How is encapsulation handled in functional programming languages?

Encapsulation in functional programming is achieved through the use of pure functions and immutable data.

In object-oriented programming, encapsulation is a principle that binds together the data and the functions that manipulate the data, and that keeps both safe from outside interference and misuse. The idea is to bundle the data (attributes) and the methods (functions) that operate on the data into a single unit, known as a class. This way, the data is hidden from the outside world and can only be accessed via the methods of the class, ensuring that it cannot be changed in unexpected ways.

However, functional programming takes a different approach. Instead of bundling data and functions together, functional programming aims to separate them. In functional programming, data and functions are distinct and each function operates on the input data and produces output data. The key here is that the data is immutable, meaning it cannot be changed once it's created. This is a form of encapsulation because it ensures that data cannot be modified in unexpected ways.

The functions in functional programming are also 'pure', meaning they do not have any side effects. A side effect is any change that a function makes to the state of the program outside of producing its output, such as modifying a global variable or changing the value of its input arguments. By avoiding side effects, pure functions provide another layer of encapsulation. They ensure that the function's behaviour is entirely determined by its input values and not by the state of the rest of the program.

In functional programming, encapsulation is also achieved through the use of higher-order functions and closures. A higher-order function is a function that takes other functions as arguments or returns a function as its result. A closure is a function that 'encloses' or 'captures' its surrounding state, meaning it has access to variables from its outer scope. This allows data to be 'hidden' inside the closure, providing a form of encapsulation.

In summary, while functional programming does not use the same mechanisms for encapsulation as object-oriented programming, it achieves a similar goal of protecting data and behaviour from unwanted interference. It does this through the use of pure functions, immutable data, higher-order functions and closures.