda(F f, typename std::enable_if<internal::is_fun<F>::has_args>::type* =0):
-      fun(f)
+  lambda_t(F f, typename std::enable_if<is_fun<F, N>::has_args>::type* = 0):
+      fun([f](node_renderer<N> renderer, const std::string& text) {
+        return renderer(f(text));
+      })
   {
   }
 
-  std::string operator()(const std::string& text = "") const {
-    return fun(text);
+  std::string operator()(node_renderer<N> renderer,
+      const std::string& text = "") const
+  {
+    return fun(renderer, text);
   }
 
  private:
-  std::function<std::string(const std::string&)> fun;
+  std::function<std::string(node_renderer<N> renderer, const std::string&)> fun;
 };
 
+}
+
 using node = boost::make_recursive_variant<
-    std::nullptr_t, std::string, int, double, bool, lambda,
+    std::nullptr_t, std::string, int, double, bool,
+    internal::lambda_t<boost::recursive_variant_>,
     std::shared_ptr<internal::object_t<boost::recursive_variant_>>,
     std::map<const std::string, boost::recursive_variant_>,
     std::vector<boost::recursive_variant_>>::type;
 using object = internal::object_t<node>;
+using lambda = internal::lambda_t<node>;
 using map = std::map<const std::string, node>;
 using array = std::vector<node>;
 

src/visitor/render_node.hpp

@@ -37,7 +37,10 @@ class render_node: public boost::static_visitor<std::string> {
   }
 
   std::string operator()(const lambda& value) const {
-    auto rendered = render_context::push(ctx).render(template_type{value()});
+    template_type interpreted{value([this](const mstch::node& n) {
+      return visit(render_node(ctx), n);
+    })};
+    auto rendered = render_context::push(ctx).render(interpreted);
     return (m_flag == flag::escape_html) ? html_escape(rendered) : rendered;
   }
 

src/visitor/render_section.hpp

@@ -5,6 +5,7 @@
 #include "render_context.hpp"
 #include "mstch/mstch.hpp"
 #include "utils.hpp"
+#include "render_node.hpp"
 
 namespace mstch {
 
@@ -28,8 +29,10 @@ class render_section: public boost::static_visitor<std::string> {
     std::string section_str;
     for(auto& token: section)
       section_str += token.raw();
-
-    return render_context::push(ctx).render(template_type{fun(section_str)});
+    template_type interpreted{fun([this](const mstch::node& n) {
+      return visit(render_node(ctx), n);
+    }, section_str)};
+    return render_context::push(ctx).render(interpreted);
   }
 
   std::string operator()(const array& array) const {

test/data/comments.hpp

@@ -1,3 +1,3 @@
 const mstch::node comments_data = mstch::map{
-    {"title", mstch::lambda{[](){return std::string{"A Comedy of Errors"};}}}
+    {"title", mstch::lambda{[]()->mstch::node{return std::string{"A Comedy of Errors"};}}}
 };

test/data/escaped.hpp

@@ -1,4 +1,4 @@
 const mstch::node escaped_data = mstch::map{
-    {"title", mstch::lambda{[](){ return std::string{"Bear > Shark"}; }}},
-    {"entities", mstch::lambda{[](){ return std::string{"&quot; \"'<>/"}; }}}
+    {"title", mstch::lambda{[]()->mstch::node{ return std::string{"Bear > Shark"}; }}},
+    {"entities", mstch::lambda{[]()->mstch::node{ return std::string{"&quot; \"'<>/"}; }}}
 };

test/data/higher_order_sections.hpp

@@ -19,7 +19,7 @@ class higher_order_sections: public mstch::object {
   }
 
   mstch::node bolder() {
-    return mstch::lambda{[this](const std::string& text) {
+    return mstch::lambda{[this](const std::string& text) -> mstch::node {
       return "<b>" + text + "</b> " + m_helper;
     }};
   }

test/data/nested_higher_order_sections.hpp

@@ -1,5 +1,5 @@
 const mstch::node nested_higher_order_sections_data = mstch::map{
-  {"bold", mstch::lambda{[](const std::string& text) {
+  {"bold", mstch::lambda{[](const std::string& text) -> mstch::node {
     return std::string{"<b>"} + text + std::string{"</b>"};
   }}},
   {"person", mstch::map{{"name", std::string{"Jonas"}}}}

test/data/partial_template.hpp

@@ -1,4 +1,4 @@
 const mstch::node partial_template_data = mstch::map{
-    {"title", mstch::lambda{[](){ return std::string{"Welcome"}; }}},
-    {"again", mstch::lambda{[](){ return std::string{"Goodbye"}; }}},
+    {"title", mstch::lambda{[]()->mstch::node{ return std::string{"Welcome"}; }}},
+    {"again", mstch::lambda{[]()->mstch::node{ return std::string{"Goodbye"}; }}},
 };

test/data/section_functions_in_partials.hpp

@@ -1,5 +1,5 @@
 const mstch::node section_functions_in_partials_data = mstch::map{
-  {"bold", mstch::lambda{[](const std::string& text) {
+  {"bold", mstch::lambda{[](const std::string& text) -> mstch::node {
     return std::string{"<b>"} + text + std::string{"</b>"};
   }}}
 };

test/data/unescaped.hpp

@@ -1,3 +1,3 @@
 const mstch::node unescaped_data = mstch::map{
-    {"title", mstch::lambda{[](){ return std::string{"Bear > Shark"}; }}}
+    {"title", mstch::lambda{[]()->mstch::node{ return std::string{"Bear > Shark"}; }}}
 };

test/specs_lambdas.hpp

@@ -1,26 +1,26 @@
-std::map<std::string,std::function<std::string(const std::string&)>> specs_lambdas {
+std::map<std::string,std::function<mstch::node(const std::string&)>> specs_lambdas {
     {"Interpolation", [](const std::string&) {
-      return "world";
+      return std::string{"world"};
     }},
     {"Interpolation - Expansion", [](const std::string&) {
-      return "{{planet}}";
+      return std::string{"{{planet}}"};
     }},
     {"Interpolation - Multiple Calls", [](const std::string&) {
-      static int calls = 0; return std::to_string(++calls);
+      static int calls = 0; return ++calls;
     }},
     {"Escaping", [](const std::string&) {
-      return ">";
+      return std::string{">"};
     }},
     {"Section", [](const std::string& txt) {
-      return (txt == "{{x}}") ? "yes" : "no";
+      return std::string{(txt == "{{x}}") ? "yes" : "no"};
     }},
     {"Section - Expansion", [](const std::string& txt) {
-      return txt + "{{planet}}" + txt;
+      return txt + std::string{"{{planet}}"} + txt;
     }},
     {"Section - Multiple Calls", [](const std::string& txt) {
       return "__" + txt + "__";
     }},
     {"Inverted Section", [](const std::string& txt) {
-      return "";
+      return false;
     }}
 };