How do higher-order functions facilitate code reusability?

Higher-order functions facilitate code reusability by allowing functions to be passed as arguments and returned as results.

In more detail, higher-order functions are a key feature of functional programming, a paradigm where functions are first-class citizens. This means that functions can be assigned to variables, stored in data structures, passed as arguments to other functions, and returned as values from other functions. This flexibility allows for a high degree of code reusability.

For instance, consider a scenario where you have a list of numbers and you want to perform various operations on this list, such as finding the square of each number, or filtering out numbers that are less than a certain value. Without higher-order functions, you would need to write separate loops for each operation. However, with higher-order functions, you can write a single function that takes another function as an argument and applies it to each element in the list. This way, you can reuse the same function for different operations by simply passing different functions as arguments.

Moreover, higher-order functions can also return functions as results. This allows for the creation of function factories, i.e., functions that generate other functions with specific behaviours. For example, you could have a function that takes a number as an argument and returns a new function that multiplies its argument by the given number. This way, you can generate as many multiplication functions as you need, each with a different multiplier, thus reusing the same function factory.

In addition, higher-order functions can be used to create control structures, such as loops or conditionals, which can be reused across different parts of a program. For example, you could write a higher-order function that takes a condition function and a body function as arguments, and repeatedly executes the body function as long as the condition function returns true. This way, you can reuse the same control structure for different conditions and bodies, thus reducing code duplication.

In conclusion, by allowing functions to be passed as arguments and returned as results, higher-order functions provide a powerful mechanism for code reusability, enabling more concise, flexible, and modular code.