Why is None represented as null?

Question

CompilationRepresentationFlags.UseNullAsTrueValue can be used to

Permit the use of null as a representation for nullary discriminators in a discriminated union

Option.None is the most prominent example of this.

Why is this useful? How is a null check better than the traditional mechanism for checking union cases (the generated Tag property)?

It leads to perhaps unexpected behavior:

Some(1).ToString() //"Some(1)"
None.ToString()    //NullReferenceException

EDIT

I tested Jack's assertion that comparing to null instead of a static readonly field is faster.

[<CompilationRepresentation(CompilationRepresentationFlags.UseNullAsTrueValue)>]
type T<'T> =
  | Z
  | X of 'T

let t = Z

Using ILSpy, I can see t compiles to null (as expected):

public static Test.T<a> t<a>()
{
    return null;
}

The test:

let mutable i = 0
for _ in 1 .. 10000000 do
  match t with
  | Z -> i <- i + 1
  | _ -> ()

The results:

Real: 00:00:00.036, CPU: 00:00:00.046, GC gen0: 0, gen1: 0, gen2: 0

If the CompilationRepresentation attribute is removed, t becomes a static readonly field:

public static Test.T<a> t<a>()
{
    return Test.T<a>.Z;
}

public static Test.T<T> Z
{
    [CompilationMapping(SourceConstructFlags.UnionCase, 0)]
    get
    {
        return Test.T<T>._unique_Z;
    }
}

internal static readonly Test.T<T> _unique_Z = new Test.T<T>._Z();

And the results are the same:

Real: 00:00:00.036, CPU: 00:00:00.031, GC gen0: 0, gen1: 0, gen2: 0

The pattern match is compiled as t == null in the former case and t is Z in the latter.

Answer 1

The F# compiler sometimes uses null as a representation for None because it's more efficient than actually creating an instance of FSharpOption<'T> and checking the Tag property.

Think about it -- if you have a normal F# type (like a record) that's not allowed to be null, then any pointer to an instance of that type (the pointer used internally by the CLR) will never be NULL. At the same time, if T is a type which can represent n states, then T option can represent n+1 states. So, using null as a representation for None simply takes advantage of that one extra state value which is available by the fact that F# types aren't allow to be null.

If you want to try turning this behavior off (for normal F# types), you can apply [<AllowNullLiteral(true)>] to them.