Introduction to Programming Languages/Concepts of Functional Languages

Concepts of Functional Languages
Functional programming is a form of declarative programming, a paradigm under which the computation of a program is described by its essential logic. This approach is in contrast to imperative programming, where specific instructions describe how a computation is to be performed. The only form of computation in a strictly functional language is a function. As in mathematics, a function establishes, by means of an equation, a relation between certain input and output. To the functional programmer, the exact implementation underneath a given computation is not visible. In this sense, we tend to say that functional programming allows one to focus on what is to be computed.

While functional programming languages have not traditionally joined the success of imperative languages in the industry, they have gained more traction in the recent years. This increasing popularity of the functional programming style is due to numerous factors. Among the virtues of typical functional languages like Haskell is the absence of mutable data and associated side-effects, a characteristic referred to as purity, which we shall study further. Together with the introduction of novel parallel architectures, we have seen an accompanying growth of concurrent programming techniques. Because functional languages are agnostic of global state, they provide a natural framework for implementations free of race conditions. In addition, they ease the design fault-tolerant functions, since only local data is of concern.

Another reason that has contributed to the emergence of the functional style is the appearance of the so-called hybrid languages. A prominent example among those is the Scala programming language. Scala offers a variety of features from the functional programming world like high-order functions and pattern matching, yet with a closely resembling object-oriented appearance. Even imperative constructs such as for loops can be found in Scala, a characteristic that helps reducing the barrier between functional and imperative programming. In addition, Scala compiles to bytecode of the Java Virtual Machine (JVM), enabling easy integration between the two languages and, consequently, allowing programmers to gradually move from one to the other. The same approach of compiling to the JVM was followed by Closure, a relatively new general-purpose language. Finally, both Java and C++, which were originally designed under strong imperative principles, feature nowadays lambdas and other functional programming constructs.