4.2. The Roots of Behavior

4.2.1. Default Behavior

Certain common behavior is shared by nearly all objects in the Self world. This basic behavior is defined in the defaultBehavior slot of the lobby and includes:

  • identity comparisons (== and !==)

  • inequality (!=)

  • default behavior for printing (reimplement printString in descendants)

  • mirror creation (reflect:)

  • support for point, and list construction (@ and &)

  • behavior that allows blocks to ignore extra arguments

  • behavior that allows an object to behave like a block that evaluates to that object (this permits a non-block object to be passed to a method that expects a block)

  • behavior that allows an object to be its own key in a collection (key)

  • default behavior for doubly-dispatched messages

  • behavior for printing error messages and stack dumps (error: and halt)

It is important to note that not all objects in the system inherit this default behavior. It is entirely permissible to construct objects that do not inherit from the lobby, and the Self world contains quite a few such objects. For example, the objects used to break a namespace into separate categories typically do not inherit from the lobby. Any program intended to operate on arbitrary objects, such as a debugger, must therefore assume that the objects it manipulates do not understand even the messages in defaultBehavior.

Modules: defaultBehavior, errorHandling

4.2.2. The Root Traits: Traits Clonable and Traits Oddball

Most concrete objects in the Self world are descendants of one of two top-level traits objects: traits clonable and traits oddball. The distinction between the two is based on whether or not the object is unique. For example, true is a unique object. There is only one true object in the entire system, although there are many references to it. On the other hand, a list object is not unique. There may be many lists in the system, each containing different elements. A unique object responds to the message copy by returning itself and uses identity to test for equality. The general rule is:

  • unique objects usually inherit from traits oddball

  • non-unique objects usually inherit from traits clonable

Module: rootTraits

4.2.3. Mixins

Like traits objects, mixin objects encapsulate a bundle of shared behavior. Unlike traits objects, however, mixin objects are generally parentless to allow their behavior to be added to an object without necessarily also adding unwanted behavior (such as access to the lobby namespace). Mixins are generally used in objects that also have other parents. An example is mixins identity.

4.2.4. The Identity Mixin

Two objects are usually tested for equality based on whether they have “the same value” within a common domain. For example, 3.0 = 3 within the domain of numbers, even though they are not the same object or even the same kind of object. In some domains, however, two objects are equal if and only if they are the exact same object. For example, even two process objects with the same state are not considered equal unless they are identical. In such cases, identity comparison is used to implement equality tests, and mixins identity can be mixed in to get the desired behavior.

Module: rootTraits