LDM Notes for January 13th, 2025

[333fred]

https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-01-13.md

Agenda

• Discriminated Unions

[orthoxerox]

I am really happy that you have decided to go ahead with class unions before finalizing the whole design.

Speaking of struct union challenges, how does F# work around the problem of large structs? As far as I know, it performs no optimizations to reduce the size of the union.

There are ways of compacting the representation, e.g. by overlapping fields, but the more you do, the more time is spent packing and unpacking values. This starts to offset the benefits of avoiding allocation.

If compaction uses unsafe techniques, the runtime might get confused and turn off its own optimizations. Runtime work to address this directly would likely be costly.

I thought the bigger problem was that generic types couldn't have explicit locations. Runtime doesn't get confused, it flat out refuses to load a struct union where you try to pack all kinds into [FieldOffset(1)] if any of them is generic.