Null Coalescing Operator in F#?

Question

When interacting with C# libraries, I find myself wanting C#'s null coalescing operator both for Nullable structs and reference types.

Is it possible to approximate this in F# with a single overloaded operator that inlines the appropriate if case?

Answer 1

Yes, using some minor hackery found in this SO answer "Overload operator in F#".

At compiled time the correct overload for an usage of either ('a Nullable, 'a) ->'a or ('a when 'a:null, 'a) -> 'a for a single operator can be inlined. Even ('a option, 'a) -> 'a can be thrown in for more flexibility.

To provide closer behavior to c# operator, I've made default parameter 'a Lazy so that it's source isn't called unless the original value is null.

Example:

let value = Something.PossiblyNullReturned()
            |?? lazy new SameType()

Implementation:

NullCoalesce.fs [Gist]:

//https://gist.github.com/jbtule/8477768#file-nullcoalesce-fs
type NullCoalesce =  

    static member Coalesce(a: 'a option, b: 'a Lazy) = 
        match a with 
        | Some a -> a 
        | _ -> b.Value

    static member Coalesce(a: 'a Nullable, b: 'a Lazy) = 
        if a.HasValue then a.Value
        else b.Value

    static member Coalesce(a: 'a when 'a:null, b: 'a Lazy) = 
        match a with 
        | null -> b.Value 
        | _ -> a

let inline nullCoalesceHelper< ^t, ^a, ^b, ^c when (^t or ^a) : (static member Coalesce : ^a * ^b -> ^c)> a b = 
        // calling the statically inferred member
        ((^t or ^a) : (static member Coalesce : ^a * ^b -> ^c) (a, b))

let inline (|??) a b = nullCoalesceHelper<NullCoalesce, _, _, _> a b

Alternatively I made a library that utilizes this technique as well as computation expression for dealing with Null/Option/Nullables, called FSharp.Interop.NullOptAble

It uses the operator |?-> instead.