Higher-order functions are functions that take one or more functions as input parameters, or return a function as output. Scala is a functional programming language that supports higher-order functions, and provides a number of built-in higher-order functions that make it easy to write concise and expressive code.
One of the most commonly used higher-order functions in Scala is `map()`, which is used to transform the elements of a collection. The `map()` function takes a function as input that defines the transformation to be applied to each element of the collection, and returns a new collection with the transformed elements. For example:
scala val numbers = List(1, 2, 3, 4, 5) val doubledNumbers = numbers.map(x => x * 2)
In this example, the `map()` function takes a lambda function that multiplies each element of the `numbers` list by 2, and returns a new list with the transformed elements.
Another common higher-order function is `filter()`, which is used to select elements from a collection that satisfy a certain condition. The `filter()` function takes a function as input that defines the condition to be checked for each element of the collection, and returns a new collection with only the elements that satisfy the condition. For example:
scala val numbers = List(1, 2, 3, 4, 5) val evenNumbers = numbers.filter(x => x % 2 == 0)
In this example, the `filter()` function takes a lambda function that checks whether each element of the `numbers` list is even (i.e., whether its remainder when divided by 2 is 0), and returns a new list with only the even elements.
Scala also provides a number of other higher-order functions, such as `flatMap()`, `foldLeft()`, and `reduce()`, that can be used to perform more complex transformations and aggregations on collections. Additionally, Scala supports the creation of custom higher-order functions using lambda functions and closures, which can be used to implement domain-specific abstractions and simplify code.