F# Option equivalent of C#'s ?? operator

Question

I am looking for a way to get the value of an F# option or use a default value if it is None. This seems so common I can\'t believe something predefined doesn\'t exist. Here

Answer 1

You could easily create your own operator to do the same thing.

let (|?) = defaultArg

Your C# example would then become

let getString() = (None:string option) 
let test = getString() |? "This will be used if the result of getString() is None.";;

val getString : unit -> string option 
val test : string = "This will be used if the result of getString() is None."

Here's a blog post that goes into a little more detail.

Edit: Nikon the Third had a much better implementation for the operator, so I updated it.

Answer 2

You're looking for defaultArg [MSDN] ('T option -> 'T -> 'T).

It's often used to provide a default value for optional arguments:

type T(?arg) =
  member val Arg = defaultArg arg 0

let t1 = T(1) 
let t2 = T() //t2.Arg is 0