`'a option` generates `null`

What is the best way to test Option values? I can use obj.ReferenceEquals to make sure the Option isn’t null, but it just doesn’t seem right.

// The type ''a option' does not have 'null' as a proper value
let f (x : 'a option) = isNull x

// Ok, passed 100 tests (99% <null>).
[<Property>]
let testOption (x : int option) =
    obj.ReferenceEquals(x, null) ==> Prop.collect x true

[kurtschelfthout]

F# has eager evaluation, so FsCheck can’t prevent it from evaluating the right hand side of ==> even if the left hand side is false. You should be able to wrap the right side in lazy to prevent this:

[<Property>]
let ``If set has length gteq 0`` (t : T) =
    (t.X.IsSome && t.X.Value <> null) ==>
        lazy (let i = (t.X.Value.Length) in i >= 0)
    |> Prop.collect t

[simendsjo]

Of course! Thanks 😃 The bug was in my head as usual.