Welcome back! You've previously learned about higher-order functions and immutability. These concepts are at the heart of functional programming. Now, let's dive into a practical application of these ideas by working with lists using higher-order functions. Lists are fundamental in Elixir and functional programming, allowing you to store and manipulate collections of data efficiently.

In this unit, you will learn how to work with lists using higher-order functions in Elixir. Higher-order functions can take other functions as arguments, return them as results, or both. This powerful feature lets you easily process lists in a concise and expressive way.

Consider the following example where we aim to filter a list, keeping only the even numbers:

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1list = [1, 2, 3, 4]
2
3filtered = Enum.filter(list, fn x -> rem(x, 2) == 0 end)
4IO.inspect(filtered)

In this code, Enum.filter is a higher-order function that takes a list and a function as arguments. The function (a lambda, in this case) specifies the condition each element must meet to be included in the resulting list. Here, rem(x, 2) == 0 checks if a number is even.

Now, let's look at another example using Enum.reduce to calculate the product of all elements in the list:

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1product = Enum.reduce(list, 1, fn x, acc -> x * acc end)
2IO.puts(product)

In this example, Enum.reduce is used to accumulate a result by applying the function to each element of the list and an accumulator, which starts at 1. The function multiplies each element by the accumulator to compute the product of all numbers in the list. Let's also pay attention to the arguments passed to Enum.reduce:

• The list to process (list).
• The initial value of the accumulator, 1 in this case to properly compute the product.
• The function that takes two arguments: the current element (x) and the accumulator (acc). It updates the accumulator by multiplying it with the current element. Thus the final value of the accumulator will be the product of all elements in the list.

Mastering list processing with higher-order functions is crucial for several reasons:

1. Efficiency: By using higher-order functions, you can perform complex operations on lists with minimal code. This makes your code more readable and maintainable.

2. Expressiveness: Higher-order functions allow you to express your intentions more clearly. Instead of writing loops, you can use functions like filter and reduce to convey what you want to achieve directly.

3. Reusability: Functions that process lists can often be reused across different parts of your program. By learning how to create and use these functions, you can write more modular and flexible code.

By understanding how to work with lists using higher-order functions, including filter and reduce, you'll enhance your ability to solve complex problems efficiently. Let's get started with the practice section and see these concepts in action!