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The option type in F# is used in calculations when there may or may not exist a value for a variable or function. Option types are used for representing optional values in calculations. They can have two possible values − Some(x) or None.
For example, a function performing a division will return a value in normal situation, but will throw exceptions in case of a zero denominator. Using options here will help to indicate whether the function has succeeded or failed.
An option has an underlying type and can hold a value of that type, or it might not have a value.
Using Options
Let us take the example of division function. The following program explains this −
Let us write a function div, and send two arguments to it 20 and 5 −
let div x y = x / y let res = div 20 5 printfn "Result: %d" res
When you compile and execute the program, it yields the following output −
Result: 4
If the second argument is zero, then the program throws an exception −
let div x y = x / y let res = div 20 0 printfn "Result: %d" res
When you compile and execute the program, it yields the following output −
Unhandled Exception: System.DivideByZeroException: Division by zero
In such cases, we can use option types to return Some (value) when the operation is successful or None if the operation fails.
The following example demonstrates the use of options −
Example
let div x y = match y with | 0 -> None | _ -> Some(x/y) let res : int option = div 20 4 printfn "Result: %A " res
When you compile and execute the program, it yields the following output −
Result: Some 5
Option Properties and Methods
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 NullReferenceException if the value is None. |
Example 1
let checkPositive (a : int) =
if a > 0 then
Some(a)
else
None
let res : int option = checkPositive(-31)
printfn "Result: %A " res
When you compile and execute the program, it yields the following output −
Result: <null>
Example 2
let div x y = match y with | 0 -> None | _ -> Some(x/y) let res : int option = div 20 4 printfn "Result: %A " res printfn "Result: %A " res.Value
When you compile and execute the program, it yields the following output −
Result: Some 5 Result: 5
Example 3
let isHundred = function | Some(100) -> true | Some(_) | None -> false printfn "%A" (isHundred (Some(45))) printfn "%A" (isHundred (Some(100))) printfn "%A" (isHundred None)
When you compile and execute the program, it yields the following output −
false true false
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