Formatter does not work on generic type

Describe the bug

I tried to format a record with generic type 'T. See below the type definition and register function.

type TestType<'T> =
  {
    Name: string
    Value: 'T
  }

let v1:TestType<float> = { Name = "Test"; Value = 1.0 }
let v2:TestType<string> = { Name = "Test"; Value = "String" }

let registerTestType (s:TestType<_>) (writer:TextWriter) =
    let headers = new ResizeArray<IHtmlContent> ()
    headers.Add(th.innerHTML(i.innerHTML("TestType")))
    let rows = ResizeArray<ResizeArray<IHtmlContent>>()
    let cells = ResizeArray<IHtmlContent>()
    cells.Add(td.innerHTML(s.Name + ": " + (s.Value.ToString())))
    rows.Add(cells)
    let t =
        table.innerHTML(
            [|
              thead.innerHTML(headers)
              tbody.innerHTML(rows.Select(fun r -> tr.innerHTML(r)))
            |])    
    writer.Write(t)

See the screenshot below for the Jupyter output. The following code does not register the format of the generic type

Formatter<TestType<_>>.Register( (fun x writer -> registerTestType x writer), mimeType = "text/html")

I had to register types explicitly to get it working

Formatter<TestType<float>>.Register( (fun x writer -> registerTestType x writer), mimeType = "text/html")
Formatter<TestType<string>>.Register( (fun x writer -> registerTestType x writer), mimeType = "text/html")

Did this error occur while using dotnet try or online?

• dotnet-try
• online

What kind of error was it?

• User Interface (UI): For example the output never displayed
• Service Error: For example “The service is temporarily unavailable. We are working on it”
• Other:

Screenshots

Please complete the following:

• OS
  • Windows 10
  • macOS
  • Linux (Please specify distro)
  • iOS
  • Android
• Browser
  • Chrome
  • Edge
  • Safari

[jonsequitur]

We didn’t anticipate this usage but it makes sense. @brettfo and I were discussing how this should be improved.

In C#, this doesn’t compile:

Formatter<TestType<>>.Register((x, writer) => { } );

We do have an API that looks like this:

Formatter.Register(typeof(TestType<>), (x, writer) => { } );

BUT this API doesn’t work yet for non-instantiable types (e.g. interfaces and open generic types).

I’m working on changing that in dotnet/try#611.

[jonsequitur]

Work on the extensibility model is in progress and we’d appreciate feedback. You can see an example here: https://github.com/eerhardt/DotNetInteractiveExtension