Support provided union and record types

[baronfel]

Submitted by Gustavo Guerra on 3/21/2014 12:00:00 AM
111 votes on UserVoice prior to migration

Title says it all

Original UserVoice Submission
Archived Uservoice Comments

[dsyme]

** by fslang-admin on 2/4/2016 12:00:00 AM **

Marking as “approved in principle” (it will show as “planned”). However, in reality it is both relatively low priority and relatively hard to implement, so I don’t expect this to happen any time soon. For example, we would have to reverse-map the metadata associated with F#-specific types. This is already done in F# reflection FSharpType.IsRecord and so on.

[7sharp9]

I propose we split this in two, add provided records, add provided unions.

@dsyme I know you said provided active patterns could be more generally usable than unions, maybe this could be fleshed out?

[dsyme]

I propose we split this in two, add provided records, add provided unions.

For RFCs yes, we can do two separate ones

@dsyme I know you said provided active patterns could be more generally usable than unions, maybe this could be fleshed out?

Here's the quick summary: APs are defined as module-let functions. Provided module-let functions and values are not supported. Hence the first step would probably be to allow those, then APs should drop out as part of that.

[7sharp9]

@dsyme I actually have ProvidedUnions and ProvidedRecords in Falanx :-)
obviously I have different parameters and constraints

[dsyme]

Added a relevant comment to one of the above discussions:

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

[7sharp9]

Yeah I bake in all the attributes to get reflection to work for ProvidedUnions and ProvidedRecords in Falanx.

[realvictorprm]

I just noticed, we don't have an RFC for this.

I'll dig into some stuff and see whether I can get anything together. I have the feeling this is worth having for FS-1023