Copy-downs
Used to propagate structure from one prototype to another:
Copied slots are shown in pink.
Need to specify:
- copy-down parent
- selector to use to make copy
- slots to omit (optional)
Programming environment propagates changes to copied-down descendants
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Copy-downs
Because Self eschews classes, it is necessary to copy down slots when
refining an object. For example, the prototypical dictionary object is derived from
the prototypical set, and contains many slots that were prescribed by the
implementation of set. Some mechanism is needed to ensure that the relationship
between the two objects is maintained, should set be changed.
In a class-based language, this need is met by a rule ensuring that subclasses
include any instance variables defined in their superclasses. In Self, this
inheritance of structure is separated from the inheritance of information
performed by the normal hierarchy of parent slots.
Instead of including in the language a facility for inheriting structure, Self
4.0 implements a facility in the environment, called "copy-down." An object's
annotation can contain a copy-down parent, copy-down selector, and set of slots
to omit. The copy-down parent is sent the message given by the copy-down
selector, and (except for the slots-to-omit), the slots in the result are added to the
object. Copied-down slots are shown in pink.
In creating dictionary from set, the slots parent and prototype were omitted because
their contents had to be different; copied-down slots are copied, they cannot be
specially initialized in Self 4.0. It is also possible to omit a slot because it is
simply not needed; most other object-oriented programming systems would not
allow a subclass to avoid inheriting an instance variable.
The Self 4.0 programming environment uses the copy-down information
to allow the programmer to use a classical style when appropriate. For example, if
the programmer adds a slot to set, the environment will offer to add it to dictionary,
too.
The least convenient aspect of using copy-downs is that to do the moral equivalent
of creating a subclass, the programmer has to create two objects: a new traits object,
and a new prototype, and then specify that the new prototype is to be copied
down.
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