Reimplementation of C# Nullable type

// XXX Uncomment the following line:
//[<Struct>]
type FSharpNullable<'T when 'T : struct> =
    | Value of 'T
    | Null

    member this.HasValue =
        match this with
        | Null -> false
        | Value _ -> true

    static member inline op_Explicit(this) : 'T =
        match this with
        | Null -> raise <| System.InvalidOperationException(
                               "Nullable object must have a value.")
        | Value x -> x

    static member inline (<??>) (lhs, rhs) =
        match lhs with
        | Null -> rhs
        | Value x -> x

module FSharpNullable =
    let bind f x =
        match x with
        | Null -> Null
        | Value x' -> f x'

type FSharpNullableBuilder() =
    member __.Bind(x, f) = x |> FSharpNullable.bind f
    member __.Return(x) = Value x

[<AutoOpen>]
module FSharpNullableComputationExpression =
    let nullable = FSharpNullableBuilder()

[<AutoOpen>]
module private FSharpNullableOps =
    let inline add (a: FSharpNullable<'T>) (b: FSharpNullable<'T>) : FSharpNullable<'T> =
        nullable {
            let! x = a
            let! y = b
            return x + y
        }

    let inline subtract (a: FSharpNullable<'T>) (b: FSharpNullable<'T>) : FSharpNullable<'T> =
        nullable {
            let! x = a
            let! y = b
            return x - y
        }

    let inline multiply (a: FSharpNullable<'T>) (b: FSharpNullable<'T>) : FSharpNullable<'T> =
        nullable {
            let! x = a
            let! y = b
            return x * y
        }

    let inline divide (a: FSharpNullable<'T>) (b: FSharpNullable<'T>) : FSharpNullable<'T> =
        nullable {
            let! x = a
            let! y = b
            return x / y
        }

type FSharpNullable<'T when 'T : struct> with
    static member inline ( + ) (lhs, rhs) = add lhs rhs
    static member inline (.+.) (lhs, rhs) = add lhs rhs
    static member inline ( +.) (lhs, rhs) = add (Value lhs) rhs
    static member inline (.+ ) (lhs, rhs) = add lhs (Value rhs)

    static member inline ( - ) (lhs, rhs) = subtract lhs rhs
    static member inline (.-.) (lhs, rhs) = subtract lhs rhs
    static member inline ( -.) (lhs, rhs) = subtract (Value lhs) rhs
    static member inline (.- ) (lhs, rhs) = subtract lhs (Value rhs)

    static member inline ( * ) (lhs, rhs) = multiply lhs rhs
    static member inline (.*.) (lhs, rhs) = multiply lhs rhs
    static member inline ( *.) (lhs, rhs) = multiply (Value lhs) rhs
    static member inline (.* ) (lhs, rhs) = multiply lhs (Value rhs)

    static member inline ( / ) (lhs, rhs) = divide lhs rhs
    static member inline (./.) (lhs, rhs) = divide lhs rhs
    static member inline ( /.) (lhs, rhs) = divide (Value lhs) rhs
    static member inline (./ ) (lhs, rhs) = divide lhs (Value rhs)

// Prepend/append `.` to the operator to let the compiler know that the left/right hand side of operand is of FSharpNullable.
let a = Value 13 .+. Value 29 // val b : FSharpNullable<int> = Value 42

// If both sides of the operator are of Nullable, you can omit `.`.
let b = Value 19 + Value 23 // val a : FSharpNullable<int> = Value 42

// Supports structural equality.
a = b // val it : bool = true

// Supports structural comparison.
Value 2 < Value 3 // val it : bool = true

// `.+` denotes the LHS is of FSharpNullable and the RHS non-nullable.
let c = a .+ 42 // val c : FSharpNullable<int> = Value 84

// `+.` denotes the LHS is of non-nullable and the RHS of FSharpNullable.
let c = 42 +. a // val c : FSharpNullable<int> = Value 84

// Null is such a scary thing so that it turns everything it touches into Null.
let n = a + Null // val n : FSharpNullable<int> = Null

// Null-coalescing operator evaluates to the LHS if the LHS is not Null..
a + c <??> 42 // val it : int = 126

// Null-coalescing operator evaluates to the RHS if the LHS is Null.
a + c + n <??> 42 // val it : int = 42

// Supports C# Nullable-like property.
a.HasValue // val it : bool = true
n.HasValue // val it : bool = false

// Explicit type conversion to the underlying type.
int a // val it : int = 42

// Type-safety.
int64 a // error FS0001: The type 'FSharpNullable<int>' does not support a conversion to the type 'int64'

int64 (Value 42L) // val it : int64 = 42L

// Can't type-convert Null in any case.
int n // System.InvalidOperationException: Nullable object must have a value.