Prototype-based Programming - Concatenation

Concatenation

Under pure prototyping, which is also referred to as concatenative prototypes, and is exemplified in the Kevo language, there are no visible pointers or links to the original prototype from which an object is cloned. The prototype object is copied exactly, but given a different name (or reference). Behavior and attributes are simply duplicated as-is.

One advantage of this approach is that object authors can alter the copy without worrying about side-effects across other children of the parent. Another advantage is that method lookup during dispatch is much cheaper computationally than with delegation, where an exhaustive search must be made of the entire delegation chain before failure to find a method or slot can be admitted.

Disadvantages to the concatenative approach include the organizational difficulty of propagating changes through the system; if a change occurs in a prototype, it is not immediately or automatically available on its clones. However, Kevo does provide additional primitives for publishing changes across sets of objects based on their similarity (so-called family resemblances) rather than through taxonomic origin, as is typical in the delegation model.

Another disadvantage is that, in the most naive implementations of this model, additional memory is wasted (versus the delegation model) on each clone for the parts that have stayed the same between prototype and clone. However, it is possible to provide concatenative behavior to the programming while sharing implementation and data behind-the-scenes; such an approach is indeed followed by Kevo.

An alternative quasi-solution to the problem of clones interfering with the behavior of the parent is to provide a means whereby the potential parent is flagged as being clonable or not. In MOO, this is achieved with the "f" ("fertile") flag. Only objects with the "f" flag can be cloned. In practice, this leads to certain objects serving as surrogate classes; their properties are kept constant to serve as initial values for their children. These children then tend to have the "f" flag not set.