The ‘manage’ statement

The manage statement provides a mechanism for performing actions automatically at the beginning and end of a scope.

The syntax of the manage statement is inspired by its Python counterpart. The main difference is the use of the manage keyword to open the statement instead of with:

manage new myManager() as myResource do
  myResource.doSomething();

The manage statement accepts a manager (the myManager() call in the above example). The statement calls a special method on the manager which lets it perform actions before executing the managed block. For a type to be recognized as a valid manager, it must implement the contextManager interface.

The manager may optionally return a resource. If a developer wants to make use of the resource, they may capture it after the manager expression (the myResource declaration in the above example). The resource may be of any type, and is the value returned by the special method called enterContext().

Any aggregate type may be used as a manager as long as it implements the contextManager interface, which requires implementing two methods called enterContext() and exitContext():

record myManager : contextManager {
  var x: int = 0;

  proc ref enterContext() ref: int {
    writeln('x is: ', x);
    return x;
  }

  proc exitContext(in err: owned Error?) {
    if err then halt(err!.message());
    writeln('x is: ', x);
  }
}

var m = new myManager();
manage m as myResource {
  // Prints '0'
  myResource = 8;
  // Prints '8'
}

The enterContext() method is called on the manager before entering the managed block. The exitContext() method is called on the manager before leaving the block. It accepts a nilable owned Error? by in intent in order to take ownership of it. The type author may decide to rethrow the error or suppress it.

Status and Future Work

The behavior of several aspects of the manage statement have yet to be formalized, such as:

• If multiple overloads of enterContext() exist that have different return intents, it is unclear what the disambiguation order used to select an overload should be.

• It is uncertain whether or not the storage kind (e.g. var) of a resource should be allowed to be explicitly specified. For example:

  manage foo as var bar do writeln(bar);
  

  Should explicit declaration of the var storage kind for bar be allowed?

• It is unclear how the exitContext() method of a manager should interact with errors beyond guaranteeing that exitContext() is called even if an error is thrown from within a managed block.

• The names of the special methods enterContext() and exitContext() are unstable and subject to change.