10.1. Glossary

• A slot is a name-value pair. The value of a slot is often called its contents.

• An object is composed of a (possibly empty) set of slots and, optionally, a series of expressions called code. The Self implementation provides objects with indexable slots (vectors) via a set of primitives.

• A data object is an object without code.

• A data slot is a slot holding a data object. An assignment slot is a slot containing the assignment primitive. An assignable data slot is a data slot for which there is a corresponding assignment slot whose name consists of the data slot’s name followed by a colon. When an assignment slot is evaluated its argument is stored in the corresponding data slot.

• An ordinary method (or simply method) is an object with code and is stored as the contents of a slot. The method’s name (also called its selector) is the name of the slot in which it is stored.

• A block is an object representing a lexically-scoped closure (similar to a Smalltalk block).

• A block method is the method that is executed when a block is evaluated by sending it value, value:, value:With:, etc. A block method is a special kind of method that is evaluated within the scope of its method and any lexically enclosing blocks.

• An activation object records the state of an executing method or block method. It is a clone of the method prototype used to store the method’s arguments and local slots during execution. There are two kinds of activation objects: ordinary method activation objects (or simply method activation objects) and block method activation objects.

• A non-lifo block is a block that is evaluated after the activation of its lexically enclosing block or method has returned. This results in an error in the current implementation.

• A non-local return is a return from a method activation resulting from performing a return (i.e., evaluating an expression preceded by the ‘^’ operator) from within a lexically enclosed block. A non-local return forces returns from all activations between the method activation and the activation of the block performing the return.

• The method holder of a method is the object containing the slot holding that method.

• The sending method holder of a message is the method holder of the method that sent it.

• A message is a request to an object to perform some operation. The object to which the request is sent is called the receiver. A message send is the action of sending a message to a receiver.

• A primitive send is a message handled by invoking a primitive, a predefined function provided by the Self implementation.

• Messages that do not have an explicit receiver are known as implicit-receiver messages. The receiver is bound to self.

• A unary message is a message consisting of a single identifier sent to a receiver. A binary message is a message consisting of an operator and a single argument sent to a receiver. A keyword message is a message consisting of one or more identifiers with trailing colons, each followed by an argument, sent to a receiver.

• Unary, binary, and keyword slots are slots with selectors that match unary, binary, and keyword messages, respectively.

• An argument slot is a slot in a method filled in with a value when the method is invoked.

• Message lookup is the process by which objects determine how to respond to a message (which slot to evaluate), by searching objects for slots matching the message.

• Inheritance is the mechanism by which message lookup searches objects for slots when the receiver’s slots are exhausted. An object’s parent slots contain objects that it inherits from.

• Dynamic inheritance is the modification of object behavior by setting an assignable parent slot.

• A resend allows a method to invoke the method that the first method (the one that invokes the resend) is overriding. A directed resend constrains the lookup to search a single parent slot.

• Cloning is the primitive operation returning an exact shallow copy (a clone) of an object, i.e. a new object containing exactly the same slots and code as the original object.

• A prototype is an object that is used as a template from which new objects are cloned.

• A traits object is a parent object containing shared behavior, playing a role somewhat similar to a class in a class-based system. Any Self implementation is required to provide traits objects for integers, floats, strings, and blocks (i.e. one object which is the parent of all integers, another object for floats, etc.).

• The root context is the object that provides the context (i.e., set of bindings) in which slot initializers are evaluated. This object is known as the lobby. During slot initialization, self is bound to the lobby. The lobby is also the sending method holder for any sends in the initializing expression.

• nil is the object used to initialize slots without explicit initializers. It is intended to indicate “not a useful object.” This object is provided by the Self implementation.