Lens cannot deduce setter from this getter

[AlexPeret]

hi!

I'm facing the following message while trying to use the Lens function:

erro FS9001: Macro error in WebSharper.UI.Macros+LensFunc.TranslateCall: Cannot deduce setter from this getter

After testing it on a debug project, I figure it is working. The main difference between both projects is the failing one references WebSharper.UI assembly (WebSharper.UI.4.2.1.86\lib\net461\WebSharper.UI.dll).

After removing such reference, my code breaks due UI.Next namespace not defined.

In summary, the failing project uses:

open WebSharper
open WebSharper.UI
open WebSharper.UI.Html  // not defined
open WebSharper.UI.Client  // not defined

and the working project uses:

open WebSharper
open WebSharper.UI.Next
open WebSharper.UI.Next.Client
open WebSharper.UI.Next.Html

I recall changing the namespace of UI.Next lib after migrating to WebSharper 4.2 and I'm not sure now what to do.

Should I use the original namespace and get rid of WebSharper.UI.dll or is it a bug on Macro module (WS 4.2)?

Thanks,
Alex

[AlexPeret]

Adding to it, below the list of assemblies both projects reference to:

Failing Project

• WebSharper.UI.Next (WebSharper.UI.Next.4.2.0.187)
• WebSharper.UI.Next.CSharp (WebSharper.UI.Next.4.2.0.187)
• WebSharper.UI.Next.Templating (WebSharper.UI.Next.4.2.0.187)
• WebSharper.UI.Next.Templating.Common (WebSharper.UI.Next.4.2.0.187)
• WebSharper.UI.Next.Templating.Runtime (WebSharper.UI.Next.4.2.0.187)
• WebSharper.UI.Templating (WebSharper.UI.4.2.1.86)
• WebSharper.UI.Templating.Common (WebSharper.UI.4.2.1.86)
• WebSharper.UI.Templating.Runtime (WebSharper.UI.4.2.1.86)

Working (debug) project

• WebSharper.UI.Next (WebSharper.UI.Next.4.2.0.187)
• WebSharper.UI.Next.CSharp (WebSharper.UI.Next.4.2.0.187)
• WebSharper.UI.Next.Templating (WebSharper.UI.Next.4.2.0.187)
• WebSharper.UI.Next.Templating.Common (WebSharper.UI.Next.4.2.0.187)
• WebSharper.UI.Next.Templating.Runtime (WebSharper.UI.Next.4.2.0.187)

I've tried removing all references to WebSharper.UI.Next.*.dll family, but issue remains.

[Tarmil]

You definitely need to either only use WebSharper.UI.Next.* assemblies and namespaces, or only WebSharper.UI.*.

What does your call to Lens look like, and the types involved?

[AlexPeret]

>> What does your call to Lens look like, and the types involved?

Looks like this issue relates to templating system. The snippet below raises the exception:

    type template = Templating.Template<"ListModelTest.html">

    let ListModelTest () =
        let aliases = 
            ListModel.Create (fun a -> a.Key) [ { Key = 1; Value = "Bill" }; { Key = 2; Value = "Joe" } ]
        
        let items =
            aliases.View
            |> Doc.BindSeqCachedView (fun aliasV ->
                template.EditItem()
                    .Value(Lens aliasV.V.Value)
                    .Doc())
        items

This is the template file (ListModelTest.html):

<div class="table-responsive">
    <table class="table table-striped table-bordered table-hover">
        <thead>
            <tr>
                <th>Value</th>
            </tr>
        </thead>
        <tbody ws-hole="ListContainer">

            <tr ws-template="EditItem">
                <td>
                    <input class="form-control" type="text" ws-var="Value" />
                </td>
            </tr>

        </tbody>
    </table>
</div>

Thanks,
Alex

[Tarmil]

Oh, right. For Lens, you need to use .V on a Var<'T>, but here you're using it on a View<'T>. The way you can obtain a Var here is by using Doc.BindListModelLens (or equivalently, the DocLens method on your ListModel) instead of Doc.BindSeqCachedView:

        let items =
            aliases
            |> Doc.BindListModelLens (fun _ aliasVar ->
                template.EditItem()
                    .Value(Lens aliasVar.V.Value)
                    .Doc())

I'll work on improving the error message so that this is clearer.