Everything about Prototype-based Languages totally explained
Prototype-based programming is a style of
object-oriented programming in which
classes are not present, and behavior reuse (known as
inheritance in class-based languages) is performed via a process of
cloning existing
objects that serve as
prototypes. This model can also be known as
class-less,
prototype-oriented or
instance-based programming.
The original (and most canonical) example of a prototype-based language is the programming language
Self developed by
David Ungar and
Randall Smith. However, the classless programming style has recently grown increasingly popular, and has been adopted for the
programming languages
JavaScript,
Squeak (when using the Viewer framework to manipulate
Morphic components),
Cecil,
NewtonScript,
Io,
MOO,
REBOL and several others.
Comparison with class-based models
With class-based languages,
objects come in two general types.
Classes define the basic layout and functionality of objects, and
instances are "usable" objects based on the patterns of a particular class. In this model,
classes act as collections of behavior (
methods) and structure that are the same for all instances, whereas
instances carry the objects' data. The role distinction is thus primarily based on a distinction between structure and behavior on the one hand, and
state on the other.
Advocates of prototype-based programming often argue that class-based languages encourage a model of development that focuses first on the taxonomy and relationships between classes. In contrast, prototype-based programming is seen as encouraging the programmer to focus on the behavior of some set of examples and only later worry about classifying these objects into archetypal objects that are later used in a fashion similar to classes. As such, many prototype-based systems encourage the alteration of prototypes during
runtime, whereas only very few class-based object-oriented systems (such as the first dynamic object-oriented system,
Smalltalk) allow classes to be altered during the execution of a program.
While the vast majority of prototype-based systems are based around interpreted and
dynamically typed programming languages, it's important to point out that
statically typed systems based around prototypes are technically feasible. The
Omega programming language discussed in
Prototype-Based Programming is an example of such a system, though according to Omega's website even Omega isn't exclusively static but rather its "compiler may choose to use static binding where this is possible and may improve the efficiency of a program."
Object construction
In class-based languages a new instance is constructed through the class's
constructor and an optional set of constructor
arguments. The resulting instance is modeled on the layout and behavior dictated by the chosen class.
In prototype-based systems there are two methods of constructing new objects, through
cloning of an existing object, and through
ex nihilo ("from nothing") object creation. While most systems support a variety of cloning,
ex nihilo object creation isn't as prominent.
Systems that support
ex nihilo object creation allow new objects to be created from scratch without cloning from an existing prototype. Such systems provide a special syntax for specifying the properties and behaviors of new objects without referencing existing objects. In many prototype languages, there's often a basic
Object prototype that carries commonly needed methods and is used as a master prototype for all other objects. One useful aspect of
ex nihilo object creation is to ensure that a new object's slot names don't have
namespace collisions with the top-level
Object object. (In the Mozilla
JavaScript implementation, one can accomplish this by setting a newly constructed object's
property to null.)
Cloning refers to a process whereby a new object is constructed by copying the behavior of an existing object (its prototype). The new object then carries all the qualities of the original. From this point on, the new object can be modified. In some systems the resulting child object maintains an explicit link (via delegation or resemblance) to its prototype, and changes in the prototype cause corresponding changes to be apparent in its clone. Other systems, such as the Forth-like programming language Kevo, don't propagate change from the prototype in this fashion, and instead follow a more concatenative model where changes in cloned objects don't automatically propagate across descendants.
Delegation
In prototype-based languages that use delegation, the language runtime is capable of dispatching the correct method or finding the right piece of data simply by following a series of delegation pointers (from object to its prototype) until a match is found. All that's required to establish this behavior-sharing between objects is the delegation pointer. Unlike the relationship between class and instance in class-based object-oriented languages, the relationship between the prototype and its offshoots doesn't require that the child object have a memory or structural similarity to the prototype beyond this link. As such, the child object can continue to be modified and amended over time without rearranging the structure of its associated prototype as in class-based systems. It is also important to note that not only data but also methods can be added or changed. For this reason, most prototype-based languages refer to both data and methods as "slots".
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.
Advantages to this approach include the fact that object authors can alter the copy without worrying about side-effects across other children of the parent. A further advantage is that the computational cost of method lookup during dispatch is drastically reduced when compared to 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 isn't 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's 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" 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.
Criticism
Advocates of class-based object models who criticize prototype-based systems often have concerns that could be seen as similar to those concerns that proponents of static type systems for programming languages have of dynamic type systems (see Datatype). Usually, such concerns involve: correctness, safety, predictability, and efficiency.
On the first three points, classes are often seen as analogous to types (in most statically typed object-oriented languages they serve that role) and are proposed to provide contractual guarantees to their instances, and to users of their instances, that that'll behave in some given fashion.
On the last point, efficiency, the declaration of classes simplifies many compiler optimizations that allow developing efficient method and instance variable lookup. For the Self language, much development time was spent on developing, compiling, and interpreting techniques to improve the performance of prototype-based systems versus class-based systems. For example, the Lisaac compiler produces code almost as fast as C. Tests have been run with an MPEG-2 codec written in Lisaac, copied from a C version. These tests show the Lisaac version is 1.9% slower than the C version with 37% fewer lines of code. However, the C programming language isn't a class-based object-oriented language, but rather a procedural language. A Lisaac to C++ comparison would be more appropriate.
The most common criticism made against prototype-based languages is that the community of software developers isn't familiar with them, despite the popularity and market permeation of JavaScript. This knowledge level of prototype based systems seems to be changing with the proliferation of JavaScript frameworks and increases in the complex use of JavaScript as "Web 2.0" matures.
Possibly because of these reasons, work on a fourth edition of the ECMAScript standard seeks to provide JavaScript with class-based language constructs. It is worth noting that according to this specification, the prototype for an object still exists when traditional class based syntax is used.
Languages
Further Information
Get more info on 'Prototype-based Languages'.
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