A list in F# is an ordered, immutable series of elements of the same type.
Microsoft.FSharp.Collections.List module that is imported automatically by F# and is accessible from every F# application.There are a variety of ways to create lists in F#. The most straightforward way is to define a list by explicitly listing out the elements, separated by semicolons and enclosed in square brackets.
let evenNumbers = [ 0; 2; 4; 6; 8; 10; ]
You can also put line breaks between elements, in which case the semicolons are optional.
let oddNumber = [
1
3
5
7
9 ]
The above syntax in more readable code when the element initialization expressions are longer or when you want to include a comment for each element.
You can also define list elements by using a range indicated integers separated by the range operator (..).
let numbers = [ 1 .. 100 ]
You can also use a sequence expression to create a list. The following example creates a list of squares of integers from 1 to 10.
let listOfSquares = [ for i in 1 .. 10 -> i*i ]
You can attach elements to a list by using the :: (cons) operator.
let list1 = [1; 2; 3;]
let list2 = 0 :: list1
The above code creates list2 as [0; 1; 2; 3].
You can concatenate lists that have compatible types by using the @ operator as shown below.
let list1 = [1; 2;]
let list2 = [13; 14;]
let list3 = list1 @ list2
The above example creates list3 as [1; 2; 13; 14].
The list type supports the following properties.
| Property | Type | Description |
|---|---|---|
| Head | 'T |
The first element. |
| Empty | 'T list |
A static property that returns an empty list of the appropriate type. |
| IsEmpty | bool |
true if the list has no elements. |
| Item | 'T |
The element at the specified index (zero-based). |
| Length | int |
The number of elements. |
| Tail | 'T list |
The list without the first element. |
The following code uses different properties of a list type.
let mylist = [ 1; 2; 3; 4; 5 ]
Console.WriteLine("mylist.IsEmpty is {0}", mylist.IsEmpty)
Console.WriteLine("mylist.Length is {0}", mylist.Length)
Console.WriteLine("mylist.Head is {0}", mylist.Head)
Console.WriteLine("mylist.Tail.Head is {0}", mylist.Tail.Head)
Console.WriteLine("mylist.Tail.Tail.Head is {0}", mylist.Tail.Tail.Head)
Console.WriteLine("mylist.Item(1) is {0}", mylist.Item(1))
It will display the following output.
list1.IsEmpty is False
list1.Length is 5
list1.Head is 1
list1.Tail.Head is 2
list1.Tail.Tail.Head is 3
list1.Item(1) is 2