By John Paul Mueller

Functional programming has somewhat different goals and approaches than other paradigms use. Goals define what the functional programming paradigm is trying to do in forging the approaches used by languages that support it. However, the goals don’t specify a particular implementation; doing that is within the purview of the individual languages.

The main difference between the functional programming paradigm and other paradigms is that functional programs use math functions rather than statements to express ideas. This difference means that rather than write a precise set of steps to solve a problem, you use math functions, and you don’t worry about how the language performs the task. In some respects, this makes languages that support the functional programming paradigm similar to applications such as MATLAB.

functional programming

Of course, with MATLAB, you get a user interface, which reduces the learning curve. However, you pay for the convenience of the user interface with a loss of power and flexibility, which functional languages do offer. Using this approach to defining a problem relies on the declarative programming style, which you see used with other paradigms and languages, such as Structured Query Language (SQL) for database management.

In contrast to other paradigms, the functional programming paradigm doesn’t maintain state. The use of state enables you to track values between function calls. Other paradigms use state to produce variant results based on environment, such as determining the number of existing objects and doing something different when the number of objects is zero. As a result, calling a functional program function always produces the same result given a particular set of inputs, thereby making functional programs more predictable than those that support state.

Because functional programs don’t maintain state, the data they work with is also immutable, which means that you can’t change it. To change a variable’s value, you must create a new variable. Again, this makes functional programs more predictable than other approaches and could make functional programs easier to run on multiple processors. Keep reading for additional information on how the functional programming paradigm differs.

Understanding the goals of functional programming

Imperative programming, the kind of programming that most developers have done until now, is akin to an assembly line, where data moves through a series of steps in a specific order to produce a particular result. The process is fixed and rigid, and the person implementing the process must build a new assembly line every time an application requires a new result. Object-oriented programming (OOP) simply modularizes and hides the steps, but the underlying paradigm is the same. Even with modularization, OOP often doesn’t allow rearrangement of the object code in unanticipated ways because of the underlying interdependencies of the code.

Functional programming gets rid of the interdependencies by replacing procedures with pure functions, which requires the use of immutable state. Consequently, the assembly line no longer exists; an application can manipulate data using the same methodologies used in pure math. The seeming restriction of immutable state provides the means to allow anyone who understands the math of a situation to also create an application to perform the math.

Using pure functions creates a flexible environment in which code order depends on the underlying math. That math models a real-world environment, and as our understanding of that environment changes and evolves, the math model and functional code can change with it — without the usual problems of brittleness that cause imperative code to fail. Modifying functional code is faster and less error prone because the person implementing the change must understand only the math and doesn’t need to know how the underlying code works. In addition, learning how to create functional code can be faster as long as the person understands the math model and its relationship to the real world.

Functional programming also embraces a number of unique coding approaches, such as the capability to pass a function to another function as input. This capability enables you to change application behavior in a predictable manner that isn’t possible using other programming paradigms.

Using the pure approach to functional programming

Programming languages that use the pure approach to the functional programming paradigm rely on lambda calculus principles, for the most part. In addition, a pure-approach language allows the use of functional programming techniques only, so that the result is always a functional program. The pure-approach language is Haskell because it provides the purest implementation, according to articles about functional programming. Haskell is also a relatively popular language, according to the TIOBE index. Other pure-approach languages include Lisp, Racket, Erlang, and OCaml.

As with many elements of programming, opinions run strongly regarding whether a particular programming language qualifies for pure status. For example, many people would consider JavaScript a pure language, even though it’s untyped. Others feel that domain-specific declarative languages such as SQL and Lex/Yacc qualify for pure status even though they aren’t general programming languages. Simply having functional programming elements doesn’t qualify a language as adhering to the pure approach.

Using the impure approach to functional programming

Many developers have come to see the benefits of functional programming. However, they also don’t want to give up the benefits of their existing language, so they use a language that mixes functional features with one of the other programming paradigms. For example, you can find functional programming features in languages such as C++, C#, and Java. When working with an impure language, you need to exercise care because your code won’t work in a purely functional manner, and the features that you might think will work in one way actually work in another. For example, you can’t pass a function to another function in some languages.

At least one language, Python, is designed from the outset to support multiple programming paradigms. In fact, some online courses in programming make a point of teaching this particular aspect of Python as a special benefit. The use of multiple programming paradigms makes Python quite flexible but also leads to complaints and apologists. Python is great for to demonstrating the impure approach to functional programming because it’s both popular and flexible, plus it’s easy to learn.