java-design-patterns/function-composition/README.md

---
title: Function Composition
category: Functional
language: en
tag:
    - Code simplification
    - Composition
    - Decoupling
    - Functional decomposition
    - Lambda
    - Reusability
---

## Also known as

* Function Chaining
* Function Pipelining
* Functional Composition

## Intent

To enable creating complex functions by composing simpler ones, enhancing modularity and reusability of function-based logic.

## Explanation

Real-world example

> Imagine a fast-food restaurant where the process of making a burger is broken down into several steps: grilling the patty, toasting the bun, adding condiments, and assembling the burger. Each of these steps can be seen as a function.
>
> In the Functional Composition design pattern, these individual steps (functions) can be composed into a complete burger-making process. Each step remains simple and reusable. For instance, the grilling function could be reused for making sandwiches or other dishes that require a grilled patty. This modular approach allows the restaurant to efficiently create various menu items by reusing and combining simple, predefined steps.

In plain words

> The Function Composition pattern allows building complex functions by combining simpler ones, making it easier to manage, test, and reuse individual pieces of functionality.

Wikipedia says

> Function composition is an act or mechanism to combine simple functions to build more complicated ones. Like the usual composition of functions in mathematics, the result of each function is passed as the argument of the next, and the result of the last one is the result of the whole.

**Programmatic Example**

Let's start with defining two simple functions. In this case, we have a function `timesTwo` that multiplies its input by 2, and a function `square` that squares its input.

```java
Function<Integer, Integer> timesTwo = x -> x * 2;
Function<Integer, Integer> square = x -> x * x;
```

Next, we use the `FunctionComposer` class to compose these two functions into a new function. The `composeFunctions` method takes two functions as arguments and returns a new function that is the composition of the input functions.

```java
Function<Integer, Integer> composedFunction = FunctionComposer.composeFunctions(timesTwo, square);
```

Finally, we apply the composed function to an input value. In this case, we apply it to the number 3. The result is the square of the number 3 multiplied by 2, which is 36.

```java
public static void main(String[] args) {
    final var logger = LoggerFactory.getLogger(App.class);
    Function<Integer, Integer> timesTwo = x -> x * 2;
    Function<Integer, Integer> square = x -> x * x;

    Function<Integer, Integer> composedFunction = FunctionComposer.composeFunctions(timesTwo, square);

    int result = composedFunction.apply(3);
    logger.info("Result of composing 'timesTwo' and 'square' functions applied to 3 is: " + result);
}
```

This will output:

```
Result of composing 'timesTwo' and 'square' functions applied to 3 is: 36
```

This example demonstrates how the Function Composition pattern can be used to create complex functions by composing simpler ones, enhancing modularity and reusability of function-based logic.

## Sequence diagram

![Functional Composition Diagram](./etc/function.composition.urm.png "Functional Composition")

## Applicability

Use the Function Composer pattern when:

* You want to create a pipeline of operations where the output of one function is the input to another.
* You need to enhance the clarity and quality of your code by structuring complex function logic into simpler, reusable components.
* You are working in a functional programming environment or a language that supports higher-order functions.
* When you want to avoid deep nesting of function calls and instead build a pipeline of operations.
* When aiming to promote immutability and side-effect-free functions in your design.

## Tutorials

* [Function Composition in Java (Medium)](https://functionalprogramming.medium.com/function-composition-in-java-beaf39426f52)
* [Functional Programming in Java (Baeldung)](https://www.baeldung.com/java-functional-programming)

## Known uses

* Stream processing in Java 8 and above
* Query builders in ORM libraries
* Middleware composition in web frameworks

## Consequences

Benefits:

* High reusability of composed functions.
* Increased modularity, making complex functions easier to understand and maintain.
* Flexible and dynamic creation of function pipelines at runtime.
* Enhances readability by structuring code in a linear, declarative manner.
* Facilitates easier testing of individual functions.

Trade-offs:

* Potentially higher complexity when debugging composed functions. 
* Overhead from creating and managing multiple function objects in memory-intensive scenarios.
* May require a paradigm shift for developers unfamiliar with functional programming concepts.

## Related patterns

* [Chain of Responsibility](https://java-design-patterns.com/patterns/chain-of-responsibility/) - Both patterns allow processing to be broken down into a series of steps, but Functional Composition focuses on function composition rather than responsibility delegation.
* [Decorator](https://java-design-patterns.com/patterns/decorator/) - Similar in combining behaviors, but Decorator applies additional behavior to objects, while Functional Composition builds new functions.
* [Strategy](https://java-design-patterns.com/patterns/strategy/) - Provides interchangeable functions (strategies), which can be composed in Functional Composition.

## Credits

* [Effective Java](https://amzn.to/4cGk2Jz)
* [Functional Programming in Java](https://amzn.to/3JUIc5Q)
* [Java 8 in Action: Lambdas, Streams, and functional-style programming](https://amzn.to/3QCmGXs)