scope.md

# What is a scope.

## Overview

Scope is an important concept in programming languages, which defines a program region that a set of bindings between names and entities applies. In a specific scope, a valid name is uniquely associated with an entity, such as a variable. And in another scope, this name may refer to other entity or nothing at all. It clearly restricts the visibility and validity of names in a program. Hence **Scope** is introduced to PaddlePaddle to manage variables in context. But different from the original abstract concept, Scope now becomes an object with two important attributes:

- Scope is an association of a name to variable.
- Variables in a parent scope can be retrieved from local scope.

A detailed explanation of these two attributes goes as following.


## Scope is a Container of Variables.

Scope is used to provide a running environment for Net.

1. Scope mainly has Variables as it's data member.

    Scope is a running environment for Net. Net should get all it need to do computation from a scope, such as data buffer, state(momentum) etc.
    All these data/state can be abstracted and create as variable in Paddle, so the only thing Scope need to care about is Variable.
1. Variable can only be created by Scope.
1. Variable can only be got from Scope.
1. Scope contains methods that are used to manage Variables, such as Create/Get/Delete.

    Because we only need to care about Variable, we only need method to manage the lifecycle of Variable.
    - `Create` is used to create a Variable by its name and add the mapping relation.
    - `Get` is used to find a Variable by name.
    - `Delete` is used to remove a Variable because sometimes we want to release memory or other resources.

1. Every variable only belongs to one certain Scope.

    Variable can not be shared between nets, if we want to use variables from different scope we can use `Parent scope`.

1. Scope should destruct all Variables within it when itself is destructed.

    Because Variable can only be got from Scope, when destroying Scope, we also need to destroy all the Vars in it.

1. Scope do not contain Operators and have no information to run them.

    Net is designed to drive the computation, Scope is only used to provide a running environment.

```cpp
class Scope {
 public:
  Variable* CreateVariable(const std::string& name);
  const Variable* GetVariable(const std::string& name) const;
  bool DeleteVariable(const std::string& name);

 private:
    std::unordered_map<std::string, std::shared_ptr<Vairable>> variable_map_;
};
```


## Parent scope and local scope

Just like [scope](https://en.wikipedia.org/wiki/Scope_(computer_science)) in programming languages, `Scope` in the neural network can also be a local scope. There are two attributes about local scope.

1.  We can create local variables in a local scope. When that local scope are destroyed, all local variables should also be destroyed.
2.  Variables in a parent scope can be retrieved from local scopes of that parent scope, i.e., when user get a variable from a scope, it will try to search this variable in current scope. If there is no such variable in the local scope, `scope` will keep searching from its parent, until the variable is found or there is no parent.

```cpp
class Scope {
public:
  Scope(const std::shared_ptr<Scope>& scope): parent_(scope) {}

  Variable* GetVar(const std::string& name) const {
    Variable* var = GetVarLocally(name);
    if (var != nullptr) {
      return var;
    } else if (parent_ != nullptr) {
      return parent_->Get(name);
    } else {
      return nullptr;
    }
  }

private:
  std::shared_ptr<Scope> parent_ {nullptr};
};
```

In `Scope` class, there is a private data member called `parent_`. `parent_` is a smart pointer to its parent scope. When user `Get` a variable by its `name`, the `name` will be searched inside the current scope. If the variable cannot be found locally and parent scope is not a `nullptr`, the variable will be searched inside that parent scope. `parent_` pointer's default value is `nullptr`. It means that the scope is a global scope when `parent_` is nullptr.

A local scope is very useful when we implement Recurrent Neural Network. Each timestep of an RNN should be a `Net`. Each `Net` of timestep (`StepNet` for short) should use an independent local scope. Just like variables in a while loop is inside a local scope in programming languages. By using a single `StepNet` and changing local scope, we can implement an RNN easily.

# Interface Design

```cpp
class Variable {
private:
  Variable() = default;
  friend class Scope;
};

using VariablePtr = std::weak_ptr<Variable>;

class Scope {
private:
  Scope(const std::shared_ptr<Scope>& parent = nullptr);

public:
  static std::shared_ptr<Scope> Create(const std::shared_ptr<Scope>& parent = nullptr);

  // return nullptr if not found.
  VariablePtr GetVariable(const std::string& name) const;

  // return Error if already contains same name variable.
  Error CreateVariable(const std::string& name);

private:
  std::shared_ptr<Scope> parent_;
  std::unordered_map<std::string, std::shared_ptr<Scope>> attrs_;
};
```
## Only scope can create a variable

To ensure `only scope can create a variable`, we should mark `Variable`'s constructor as a private member function, and Scope is a friend class of Variable. And then only `CreateVariable` can construct `Variable`.

## When scope destroyed, all variables inside this scope should be destroyed together

The `VariablePtr` is a `weak_ptr`. `Net` and `Op` can only get a Variable from `Scope`, but cannot hold it. When scope is destroyed, all `VariablePtr`s belong to this Scope will be changed to `nullptr`.

## Sharing a parent scope

Local scope contains a `parent_` pointer. It is a linked-list for scopes. Using a `shared_ptr` because when a local scope is using, its parents cannot be destroyed.

Also, as the parent scope is a `shared_ptr`, we can only `Create()` a scope shared pointer. We cannot construct a scope variable, because it cannot be passed to other scope as `parent` pointer.

## Orthogonal interface

`GetVariable` will return `nullptr` when `name` is not found. It can be used as `Contains` method. `CreateVariable` will return a `Error` when there is a name conflict locally. Combine `GetVariable` and `CreateVariable`, we can implement `CreateOrGetVariable` easily.