How to identify an FSCheck generator so that it can be detected

I am writing an FSCheck generator to generate strings with the following properties:

• They are not zero.
• Trimming them does not affect the length
• They do not contain spaces.

Here's my generator code:

namespace Example

open FsCheck.Arb

module public Generation = 

    let hasChars (s : string) =
        (isNull s |> not)
        && s.Length > 0

    let isTrimmed (s : string) =
        s.Trim().Length = s.Length

    let isContinuous (s : string) =
        s
        |> Seq.exists ((=) ' ')
        |> not

    [<AbstractClass; Sealed>]
    type public Generators = class end

    type public ContinuousString = ContinuousString of string with
        member x.Get = match x with ContinuousString r -> r
        override x.ToString() = x.Get

    type public Generators with

        static member ContinuousString() =
            Default.String()
            |> filter hasChars
            |> filter isTrimmed
            |> filter isContinuous
            |> convert ContinuousString string

      

        
        
        
      

    

And here is a test designed to test the generation:

[<Property(Arbitrary=[| typeof<ContinuousString> |], MaxTest=2)>]
let ``A continuous string contains no spaces`` (s: ContinuousString) =
    s.Get.Contains " " |> not

      

        
        
        
      

    

When I run this test I get:

System.Exception: No instances found on type Example.Generation+ContinuousString. Check that the type is public and has public static members with the right signature.

      

        
        
        
      

    

As far as I can tell from the FSCheck source code, the item I define should be found by the discovery filter and this method seems to be similar to the built-in one, for example NonEmptyString

.

What did I miss? Thank!