In the F# API, is there something like Option.ofNull?

Question

Many APIs that I consume from F# allow null values. I like turning them into Options. Is there a simple built-in way to do this? Here is one way I have done so:

type Option<'A> with
    static member ofNull (t:'T when 'T : equality) =
        if t = null then None else Some t

Then I'm able to use Option.ofNull like so:

type XElement with
    member x.El n = x.Element (XName.Get n) |> Option.ofNull

Is there something built-in that already does this?

Based on Daniel's answer, equality isn't needed. null constraint can be used instead.

type Option<'A> with
    static member ofNull (t:'T when 'T : null) =
        if t = null then None else Some t

I'd call it `Option.ofObj` http://stackoverflow.com/questions/24077105/is-there-conditional-access-operator-in-f-similar-to-new-in-c#comment37130992_24077479 — Ruben Bartelink, Jun 26 '14 at 13:31
I suggested that we add a common function to the F# API for this. Vote it up here: http://fslang.uservoice.com/forums/245727-f-language/suggestions/6103054-add-option-ofnull-to-help-remove-nulls — Cameron Taggart, Jun 26 '14 at 18:15
Just to point out any readers - as of F#4 this is now built into FSharp.Core. — Isaac Abraham, Feb 08 '16 at 19:49

score 6 · Accepted Answer · answered Jun 25 '14 at 21:29

6

There's nothing built-in to do this. BTW, you can do without the equality constraint:

//'a -> 'a option when 'a : null
let ofNull = function
    | null -> None
    | x -> Some x

or, if you want to handle F# values passed from other languages and Unchecked.defaultof<_>:

//'a -> 'a option
let ofNull x = 
    match box x with
    | null -> None
    | _ -> Some x

answered Jun 25 '14 at 21:29

Daniel

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As you say, for the purposes of this function, it's better not to have the `null` constraint on the type parameter, but I'm not sure boxing is the best way to remove the type constraint here -- why does it make sense for this function to accept value types? Another way to implement this would be to manually specify the `not struct` constraint on the type parameter, then use `System.Object.ReferenceEquals(null, x)` to check for null. – Jack P. Jul 06 '14 at 14:53
@JackP.: Those are reasonable improvements. Of course, you could avoid `box` with just a type annotation: `let ofNull (x: obj) = ...` – Daniel Jul 07 '14 at 03:24

In the F# API, is there something like Option.ofNull?

1 Answers1