How do I define an FSCheck generator so that it can be discovered

Question

I'm writing an FSCheck generator to create strings having the following properties:

• They are non-null
• Trimming them won't affect the length
• They contain no 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's a test intended to verify 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 looking at the FSCheck source code, the member I have defined should be found by the discovery filter, and the method seems analogous to similar built-in ones such as NonEmptyString.

What have I missed? Thanks!

Answer 1

You're passing FsCheck the wrong type. You should be passing it your Generators class, not your ContinuousString DU. I.e., this:

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

should have been:

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

The FsCheck error message was trying to tell you this as well:

Check that the type is public and has public static members with the right signature.

The type that you created that matches what it's looking for is Generators.