Emit Discriminated Unions

[JaggerJo]

I'm currently playing around with building a DSL for Avalonia UI. I looked at how Fabulous generates types one time.

It would be interesting to create the DSL dynamically using a (generative) type provider. In my case I would like to generate DU's for all implementations of IControl.

type ButtonAttr =
| Name of string
| Width of float
…

This would make it extremely simple to use 3rd party controls, because bindings are generated at compile/ design time.

Currently it is (as far as I know) not possible to emit F# specific types like Records or Discriminated Unions from a Type Provider.

1. Why isn't it possible to generate F# specific types (aren't they just nested classes with custom attributes/metadata after all) ?
2. Is there a general interest in making this possible (or is it just me wanting this) ?

EDIT: just found out there is a discussion about using a TP in Fabulous

[smoothdeveloper]

There is a language suggestion regarding provided DU & records: https://github.com/fsharp/fslang-suggestions/issues/154

And another one to be able to pass types to type providers: https://github.com/fsharp/fslang-suggestions/issues/212

[dsyme]

1. Why isn't it possible to generate F# specific types (aren't they just nested classes with custom attributes/metadata after all) ?

The main thing is that it's just a lot of work. The way TPs are architected (providing .NET System.Type/MethodInfo etc. and the compiler converts them to F# TAST nodes) effectively means we have to fully invert the compilation process from .NET IL types --> F# TAST types. This is implemented in tast.fs and infos.fs in the F# compiler for methods, types, properties, events but there is no corresponding inversion for F#-defined union/record types.

The inversion would be similar to that implemented in FSharp.Reflection.FSharpType/FSharpValue based on the attributes etc. But it's painful to implement and fully test accurately.

2. Is there a general interest in making this possible (or is it just me wanting this) ?

Yes, people are interested in this

[JaggerJo]

@dsyme Thank you for the explanation (If you don't mind I have on further question).

If generating TAST in F# would be simple enough, the TAST could be given straight to the Compiler, then compilation could happen as usual. Why is it the other way around (Type/MethodInfo -> TAST) ?

[dsyme]

If generating TAST in F# would be simple enough, the TAST could be given straight to the Compiler, then compilation could happen as usual. Why is it the other way around (Type/MethodInfo -> TAST) ?

The TAST is an internal compiler representation and not deemed to be sufficiently stable nor simple enough to allow direct generation.

[JaggerJo]

Thanks for the explanation, should I close this now ?

[dsyme]

@JaggerJo It's ok to leave it open I think

[Luiz-Monad]

What about providing the FSharp compiler services interface to a TypeProvider, so it can get better metadata? And with ProvidedDU and ProvidedRecords, that would increase the flexibility. Perhaps we could have a DSL for creating the relevant part of the TypeProvider output that will become the TAST. (of course you wouldn't expose the real TAST) I don't mind it being internally represented by Type/MethodInfo, but wouldn't it be simpler having a specific AST/DSL for TypeProviders ? (nothing impedes someone doing that and it creating the Quotation expressions needed, that's precisely how I'm thinking to tackle my TypeProvider ) [edit: after reading other topics, I guess I have my historical reasons answer]

But FSharpCompilerServiceTypeProvider, that would be a fun thing !