Option. In F# an option is a discriminated union. This is similar to a Nullable in C#. It can have a value (like an int) or be None. It is an optional int: a value that might not exist.
Avoiding errors. Options will cause some small level of performance overhead, but if we use them to prevent exceptions, they can result in an overall improvement in F#.
An example. Let us begin. Many built-in functions like List.tryFindIndex return options. This function uses a predicate (a lambda) and returns the first index where an element matches.
Here The "result" is an option int. We cannot use it as an int directly—we must extract its Value.
Detail When we print an option int to the screen, we see the type name "Some." If the inner value is 2, we see "Some 2."
Important For the inner value of the discriminated union, we access the Value property. This must be done only if the option is not None.
// An example list.
let ids = [10; 20; 30]
// Find index of element with value 30.// ... This returns an option int.
let result = List.tryFindIndex (fun y -> y = 30) ids
// Write return value.
printfn "Result = %A" result
printfn "IsNone = %A" result.IsNone
printfn "IsSome = %A" result.IsSome
// See if there is a value.
if result.IsSome then
// Get and write the value.
let num = result.Value
printfn "Value = %A" numResult = Some 2
IsNone = false
IsSome = true
Value = 2
None, Some. Here we examine how to create a discriminated union (an option) from a function. In checkNumber we receive one argument, and return None or a Some value based on that argument.
Result When we pass the argument 0 to checkNumber, we get None back. We test this with an if.
Next We pass 1 to checkNumber and get Some number back. We use IsSome to test it, and then print its value.
// If number is 0, return None.// ... Otherwise return Some number.
let checkNumber n = match n with
| 0 -> None
| _ -> Some(n)
// This returns None.
let c = checkNumber 0
if c = None then
printfn "None returned"// This returns Some number.
let n = checkNumber 1
if n.IsSome then
printfn "Some returned, value = %A" n.ValueNone returned
Some returned, value = 1
An error. We cannot use an option as we would use the underlying value. An error will occur. We must access IsSome and Value on the result of tryFindIndex.
// This returns an option int, not an int.
let index = List.tryFindIndex (fun y -> y = 10) [2; 10; 12]
// We must access the Value, not just use the option.
let result = index + 1error FS0001: The type 'int' does not match the type 'int option'
A review. Options eliminate the need to have special values mean "no value." For example, using None instead of -1 to mean "nothing found" may be clearer. Options help clarify our code.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Dec 22, 2023 (edit).