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# Pure Function

Delving into the fascinating realm of functional programming, a critical concept to grasp is that of the pure function. Intricately woven into the fabric of computer science, pure function forms the bedrock of understanding and implementing functional programming. This comprehensive guide sheds light on what exactly constitutes a pure function, demystifying it and presenting it in a tangible, accessible manner. Moreover, it contrasts pure and impure functions, enhancing your grasp of their usage and purpose, backed by relatable, real-world examples. The journey doesn't stop here; further exploration unfolds the multitude of benefits offered by pure functions, from simplifying debugging to easing testing, largely enriching your operational efficiency. Lastly, the deep dive into functional programming pure functions and dispelling common misconceptions will augment your knowledge, firmly anchoring your understanding of this crucial computer science concept.

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## Understanding Pure Function in Functional Programming

In the world of functional programming, the concept of a pure function is critical. It underpins key principles and is a stepping-stone to mastering this paradigm. A pure function is one that delivers the same result every single time given the same set of inputs. Importantly, these functions do not produce side effects, meaning they do not alter any state outside the function or rely on data that changes.

A Pure Function is a type of function where:

• The return value is based only on the passed arguments and not on any other outside state.
• The function does not modify its arguments or any global variables (i.e., there are no side effects).
• For the same input, the function will always produce the same output.

In functional programming, side effects are frowned upon. They entail changes in state that go beyond the function's scope and that can be hard to track, leading to buggy, hard-to-maintain code. Functions without side effects, like pure functions, make coding less error-prone and easier to debug, read, and understand.

## What Defines a Pure Function

When trying to grasp the concept of pure functions, it's useful to break it down into its defining characteristics. Remember, a pure function is one that gives the same output for the same input and never alters external states or produces side effects. Now, let's take a close look at these features:

Consider a simple function $$\text{{sum}}(a, b)$$ where $$\text{{a}}$$ and $$\text{{b}}$$ are numbers. The return value of this function is the sum of $$\text{{a}}$$ and $$\text{{b}}$$. This function is pure because:

1. The output of this function (the sum of $$\text{{a}}$$ and $$\text{{b}}$$) solely depends on the input values. It doesn't depend on or change any other state.
2. The function doesn't produce any side effects. It doesn't alter any state outside the function.
3. Every time the function is called with the same numbers $$\text{{a}}$$ and $$\text{{b}}$$, it will always produce the same sum.

### Pure Function vs Impure Functions

Now that we understand what a pure function is, let's distinguish it from an impure function. Impure functions are the opposite of pure functions, as they could rely on external state, produce side effects, and may yield different results even with identical input. The distinction between the two is crucial in functional programming.

Pure FunctionsImpure Functions
Output is only determined by inputOutput could be influenced by outside state
No side effectsPotentially produces side effects
Identical input leads to identical outputIdentical input may lead to different output

## Real World Pure Function Examples

Now we will explore real-world examples of pure functions. This will help solidify your understanding and give you a practical context. Besides their theoretical advantages, pure functions are frequently used in modern libraries and frameworks because they help to avoid many common bugs in programming.

An everyday example of a pure function used in front-end development is the map method in JavaScript:


const arr = [1, 2, 3, 4, 5];
const arrTimesTwo = arr.map(x => x * 2);

The map method here produces a new array where each element is multiplied by two. It doesn't affect or change the original array, and passing the same array with the function will always result in the same new array. This makes it a pure function.

## Advantages of Using Pure Functions in Computer Science

Pure functions come with a myriad of benefits that can enhance your computer programming endeavours. They make programs easier to reason about, streamline testing and debugging, facilitate parallel computing, and foster reusable, clean code. These advantages make them an asset to any coder, particularly those working within a functional programming paradigm.

### Top Benefits of Pure Functions

Being exposed to the top benefits of using pure functions will persuade you of their value in computer science. Below lies a comprehensive examination of the profound impact that implementing pure functions can have on your programming.

Deterministic: For any given input, the output will always be the same. This feature simplifies understanding and predicting code behaviour.

No Side Effects: The function doesn't change anything in the program's state, it doesn't alter any variables outside of its scope, and it doesn't produce outputs other than its return value (such as print statements or GUI commands).

Enhanced Testability: The lack of dependencies on outside variables and deterministic nature makes testing a breeze. Just call the function with various inputs and check the outputs. There's no need to set up complex environments to ensure your function works correctly.

Parallel Execution Capability: As these functions don't interact with shared states, multiple instances can be executed simultaneously on different note cores without any fears of conflicts. They can also be run independently, making them highly suitable for distributed systems and concurrent programming.

#### It Simplifies Debugging: Key Benefit of Pure Functions

One of the significant advantages of pure functions is how they streamline the debugging process. As developers, you know that debugging is an integral part, albeit often frustrating, of coding. Pure functions can alleviate some of this frustration.

 Predictability: Pure functions always produce the same output for the same input. This deterministic behaviour makes it easier to track issues since you can reliably predict what will happen. No Side Effects: With no external state dependencies, you don't have to worry about the unforeseen changes that side effects can bring. A bug in a pure function does not impact the rest of the program, limiting its reach and making it easier to fix. Facilitating 'Divide and Conquer': The isolation of pure functions allows for a 'divide and conquer' approach to debugging. If you suspect a bug in a section of your program, you can check each pure function in that section individually. By validating each one, you can eliminate culprits and narrow down the source of the problem.

#### Easier Testing with Pure Functions

Another rewarding advantage of pure functions is how they simplify the testing process. Testing is an integral part of software development, ensuring that your program behaves as expected and making it robust against potential future changes.

Suppose you have pure function $$\text{{factorial}}(n)$$ where $$n$$ is an integer. The function returns the factorial of $$n$$, which is the product of all positive integers less than or equal to $$n$$. With pure testing, a simple test could look like this:


assert factorial(5) == 120
assert factorial(0) == 1

The tests are simple, straightforward, and efficient since pure functions don't have side effects or rely on external state.

You can swiftly run through multiple sets of inputs and verify whether or not they produce the expected outputs. Furthermore, the isolation of pure functions makes it possible to unit test them effectively, providing confidence in the correctness of the code and the reliability of components before they are integrated. Overall, by making testing more straightforward, pure functions can foster a stronger, more reliable application.

## Exploring Functional Programming: Pure Functions

Functional programming is a paradigm in computer science that treats computation as an evaluation of mathematical functions. It emphasises immutability and the avoidance of side effects, unlike imperative programming that depends on mutable objects and commands. Herein, the star players are pure functions.

### A Closer Look at Functional Programming Pure Functions

As previously discussed, pure functions in functional programming adhere to certain, stringent properties. However, beyond their definitions, their integration and use in functional programming deserve a more in-depth exploration.

In functional programming, developers heavily rely on pure functions and mathematical concepts to manipulate data. This technique eliminates the need for mutable data, creating advantages such as clear, predictable, and concise code.

Observing the feature characteristics, one can understand how pure functions can be an integral part of functional programming:

• Pure functions always give the same output for the same input, making them deterministic.
• Their dependency is only on input arguments, assuring no mutable state or data change.
• Having no side effects enables them to be free of context, meaning they don't rely on external elements.
• For functional programming, pure functions are first-class entities, meaning they can be used as data, stored in variables, or passed to and returned from other functions.

For a function $$\text{{squared}}(x)$$ that returns the square of a number $$x$$, in functional programming, this function is a pure function - but there's more to it:


const squared = x => x * x;

Here, squared is not just another function that can be called. It's also a first-class entity that can be used as an argument to another function, stored in data structures, and so on.

Functional programming emphasises code clarity and simplicity, making it an excellent choice for large, complex software projects. By using pure functions, developers can expect to write code that is easier to test, debug, concurrently execute, and reason about, leading to efficient and high-quality software.

### Common Misconceptions about Pure Functions

Pure functions, while straightforward in their definition, often come with a handful of misconceptions, especially for those new to the functional programming paradigm. Here, we'll debunk some of these misconceptions and provide clarity.

• Misconception 1: Pure functions can't use variables defined outside the function.

It's not about using outside variables but changing them. Pure functions can use global constants, such as the value of pi or the speed of light because they are not altered.

• Misconception 2: Pure functions aren't practical; they create a lot of data copying.

Instead of modifying data, pure functions often produce new data. However, many functional languages optimise this to avoid the overheads of making numerous copies.

• Misconception 3: Pure functions don't allow interactivity because they don't have side effects.

Although it's true pure functions don't have side effects, it doesn't mean your program can't interact with the user or change state. Functional programming languages provide mechanisms for managing side effects in a controlled manner.

Overall, while pure functions do alter the way we traditionally think about programming, they are a powerful concept that, when rightly understood, can serve as an indispensable tool for writing robust, maintainable, and predictable code.

## Pure Function - Key takeaways

• Pure function is a critical concept in functional programming that delivers the same result every time given the same set of inputs and does not produce any side effects. A pure function does not alter any state outside the function or rely on data that changes.

• Pure functions are defined by three main characteristics: the return value relies only on passed arguments; functions do not modify any global variables; and for the same input, it will always produce the same output.

• In contrast, impure functions could depend on external state, produce side effects, and may yield different results even with identical input. This distinction is critical in functional programming.

• Pure functions make coding less error-prone and easier to debug, read, and understand. They simplify debugging and testing, and enrich operational efficiency.

• Some common misconceptions about pure functions include: they can't use variables defined outside the function; they are impractical because they create a lot of data copying; and they don't allow interactivity because they don't have side effects.

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What are pure functions?

Pure functions are functions in programming that give the same output for the same set of inputs, regardless of how many times it's called. These functions do not maintain any state between executions or produce any side effects, which means they do not change or affect external data or variables. Pure functions are deterministic, as their output is solely determined by their input. They are commonly used in functional programming and make code easier to understand and test.

What are pure functions injavascript?

Pure functions in JavaScript are functions that always return the same result given the same arguments and do not produce side effects. They are independent of any external or hidden states and rely only on their input to produce output. Pure functions also do not alter or modify the variables used within the function. Essentially, they treat their values as immutable.

What is a pure function in python?

In Python, a pure function is a function that has no side effects and provides the same output for the same set of inputs. It doesn't alter any state or data outside its scope, and its execution relies solely on its input arguments. Pure functions are a key concept in functional programming and they facilitate easy testing and debugging due to their predictability.

What are the two elements of a pure function?

The two main elements of a pure function are determinism and no side effects. Determinism means the function always produces the same output given the same input. No side effects means a function does not alter outside variables, global variables, or anything outside of its scope, it only works on the input given.

What is a pure and impure function?

A pure function is one where the return value is solely determined by its input values, without any observable side effects or dependencies on external states. In contrast, an impure function's output may depend on external states or conditions, and it may produce side effects such as modifying a global variable or changing the original input.

## Test your knowledge with multiple choice flashcards

How do pure functions streamline the debugging process?

How does the concept of a pure function contribute to functional programming?

How can functional languages manage the misconception that pure functions create a lot of data copying?

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