List Module

In addition to the basic list operations that we covered in the previous chapter, F# also provides a List module with many useful functions for working with lists. This module provides a variety of functions for manipulating lists and performing common operations.

Common Functions

There are many other functions in the List module that are beyond the scope of this chapter, but some notable ones include:

append

Concatenates two lists together

let list1 = [1; 2; 3]
let list2 = [4; 5; 6]

let concatenated = List.append list1 list2 // => val concatenated: int list = [1; 2; 3; 4; 5; 6]

length

Returns the length of a list

let myList = [1; 2; 3; 4; 5]

let length = List.length myList // => val length: int = 5

exists

Returns true if at least one element in a list satisfies a given predicate

let myList = [1; 3; 5; 7; 8; 9]

let hasEven = List.exists (fun x -> x % 2 = 0) myList // => val hasEven: bool = true

forAll

Returns true if all elements in a list satisfy a given predicate

let myList = [2; 4; 6; 8]

let allEven = List.forall (fun x -> x % 2 = 0) myList // => val allEven: bool = true

let myList2 = [2; 4; 6; 7; 8]

let allEven2 = List.forAll (fun x -> x % 2 = 0) myList2 // => val allEven2: bool = false

sortBy

Sorts a list by a given comparison function

let myList = ["apple"; "banana"; "cherry"; "date"; "elderberry"]

let sortedList = List.sortBy (fun (x : string) -> x.Length) myList 
// => val sortedList: string list = ["date"; "apple"; "banana"; "cherry"; "elderberry"]

take

Returns the first n elements of a list

let myList = [1; 2; 3; 4; 5; 6; 7]

let firstThree = List.take 3 myList // => val firstThree: int list = [1; 2; 3]

skip

Skips the first n elements of a list

let myList = [1; 2; 3; 4; 5; 6; 7]

let afterThree = List.skip 3 myList // => val afterThree: int list = [4; 5; 6; 7]