Provide default printing function for map_like and array_like native …

…objects

Summary:
`array_like` and `map_like` implementations should generally print out to similar looking trees. This diff provides a default implementation for the *very* common case.

This is useful in future diffs where we make additions to the standard library which use array-like and map-like native objects more.

Reviewed By: yoney

Differential Revision: D68007921

fbshipit-source-id: b8a4c3e379112584960e1f535b24324a671e213b

thrift/compiler/whisker/mstch_compat.cc

@@ -62,15 +62,7 @@ class mstch_array_proxy final
 
   void print_to(tree_printer::scope scope, const object_print_options& options)
       const override {
-    assert(scope.semantic_depth() <= options.max_depth);
-
-    const auto size = proxied_.size();
-    scope.println("mstch::array (size={})", size);
-    for (std::size_t i = 0; i < size; ++i) {
-      auto element_scope = scope.open_transparent_property();
-      element_scope.println("[{}]", i);
-      whisker::print_to(*at(i), element_scope.open_node(), options);
-    }
+    default_print_to("mstch::array", std::move(scope), options);
   }
 
   bool operator==(const native_object& untyped_other) const override {
@@ -129,17 +121,13 @@ class mstch_map_proxy final
 
   void print_to(tree_printer::scope scope, const object_print_options& options)
       const override {
-    assert(scope.semantic_depth() <= options.max_depth);
-    const auto size = proxied_.size();
-    scope.println("mstch::map (size={})", size);
-
+    std::vector<std::string> property_names;
+    property_names.reserve(proxied_.size());
     for (const auto& [key, _] : proxied_) {
-      auto cached = lookup_property(key);
-      assert(cached != nullptr);
-      auto element_scope = scope.open_transparent_property();
-      element_scope.println("'{}'", key);
-      whisker::print_to(*cached, element_scope.open_node(), options);
+      property_names.push_back(key);
     }
+    default_print_to(
+        "mstch::map", std::move(property_names), std::move(scope), options);
   }
 
   bool operator==(const native_object& untyped_other) const override {

thrift/compiler/whisker/object.cc

@@ -134,6 +134,39 @@ class to_string_visitor {
 
 } // namespace
 
+void native_object::map_like::default_print_to(
+    std::string_view name,
+    std::vector<std::string> property_names,
+    tree_printer::scope scope,
+    const object_print_options& options) const {
+  assert(scope.semantic_depth() <= options.max_depth);
+  const auto size = property_names.size();
+  scope.println("{} (size={})", name, size);
+
+  for (const std::string& key : property_names) {
+    auto cached = lookup_property(key);
+    assert(cached != nullptr);
+    auto element_scope = scope.open_transparent_property();
+    element_scope.println("'{}'", key);
+    whisker::print_to(*cached, element_scope.open_node(), options);
+  }
+}
+
+void native_object::array_like::default_print_to(
+    std::string_view name,
+    tree_printer::scope scope,
+    const object_print_options& options) const {
+  assert(scope.semantic_depth() <= options.max_depth);
+
+  const auto sz = size();
+  scope.println("{} (size={})", name, sz);
+  for (std::size_t i = 0; i < sz; ++i) {
+    auto element_scope = scope.open_transparent_property();
+    element_scope.println("[{}]", i);
+    whisker::print_to(*at(i), element_scope.open_node(), options);
+  }
+}
+
 void native_object::print_to(
     tree_printer::scope scope, const object_print_options&) const {
   scope.println("<native_object>");

thrift/compiler/whisker/object.h

@@ -175,6 +175,22 @@ class native_object {
      */
     virtual managed_object_ptr<> lookup_property(
         std::string_view identifier) const = 0;
+
+   protected:
+    /**
+     * A default implementation of whisker::print_to for native object
+     * subclasses of map_like to use.
+     *
+     * For the default implementation to work, property names must be explicitly
+     * enumerated.
+     *
+     * The provided name allows the caller to expose type information.
+     */
+    void default_print_to(
+        std::string_view name,
+        std::vector<std::string> property_names,
+        tree_printer::scope,
+        const object_print_options&) const;
   };
   /**
    * Returns an implementation of map_list if this object supports map-like
@@ -219,6 +235,18 @@ class native_object {
      *   - index < size()
      */
     virtual managed_object_ptr<> at(std::size_t index) const = 0;
+
+   protected:
+    /**
+     * A default implementation of whisker::print_to for native object
+     * subclasses of array_like to use.
+     *
+     * The provided name allows the caller to expose type information.
+     */
+    void default_print_to(
+        std::string_view name,
+        tree_printer::scope,
+        const object_print_options&) const;
   };
   /**
    * Returns an implementation of array_like if this object supports array-like