第7章高水準プログラミング言語

By the end of this section, you will be able to:

• Discuss characteristics of functional programming

• Explain characteristics of declarative programming

• Distinguish the characteristics of object-oriented programming

• Explain HLL constructs used to support concurrency and parallelism

• Summarize when to use scripting languages

As we have learned, HLLs can be classified into various paradigms, two of which are imperative and declarative. There are other paradigms that fall within these major categories. Some worth investigating are functional programming, declarative programming, object-oriented programming, and parallel programming utilizing concurrency.

In 1930, Alonzo Church developed the lambda calculus model of computing, which got its name from the Greek letter lambda (λ). In this model, each parameter was introduced by the lambda symbol and computation was performed by placing parameters into expressions in the same way that high-level programs transfer arguments to functions. Figure 7.20 highlights various HLLs.

The functional programming model’s key concept is that programs are constructed by composing functions and applying them. A pure function is one that will return the same result every time given the same arguments, and there is no reading of shared mutable state. In this context, shared data means that the data is available to multiple program locations or scopes, state represents data that are remembered over time, and mutable means it is changeable. If the data used by the function is shared mutable, it is possible to return different results for the same arguments. A pure function may not have any side effects, meaning it cannot modify any state that is not local to it. For this reason, functional programs do not have assignment statements, which is also known as referential transparency.

Some other necessary features and characteristics that may be missing in some imperative languages include first-class functions and high-order functions. A first-class function is one that can be assigned as a value to a variable. A high-order function is one that can take one or more first-class functions as arguments. It can also return functions as results, which gives a function the ability to apply functions to its arguments one at a time. This is very useful in making code both more flexible and simpler. It encourages the creation of smaller functions that take care of just one piece of a larger job, which makes for great maintainability as well.

In the same way that you perform tasks over and over each day, functional programming treats functions used to execute these tasks as prime citizens. Some benefits of repetition include making programs easier to understand for the programmer as well as shortening programs. Some downsides of functional programming are having to move data through the functions, effective implementation is difficult (but not impossible), and new arrays must be created when data in an element are changed.

A number of imperative languages are considered object oriented, and most modern HLLs support this paradigm to some degree. Its basic characteristic is to design software around the concept of a class, which is the blueprint from which objects are constructed. Constructing an object is known as instantiation or building an instance of a class. Objects are made up of attributes (characteristics) and behaviors (methods or functions).

The three main principles of OOP are encapsulation, inheritance, and polymorphism, which provide a very high capability of abstraction. With encapsulation, the attributes and behaviors of classes and objects are self-contained and go along with them throughout their lifetime. It also means that internal implementation is hidden from outside the class or object. In inheritance, objects take on specified attributes from their ancestors. An example is that a Dog object inherits from an Animal parent object. Polymorphism means that behaviors that are inherited may perform in different ways that depend upon their context. A Dog is an Animal object and makes noise. A Snake is also an Animal object that makes noise—a different noise.

The degree to which an HLL conforms to these principles measures how object oriented it is. Java and C# are fully object oriented. C/C++ is a hybrid of procedural characteristics and OOP characteristics. Languages such as Python, JavaScript, and PHP partially ascribe to these characteristics. Therefore, they are sometimes referred to as object based.