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Avoiding boilerplate in using Lenses
websharperui.nextlens

One of my key usecases involves creating a sort of advanced CRUD editor for an OR Mapped database. So I'm starting off with a business model which knows nothing about WebSharper and lives on the server originally.

Websharper will allow me to use those type definitions directly on the client, but if I bind individual fields to Vars and Views I need to produce Lenses for all my fields or I will end up rendering my entire model on every change.

I can't write the lens functions up front as I don't know what the model type will be in my code - it's intended to generically work with a wide variety of models built by consumers later on. And if I tell my consumers to write lenses, they won't use my system.

So I either have to write a preprocessor of some sort using reflection to create all the lens functions I may need, or find a way of achieving the same result automatically within the code.

My internal code already has to use Reflection to understand the structure of the model, but I can't work out whether/how to create lenses that way.

My problem is, I think, solved if I could have a function which takes a View(record) and a .NET property, and delivers me a View(the corresponding property of the underlying record)

Anyone done this before? Or any pointers how to do this cleanly.

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In my opinion the cleanest way to do this would be to write a WebSharper macro that simply takes the getter function and infers the setter. This is something I've wanted to implement in UI.Next for quite some time actually, as it would cut down a lot on boilerplate as you said.

Here I implemented such a macro for record fields. In the future we'll probably add to UI.Next a more advanced version which would be able to recognize more getters (field of field, tuple deconstruction, etc).

By on 5/1/2016 12:22 AM ()

Thanks for the info. Your macro looks like it's a good starting point for what I want to d, but I'm bound to say it's a bit obscure to understand, and I can't find much in the standard docs to explain the detail of all the calls. Is this stuff a matter of reading the source code, or are there some more detailed docs out there. And is the macro api likely to be stable or will it all change with the next major version?

By on 5/2/2016 11:41 AM ()

No, unfortunately the macro API will be completely different in the next major version, both due to the inner workings of WebSharper being completely revamped, and the macro API being enhanced to reduce the need to use .NET reflection like I did above at line 33. The current API is not very well documented (basically the page I linked is the only documentation), we hope to do better for the next one.

By on 5/3/2016 3:04 AM ()

Well thanks for the clarification anyway even if it's not what I hoped to hear. Meanwhile, I'm having trouble getting your sample to work. If I add in a reference in my code to the lensFirst method, the F# compiles OK, but then websharper says:

1

C:\home\Gary\Documents\Visual Studio 2015\Projects\UINextClientServer1\packages\WebSharper.3.6.11.234\build\WebSharper.targets(60,5): error : Cannot load generic type

I'm assuming that the macro can't handle the passed-in type having a generic parameter. is there a workaround?

By on 5/3/2016 9:07 AM ()

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