The option type in F# is used when an actual value might not exist for a named value or variable. An option has an underlying type and can hold a value of that type, or it might not have a value.
The following code shows a function that generates an option type.
let keepIfPositive (a : int) =
if a > 0 then
Some(a)
else
None
Console.WriteLine(keepIfPositive 3)
As you can see, if the input a is greater than 0, Some(a) is generated. Otherwise, None is generated.
None is used when an option does not have an actual value.Some(...) gives the option a value.Some and None are useful in pattern matching.The following exists function returns true if the option has a value and false if it does not.
let exists (x : int option) =
match x with
| Some(x) -> true
| None -> false
Console.WriteLine(exists (Some (11)))
Console.WriteLine(exists (Some (30)))
Console.WriteLine(exists None)
The option type supports the following properties and methods.
| Property or Method | Type | Description |
|---|---|---|
| None | 'T option |
A static property that enables you to create an option value that has the None value. |
| IsNone | bool |
Returns true if the option has the None value. |
| IsSome | bool |
Returns true if the option has a value that is not None. |
| Some | 'T option |
A static member that creates an option that has a value that is not None. |
| Value | 'T |
Returns the underlying value, or throws a System.NullReferenceException if the value is None. |
Options are commonly used when a search does not return a matching result, as shown in the following code.
let rec tryFindMatch pred list =
match list with
| head :: tail -> if pred(head)
then Some(head)
else tryFindMatch pred tail
| [] -> None
let result1 = tryFindMatch (fun elem -> elem = 100) [ 200; 100; 50; 25 ]
let result2 = tryFindMatch (fun elem -> elem = 26) [ 200; 100; 50; 25 ]
Console.WriteLine(result1)
Console.WriteLine(result2)