3 ms·

Looks like it's "just" type-erasure / syntactical sugar. E.g. value types are boxed.

AndrewDucker 6mo ago
Yes, but see the section on custom unions* - you can write non-boxing unions/generators. * https://devblogs.microsoft.com/dotnet/csharp-15-union-types/#custom-unions-for-existing-libraries https://devblogs.microsoft.com/dotnet/csharp-15-union-types/...
- mwkaufma 6mo ago
  Yes, there's a compat-shim in the stdlib/runtime, but not in the language syntax. E.g. it by-definition won't do escape-analysis and optimize discriminated value-types with the first-class keyword.
  - Izikiel43 6mo ago
    For now
celeries 6mo ago
Yes, but that's just the default behavior. You can implement your own non-boxing version for performance critical applications.
- algorithmsRcool 6mo ago
  Why on earth did they decide boxing by default was a sensible design decision... We have been pushing toward higher performance for years and this is a performance pitfall for unions would are often thought of as being lighter weight than inheritance hierarchies. F# just stores a field-per-case, with the optimization that cases with the same type are unified which is still type safe.
  - zigzag312 6mo ago
    From what I've read, this is for the first implementation of unions, to reduce amount of compiler work they need to do. They have designed them in a way they can implement enhancements like this in the future. Things like non-boxing unions and tagged unions / enhanced enums are still being considered, just not for this version.
    - CharlieDigital 6mo ago
      This is the general pattern of how the C# team operates, IME. "Never let perfect be the enemy of good" Very much what I've seen from them over the years as they iterate and improve features and propagate it through the platform. AOT as an example; they ship the feature first and then incrementally move first party packages over to support it. Runtime `async` is another example.
      - pjmlp 6mo ago
        In the meantime I still haven't done any project with nullable references, because the ecosystem has yet to move along. Same applies to ValueTask for async code.
        zigzag312 6mo ago
        Which part of the ecosystem is blocking your projects from using nullable references? I find them very helpful, but the projects were all newer or migrated to new SDK.
        pjmlp 6mo ago
        What all projects? It is relatively easy to find corporate libraries, or commercial products that still aren't using it, including Microsoft products still stuck in .NET Framework, or .NET standard 2.0. If you want name shaming of commercial products with modern .NET, here is one, can provide more. https://github.com/episerver/content-templates https://github.com/episerver/content-templates
functional_dev 6mo ago
Right, the default boxes into heap, but unions are different. Some languages pack them as a flat struct (tag + payload, no allocation). Here is visual layout if anyone is interested - https://vectree.io/c/memory-layout-tagging-and-payload-overlapping-of-union-types https://vectree.io/c/memory-layout-tagging-and-payload-overl...
- debugnik 6mo ago
  That is not what C# has just added to the language though. These union types so far are just wrappers over an `object` field which gets downcasted. F# offers actual field sharing for value-type (struct) unions by explicitly sharing field names across cases, which is as far as you can push it on the CLR without extra runtime support.

Yes, but see the section on custom unions* - you can write non-boxing unions/generators. * https://devblogs.microsoft.com/dotnet/csharp-15-union-types/#custom-unions-for-existing-libraries https://devblogs.microsoft.com/dotnet/csharp-15-union-types/...

mwkaufma 6mo ago
Yes, there's a compat-shim in the stdlib/runtime, but not in the language syntax. E.g. it by-definition won't do escape-analysis and optimize discriminated value-types with the first-class keyword.
- Izikiel43 6mo ago
  For now

celeries 6mo ago

Yes, but that's just the default behavior. You can implement your own non-boxing version for performance critical applications.

algorithmsRcool 6mo ago
Why on earth did they decide boxing by default was a sensible design decision... We have been pushing toward higher performance for years and this is a performance pitfall for unions would are often thought of as being lighter weight than inheritance hierarchies. F# just stores a field-per-case, with the optimization that cases with the same type are unified which is still type safe.
- zigzag312 6mo ago
  From what I've read, this is for the first implementation of unions, to reduce amount of compiler work they need to do. They have designed them in a way they can implement enhancements like this in the future. Things like non-boxing unions and tagged unions / enhanced enums are still being considered, just not for this version.
  - CharlieDigital 6mo ago
    This is the general pattern of how the C# team operates, IME. "Never let perfect be the enemy of good" Very much what I've seen from them over the years as they iterate and improve features and propagate it through the platform. AOT as an example; they ship the feature first and then incrementally move first party packages over to support it. Runtime `async` is another example.
    - pjmlp 6mo ago
      In the meantime I still haven't done any project with nullable references, because the ecosystem has yet to move along. Same applies to ValueTask for async code.
      - zigzag312 6mo ago
        Which part of the ecosystem is blocking your projects from using nullable references? I find them very helpful, but the projects were all newer or migrated to new SDK.
        pjmlp 6mo ago
        What all projects? It is relatively easy to find corporate libraries, or commercial products that still aren't using it, including Microsoft products still stuck in .NET Framework, or .NET standard 2.0. If you want name shaming of commercial products with modern .NET, here is one, can provide more. https://github.com/episerver/content-templates https://github.com/episerver/content-templates

functional_dev 6mo ago

Right, the default boxes into heap, but unions are different. Some languages pack them as a flat struct (tag + payload, no allocation). Here is visual layout if anyone is interested - https://vectree.io/c/memory-layout-tagging-and-payload-overlapping-of-union-types https://vectree.io/c/memory-layout-tagging-and-payload-overl...

debugnik 6mo ago
That is not what C# has just added to the language though. These union types so far are just wrappers over an `object` field which gets downcasted. F# offers actual field sharing for value-type (struct) unions by explicitly sharing field names across cases, which is as far as you can push it on the CLR without extra runtime support.