程序代写代做代考 chain Haskell ## Maybe Types

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## Maybe Types

The `Nan::MaybeLocal` and `Nan::Maybe` types are monads that encapsulate `v8::Local` handles that _may be empty_.

* **Maybe Types**
– Nan::MaybeLocal
– Nan::Maybe
– Nan::Nothing
– Nan::Just
* **Maybe Helpers**
– Nan::Call()
– Nan::ToDetailString()
– Nan::ToArrayIndex()
– Nan::Equals()
– Nan::NewInstance()
– Nan::GetFunction()
– Nan::Set()
– Nan::ForceSet()
– Nan::Get()
– Nan::GetPropertyAttributes()
– Nan::Has()
– Nan::Delete()
– Nan::GetPropertyNames()
– Nan::GetOwnPropertyNames()
– Nan::SetPrototype()
– Nan::ObjectProtoToString()
– Nan::HasOwnProperty()
– Nan::HasRealNamedProperty()
– Nan::HasRealIndexedProperty()
– Nan::HasRealNamedCallbackProperty()
– Nan::GetRealNamedPropertyInPrototypeChain()
– Nan::GetRealNamedProperty()
– Nan::CallAsFunction()
– Nan::CallAsConstructor()
– Nan::GetSourceLine()
– Nan::GetLineNumber()
– Nan::GetStartColumn()
– Nan::GetEndColumn()
– Nan::CloneElementAt()
– Nan::MakeMaybe()


### Nan::MaybeLocal

A `Nan::MaybeLocal` is a wrapper around [`v8::Local`](https://v8docs.nodesource.com/io.js-3.0/de/deb/classv8_1_1_local.html) that enforces a check that determines whether the `v8::Local` is empty before it can be used.

If an API method returns a `Nan::MaybeLocal`, the API method can potentially fail either because an exception is thrown, or because an exception is pending, e.g. because a previous API call threw an exception that hasn’t been caught yet, or because a `v8::TerminateExecution` exception was thrown. In that case, an empty `Nan::MaybeLocal` is returned.

Definition:

“`c++
template class Nan::MaybeLocal {
public:
MaybeLocal();

template MaybeLocal(v8::Local that);

bool IsEmpty() const;

template bool ToLocal(v8::Local *out);

// Will crash if the MaybeLocal<> is empty.
v8::Local ToLocalChecked();

template v8::Local FromMaybe(v8::Local default_value) const;
};
“`

See the documentation for [`v8::MaybeLocal`](https://v8docs.nodesource.com/io.js-3.0/d8/d7d/classv8_1_1_maybe_local.html) for further details.


### Nan::Maybe

A simple `Nan::Maybe` type, representing an object which may or may not have a value, see https://hackage.haskell.org/package/base/docs/Data-Maybe.html.

If an API method returns a `Nan::Maybe<>`, the API method can potentially fail either because an exception is thrown, or because an exception is pending, e.g. because a previous API call threw an exception that hasn’t been caught yet, or because a `v8::TerminateExecution` exception was thrown. In that case, a “Nothing” value is returned.

Definition:

“`c++
template class Nan::Maybe {
public:
bool IsNothing() const;
bool IsJust() const;

// Will crash if the Maybe<> is nothing.
T FromJust();

T FromMaybe(const T& default_value);

bool operator==(const Maybe &other);

bool operator!=(const Maybe &other);
};
“`

See the documentation for [`v8::Maybe`](https://v8docs.nodesource.com/io.js-3.0/d9/d4b/classv8_1_1_maybe.html) for further details.


### Nan::Nothing

Construct an empty `Nan::Maybe` type representing _nothing_.

“`c++
template Nan::Maybe Nan::Nothing();
“`


### Nan::Just

Construct a `Nan::Maybe` type representing _just_ a value.

“`c++
template Nan::Maybe Nan::Just(const T &t);
“`


### Nan::Call()

A helper method for calling [`v8::Function#Call()`](https://v8docs.nodesource.com/io.js-3.0/d5/d54/classv8_1_1_function.html#a468a89f737af0612db10132799c827c0) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::MaybeLocal Nan::Call(v8::Local fun, v8::Local recv, int argc, v8::Local argv[]);
“`


### Nan::ToDetailString()

A helper method for calling [`v8::Value#ToDetailString()`](https://v8docs.nodesource.com/io.js-3.0/dc/d0a/classv8_1_1_value.html#a2f9770296dc2c8d274bc8cc0dca243e5) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::MaybeLocal Nan::ToDetailString(v8::Local val);
“`


### Nan::ToArrayIndex()

A helper method for calling [`v8::Value#ToArrayIndex()`](https://v8docs.nodesource.com/io.js-3.0/dc/d0a/classv8_1_1_value.html#acc5bbef3c805ec458470c0fcd6f13493) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::MaybeLocal Nan::ToArrayIndex(v8::Local val);
“`


### Nan::Equals()

A helper method for calling [`v8::Value#Equals()`](https://v8docs.nodesource.com/io.js-3.0/dc/d0a/classv8_1_1_value.html#a0d9616ab2de899d4e3047c30a10c9285) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::Maybe Nan::Equals(v8::Local a, v8::Local(b));
“`


### Nan::NewInstance()

A helper method for calling [`v8::Function#NewInstance()`](https://v8docs.nodesource.com/io.js-3.0/d5/d54/classv8_1_1_function.html#a691b13f7a553069732cbacf5ac8c62ec) and [`v8::ObjectTemplate#NewInstance()`](https://v8docs.nodesource.com/io.js-3.0/db/d5f/classv8_1_1_object_template.html#ad605a7543cfbc5dab54cdb0883d14ae4) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::MaybeLocal Nan::NewInstance(v8::Local h);
Nan::MaybeLocal Nan::NewInstance(v8::Local h, int argc, v8::Local argv[]);
Nan::MaybeLocal Nan::NewInstance(v8::Local h);
“`


### Nan::GetFunction()

A helper method for calling [`v8::FunctionTemplate#GetFunction()`](https://v8docs.nodesource.com/io.js-3.0/d8/d83/classv8_1_1_function_template.html#a56d904662a86eca78da37d9bb0ed3705) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::MaybeLocal Nan::GetFunction(v8::Local t);
“`


### Nan::Set()

A helper method for calling [`v8::Object#Set()`](https://v8docs.nodesource.com/io.js-3.0/db/d85/classv8_1_1_object.html#a67604ea3734f170c66026064ea808f20) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::Maybe Nan::Set(v8::Local obj,
v8::Local key,
v8::Local value)
Nan::Maybe Nan::Set(v8::Local obj,
uint32_t index,
v8::Local value);
“`


### Nan::ForceSet()

A helper method for calling [`v8::Object#ForceSet()`](https://v8docs.nodesource.com/io.js-3.0/db/d85/classv8_1_1_object.html#a796b7b682896fb64bf1872747734e836) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::Maybe Nan::ForceSet(v8::Local obj,
v8::Local key,
v8::Local value,
v8::PropertyAttribute attribs = v8::None);
“`


### Nan::Get()

A helper method for calling [`v8::Object#Get()`](https://v8docs.nodesource.com/io.js-3.0/db/d85/classv8_1_1_object.html#a2565f03e736694f6b1e1cf22a0b4eac2) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::MaybeLocal Nan::Get(v8::Local obj,
v8::Local key);
Nan::MaybeLocal Nan::Get(v8::Local obj, uint32_t index);
“`


### Nan::GetPropertyAttributes()

A helper method for calling [`v8::Object#GetPropertyAttributes()`](https://v8docs.nodesource.com/io.js-3.0/db/d85/classv8_1_1_object.html#a9b898894da3d1db2714fd9325a54fe57) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::Maybe Nan::GetPropertyAttributes(
v8::Local obj,
v8::Local key);
“`


### Nan::Has()

A helper method for calling [`v8::Object#Has()`](https://v8docs.nodesource.com/io.js-3.0/db/d85/classv8_1_1_object.html#ab3c3d89ea7c2f9afd08965bd7299a41d) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::Maybe Nan::Has(v8::Local obj, v8::Local key);
Nan::Maybe Nan::Has(v8::Local obj, uint32_t index);
“`


### Nan::Delete()

A helper method for calling [`v8::Object#Delete()`](https://v8docs.nodesource.com/io.js-3.0/db/d85/classv8_1_1_object.html#a2fa0f5a592582434ed1ceceff7d891ef) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::Maybe Nan::Delete(v8::Local obj,
v8::Local key);
Nan::Maybe Nan::Delete(v8::Local obj, uint32_t index);
“`


### Nan::GetPropertyNames()

A helper method for calling [`v8::Object#GetPropertyNames()`](https://v8docs.nodesource.com/io.js-3.0/db/d85/classv8_1_1_object.html#aced885270cfd2c956367b5eedc7fbfe8) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::MaybeLocal Nan::GetPropertyNames(v8::Local obj);
“`


### Nan::GetOwnPropertyNames()

A helper method for calling [`v8::Object#GetOwnPropertyNames()`](https://v8docs.nodesource.com/io.js-3.0/db/d85/classv8_1_1_object.html#a79a6e4d66049b9aa648ed4dfdb23e6eb) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::MaybeLocal Nan::GetOwnPropertyNames(v8::Local obj);
“`


### Nan::SetPrototype()

A helper method for calling [`v8::Object#SetPrototype()`](https://v8docs.nodesource.com/io.js-3.0/db/d85/classv8_1_1_object.html#a442706b22fceda6e6d1f632122a9a9f4) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::Maybe Nan::SetPrototype(v8::Local obj,
v8::Local prototype);
“`


### Nan::ObjectProtoToString()

A helper method for calling [`v8::Object#ObjectProtoToString()`](https://v8docs.nodesource.com/io.js-3.0/db/d85/classv8_1_1_object.html#ab7a92b4dcf822bef72f6c0ac6fea1f0b) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::MaybeLocal Nan::ObjectProtoToString(v8::Local obj);
“`


### Nan::HasOwnProperty()

A helper method for calling [`v8::Object#HasOwnProperty()`](https://v8docs.nodesource.com/io.js-3.0/db/d85/classv8_1_1_object.html#ab7b7245442ca6de1e1c145ea3fd653ff) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::Maybe Nan::HasOwnProperty(v8::Local obj,
v8::Local key);
“`


### Nan::HasRealNamedProperty()

A helper method for calling [`v8::Object#HasRealNamedProperty()`](https://v8docs.nodesource.com/io.js-3.0/db/d85/classv8_1_1_object.html#ad8b80a59c9eb3c1e6c3cd6c84571f767) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::Maybe Nan::HasRealNamedProperty(v8::Local obj,
v8::Local key);
“`


### Nan::HasRealIndexedProperty()

A helper method for calling [`v8::Object#HasRealIndexedProperty()`](https://v8docs.nodesource.com/io.js-3.0/db/d85/classv8_1_1_object.html#af94fc1135a5e74a2193fb72c3a1b9855) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::Maybe Nan::HasRealIndexedProperty(v8::Local obj,
uint32_t index);
“`


### Nan::HasRealNamedCallbackProperty()

A helper method for calling [`v8::Object#HasRealNamedCallbackProperty()`](https://v8docs.nodesource.com/io.js-3.0/db/d85/classv8_1_1_object.html#af743b7ea132b89f84d34d164d0668811) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::Maybe Nan::HasRealNamedCallbackProperty(
v8::Local obj,
v8::Local key);
“`


### Nan::GetRealNamedPropertyInPrototypeChain()

A helper method for calling [`v8::Object#GetRealNamedPropertyInPrototypeChain()`](https://v8docs.nodesource.com/io.js-3.0/db/d85/classv8_1_1_object.html#a8700b1862e6b4783716964ba4d5e6172) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::MaybeLocal Nan::GetRealNamedPropertyInPrototypeChain(
v8::Local obj,
v8::Local key);
“`


### Nan::GetRealNamedProperty()

A helper method for calling [`v8::Object#GetRealNamedProperty()`](https://v8docs.nodesource.com/io.js-3.0/db/d85/classv8_1_1_object.html#a84471a824576a5994fdd0ffcbf99ccc0) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::MaybeLocal Nan::GetRealNamedProperty(v8::Local obj,
v8::Local key);
“`


### Nan::CallAsFunction()

A helper method for calling [`v8::Object#CallAsFunction()`](https://v8docs.nodesource.com/io.js-3.0/db/d85/classv8_1_1_object.html#a9ef18be634e79b4f0cdffa1667a29f58) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::MaybeLocal Nan::CallAsFunction(v8::Local obj,
v8::Local recv,
int argc,
v8::Local argv[]);
“`


### Nan::CallAsConstructor()

A helper method for calling [`v8::Object#CallAsConstructor()`](https://v8docs.nodesource.com/io.js-3.0/db/d85/classv8_1_1_object.html#a50d571de50d0b0dfb28795619d07a01b) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::MaybeLocal Nan::CallAsConstructor(v8::Local obj,
int argc,
v8::Local argv[]);
“`


### Nan::GetSourceLine()

A helper method for calling [`v8::Message#GetSourceLine()`](https://v8docs.nodesource.com/io.js-3.0/d9/d28/classv8_1_1_message.html#a849f7a6c41549d83d8159825efccd23a) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::MaybeLocal Nan::GetSourceLine(v8::Local msg);
“`


### Nan::GetLineNumber()

A helper method for calling [`v8::Message#GetLineNumber()`](https://v8docs.nodesource.com/io.js-3.0/d9/d28/classv8_1_1_message.html#adbe46c10a88a6565f2732a2d2adf99b9) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::Maybe Nan::GetLineNumber(v8::Local msg);
“`


### Nan::GetStartColumn()

A helper method for calling [`v8::Message#GetStartColumn()`](https://v8docs.nodesource.com/io.js-3.0/d9/d28/classv8_1_1_message.html#a60ede616ba3822d712e44c7a74487ba6) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::Maybe Nan::GetStartColumn(v8::Local msg);
“`


### Nan::GetEndColumn()

A helper method for calling [`v8::Message#GetEndColumn()`](https://v8docs.nodesource.com/io.js-3.0/d9/d28/classv8_1_1_message.html#aaa004cf19e529da980bc19fcb76d93be) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::Maybe Nan::GetEndColumn(v8::Local msg);
“`


### Nan::CloneElementAt()

A helper method for calling [`v8::Array#CloneElementAt()`](https://v8docs.nodesource.com/io.js-3.0/d3/d32/classv8_1_1_array.html#a1d3a878d4c1c7cae974dd50a1639245e) in a way compatible across supported versions of V8.

Signature:

“`c++
Nan::MaybeLocal Nan::CloneElementAt(v8::Local array, uint32_t index);
“`


### 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 someNumber = MakeMaybe(New(3.141592654));
MaybeLocal someString = MakeMaybe(New(“probably”));
“`

Signature:

“`c++
template class MaybeMaybe>
Nan::MaybeLocal Nan::MakeMaybe(MaybeMaybe v);
“`

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