Set F# list as null for serialisation - f#

Question

I'm using F# alongside a JSON data-store making use of the JSON.NET library. I'm trying to utilise F# structures and types where possible and have run into the following issue. Say I wish to store the following data structure,
type A = {
id : int
name : string
posts : string list
}
Creation works fine, but to update just the stored name field I need to send a JSON record that omits the posts field. Using the empty list won't work as the persistence system will assume that I wish to replace the existing posts with an empty list and thus overwrite them. From the JSON.NET docs I've read a field can be omitted from serialisation by setting it to null,
let updatedEntry = { id : 0, name : "Fred", posts = null }
However the F# compiler will give an error stating that the type list can not be set to null. Is there anyway to accomplish this from within F#, perhaps an attribute I'm unaware of? Thanks

Answer 1

There are two ways you could do this easily:
Option 1
Use the System.Collections.Generic.List type, which can be null:
> type A = {id: int; name:string; posts: System.Collections.Generic.List<string> };;
type A =
{id: int;
name: string;
posts: System.Collections.Generic.List<string>;}
> let a = {id=5; name="hello"; posts=null};;
val a : A = {id = 5;
name = "hello";
posts = null;}
Option 2
The other, more idiomatic way, would be to use the Option type:
> type A = {id: int; name:string; posts: string list option };;
type A =
{id: int;
name: string;
posts: string list option;}
> let a = {id=5; name="there"; posts=None};;
val a : A = {id = 5;
name = "there";
posts = null;}
Note that you'd compare the posts member to None rather than null.
Handy reading: Option types
Edit
(After some searching & experimentation) you could use boxing to still use F# types as the values:
> type A = {id: int; name:string; posts: System.Object };;
type A =
{id: int;
name: string;
posts: Object;}
> let a = {id=5; name="foo"; posts=null};;
val a : A = {id = 5;
name = "foo";
posts = null;}
> let b = {id=6; name="bar"; posts=(box [])};;
val b : A = {id = 6;
name = "bar";
posts = [];}
But I'd stick with the Option type, personally