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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 a container of variables
Scope can be inherited or shared

A detailed explanation of these two attributes goes as following.

Scope is a Container of Variables.

* Scope contains Variables as it's data member.
* Scope contains methods that are used to manage Variables, such as Create/Get/Delete.
* every variable only belong to one certain Scope.
* Scope should destruct all Variables within it when itself is destructed.
* Variable can only be created by Scope.
* Variable can only be got from Scope.

* Scope do not contains Operators and have no information to run them.

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 in programming languages, Scope in the neural network can also be a local scope. There are two attributes about local scope.

We can create local variables in a local scope. When that local scope are destroyed, all local variables should also be destroyed.
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.

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

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 VariablePtrs belong to this Scope will be changed to nullptr.

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.

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