[Maybe types] Add MakeMaybe(...)

Wraps Locals in MaybeLocals. When called with a MaybeLocal it is the identity function and returns its argument. This is useful in writing generic template code on top of NAN.

Author: agnat

README.md

@@ -185,6 +185,7 @@ The `Nan::MaybeLocal` and `Nan::Maybe` types are monads that encapsulate `v8::Lo
   - <a href="doc/maybe_types.md#api_nan_get_start_column"><b><code>Nan::GetStartColumn()</code></b></a>
   - <a href="doc/maybe_types.md#api_nan_get_end_column"><b><code>Nan::GetEndColumn()</code></b></a>
   - <a href="doc/maybe_types.md#api_nan_clone_element_at"><b><code>Nan::CloneElementAt()</code></b></a>
+  - <a href="doc/maybe_types.md#api_nan_make_maybe"><b><code>Nan::MakeMaybe()</code></b></a>
 
 ### Script
 

doc/maybe_types.md

@@ -36,6 +36,7 @@ The `Nan::MaybeLocal` and `Nan::Maybe` types are monads that encapsulate `v8::Lo
   - <a href="#api_nan_get_start_column"><b><code>Nan::GetStartColumn()</code></b></a>
   - <a href="#api_nan_get_end_column"><b><code>Nan::GetEndColumn()</code></b></a>
   - <a href="#api_nan_clone_element_at"><b><code>Nan::CloneElementAt()</code></b></a>
+  - <a href="#api_nan_make_maybe"><b><code>Nan::MakeMaybe()</code></b></a>
 
 <a name="api_nan_maybe_local"></a>
 ### Nan::MaybeLocal
@@ -478,3 +479,22 @@ Signature:
 ```c++
 Nan::MaybeLocal<v8::Object> Nan::CloneElementAt(v8::Local<v8::Array> array, uint32_t index);
 ```
+
+<a name="api_nan_make_maybe"></a>
+### Nan::MakeMaybe()
+
+Wraps a `v8::Local<>` in a `Nan::MaybeLocal<>`. When called with a `Nan::MaybeLocal<>` it just returns its argument. This is useful in generic template code that builds on NAN.
+
+Synopsis:
+
+```c++
+  MaybeLocal<v8::Number> someNumber = MakeMaybe(New<v8::Number>(3.141592654));
+  MaybeLocal<v8::String> someString = MakeMaybe(New<v8::String>("probably"));
+```
+
+Signature:
+
+```c++
+template <typename T, template <typename> class MaybeMaybe>
+Nan::MaybeLocal<T> Nan::MakeMaybe(MaybeMaybe<T> v);
+```

nan.h

@@ -2186,6 +2186,32 @@ struct Tap {
 
 #undef TYPE_CHECK
 
+//=== Generic Maybefication ===================================================
+
+namespace imp {
+
+template <typename T> struct Maybefier;
+
+template <typename T> struct Maybefier<v8::Local<T> > {
+  static MaybeLocal<T> convert(v8::Local<T> v) {
+    return MaybeLocal<T>(v);
+  }
+};
+
+template <typename T> struct Maybefier<MaybeLocal<T> > {
+  static MaybeLocal<T> convert(MaybeLocal<T> v) {
+    return v;
+  }
+};
+
+}  // end of namespace imp
+
+template <typename T, template <typename> class MaybeMaybe>
+MaybeLocal<T>
+MakeMaybe(MaybeMaybe<T> v) {
+  return imp::Maybefier<MaybeMaybe<T> >::convert(v);
+}
+
 }  // end of namespace Nan
 
 #endif  // NAN_H_

test/cpp/nannew.cpp

@@ -447,6 +447,13 @@ NAN_METHOD(newExternal) {
   info.GetReturnValue().Set(New<External>(&ttt));
 }
 
+NAN_METHOD(invokeMakeMaybe) {
+  Nan::MaybeLocal<v8::Number> number = MakeMaybe(New<v8::Number>(3.141592654));
+  Nan::MaybeLocal<v8::String> string = MakeMaybe(New<v8::String>("probably"));
+  (void)string;
+  info.GetReturnValue().Set(number.ToLocalChecked());
+}
+
 NAN_MODULE_INIT(Init) {
   NAN_EXPORT(target, testArray);
   NAN_EXPORT(target, testBoolean);
@@ -478,6 +485,8 @@ NAN_MODULE_INIT(Init) {
   NAN_EXPORT(target, newStringFromStdString);
 
   NAN_EXPORT(target, newExternal);
+
+  NAN_EXPORT(target, invokeMakeMaybe);
 }
 
 }  // end of anonymous namespace

test/js/nannew-test.js

@@ -49,3 +49,8 @@ test('strings', function (t) {
   t.end();
 });
 
+test('test MakeMaybe(...)', function (t) {
+  t.plan(1);
+  t.ok(bindings.invokeMakeMaybe() - Math.PI < 10e-8);
+  t.end();
+});