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The F# options 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 a normal situation but will throw exceptions in the 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 F# Options

Let us take the example of the 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 −

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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