How do I create an F# mutable option type?

Question

I need to create a mutable `option<T>` type in F#. I've tried writing ``` let x = ref None ``` and subsequently writing ``` x := Some(z) ``` but it doesn't work. Help!

Accepted Answer

You need to state the type explicitly to avoid "the Value Restriction" (or see "Automatic Generalization" on msdn):

```
let x : Ref<int option> = ref None

x := Some 4
```

How do I create an F# mutable option type?

Solution

Problem