CSC/ECE 517 Fall 2009/wiki2 13 StaticDynamic

Design Patterns from a Static/Dynamic Point of View

Clearly, Ruby offers more concise realizations of certain design patterns than Java does. Is this typical of dynamic o-o languages? Are there design patterns that any dynamically typed language can realize better than a statically typed language? Are there instances when a different pattern should be used in a dynamic language than a static language?

Introduction

Design Pattern

A design pattern is a general reusable solution to a commonly occurring problem in software design. A design pattern is not a finished design that can be transformed directly into code. It is a description or template for how to solve a problem that can be used in many different situations. Object-oriented design patterns typically show relationships and interactions between classes or objects, without specifying the final application classes or objects that are involved.

[1]

Name	Description
Creational Patterns
Abstract Factory	Creates an instance of several families of classes
Builder	Separates object construction from its representation
Factory Method	Creates an instance of several derived classes
Prototype	A fully initialized instance to be copied or cloned
Singleton	A class of which only a single instance can exist
Structural Patterns
Adapter	Match interfaces of different classes
Bridge	Separates an object’s interface from its implementation
Composite	A tree structure of simple and composite objects
Decorator	Add responsibilities to objects dynamically
Facade	A single class that represents an entire subsystem
Flyweight	A fine-grained instance used for efficient sharing
Proxy	An object representing another object
Behavioral Patterns
Chain of Resp.	A way of passing a request between a chain of objects
Command	Encapsulate a command request as an object
Interpreter	A way to include language elements in a program
Iterator	Sequentially access the elements of a collection
Mediator	Defines simplified communication between classes
Memento	Capture and restore an object's internal state
Observer	A way of notifying change to a number of classes
State	Alter an object's behavior when its state changes
Strategy	Encapsulates an algorithm inside a class
Template Method	Defer the exact steps of an algorithm to a subclass
Visitor	Defines a new operation to a class without change

Static/Dynamic Type Languages

Types usually have associations either with values in memory or with objects such as variables. Because any value simply consists of a sequence of bits in a computer, hardware makes no distinction even between memory addresses, instruction code, characters, integers and floating-point numbers. Assignment to a type informs programs and programmers how those bit collections should be treated.

Major functions provided by type systems include:

• Safety - Use of types may allow a compiler to detect meaningless or probably invalid code. For example, we can identify an expression "Hello, World" as invalid because the rules of arithmetic do not specify how to divide an integer by a string. As discussed below, strong typing offers more safety, but generally does not guarantee complete safety (see type-safety for more information).
• Optimization - Static type-checking may provide useful compile-time information. For example, if a type requires that a value must align in memory at a multiple of 4 bytes, the compiler may be able to use more efficient machine instructions.
• Documentation - In more expressive type systems, types can serve as a form of documentation, since they can illustrate the intent of the programmer. For instance, timestamps may be represented as integers—but if a programmer declares a function as returning a timestamp type rather than merely an integer type, this documents part of the meaning of the function.
• Abstraction (or modularity) - Types allow programmers to think about programs at a higher level than the bit or byte, not bothering with low-level implementation. For example, programmers can think of a string as a collection of character values instead of as a mere array of bytes. Or, types can allow programmers to express the interface between two subsystems. This helps localize the definitions required for interoperability of the subsystems and prevents inconsistencies when those subsystems communicate.

Static Type Languages

A programming language is said to use static typing when type checking is performed during compile-time as opposed to run-time.

C++, C#, Java

Dynamic Type Languages

A programming language is said to be dynamically typed, or just 'dynamic', when the majority of its type checking is performed at run-time as opposed to at compile-time.

Ruby, Python

Example

Many software designers view patterns as a form of language-independent design. Pattern-Oriented Software Architectures: A System of Patterns, edited by Frank Buschmann (John Wiley & Sons, 1996), for instance, divides patterns into three main groups -- architectural patterns, design patterns, and idioms. Only the idioms (defined as "low-level patterns specific to a program language") are language dependent -- the other patterns (and the implicit pattern language) rise above the level of programming language, much as the unified modeling language (UML) provides a common way to express designs.

This idea of language independence is seductive. It is also misleading. While patterns are language independent, language choice limits the patterns that are possible, easily supported, and useful. Language also affects how applications are structured.

To illustrate, I'll examine some patterns often used with Objective-C, but not usually used (or used differently) with C++. Both languages are object-oriented extensions of C. The main difference between them is that C++ has compile-time binding and fairly strong typing while Objective-C uses the Smalltalk object model of dynamic binding and weak typing. I'll start by examining two idioms commonly found in Objective-C programming. While simple, these idioms illustrate some of the key Objective-C programming techniques (although I use NextStep classes and frameworks to illustrate points, what I say also holds true for more generic Objective-C environments). I'll also examine how three standard design patterns -- Visitor, Command, and Facade -- are implemented in Objective-C.

The Facade Pattern

You might have noticed a common thread in the patterns just discussed. Both of the idioms involve ways to break out pieces of code and put them in shared libraries or bundles. The design patterns show how to decouple objects from each other for maximal flexibility. I'll now examine a commonly used architectural pattern that describes a way to structure applications to take advantage of this flexibility. In the good old days of structured programming, saving an application's state was simple. Programs were divided into data and functions, and saving consisted of calling the function that wrote the data to some storage area. In object-oriented applications, things become a little more difficult. To the extent that you practice information hiding, object serialization is the natural approach to take. Unfortunately, if m is the number of object references and n is the number of objects, serialization is O(mlog(n)). This is far too slow for serialization to be the saving mechanism in many applications.

But an Objective-C program that uses the four previous patterns will almost certainly use lots of facades as well. Notice here that Facades look a lot like cut points in the object graph. This leads to the natural Objective-C solution to the speed problem for object serialization: Serialize each facade to a separate Serializer (make each facade responsible for serializing the subsystem it abstracts). This can get complicated. If an object outside a subsystem bypasses a facade (and messages an object in a subsystem directly), then extra care must be taken during serialization (to avoid serializing objects to more than one location). And, deserializing (opening) becomes trickier as well -- objects that bypass a facade will need to find objects within the subsystem. In practice, this comes down to making the facades used in serialization Singletons and making certain that all connections to objects in the subsystem are mediated by the facade (so that, during deserialization, the connection can be restored).