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Note when a shared field needs option instead of voption #20594

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3 changes: 2 additions & 1 deletion .github/instructions/FSharp.instructions.md
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Expand Up @@ -25,7 +25,8 @@ When the IDE's F# semantic tools are unavailable, use the `F#` MCP server (`.mcp

## Values and types

- `voption` – `ValueSome`/`ValueNone` – over `option`; it is this compiler's option type. Fields, members, parameters and values shared between threads included: none of them is a reason to pick `option`. Exception: when an API hands you `'T option` and has no `voption` counterpart, unwrap with `Option.defaultValue`/`Option.defaultWith` directly – do not insert `ValueOption.ofOption` just to switch modules.
- `voption` – `ValueSome`/`ValueNone` – over `option`; it is this compiler's option type. Exception: when an API hands you `'T option` and has no `voption` counterpart, unwrap with `Option.defaultValue`/`Option.defaultWith` directly – do not insert `ValueOption.ofOption` just to switch modules.
- A field one thread writes without a lock and another reads needs an *atomic-sized* write, not `voption`'s style. `'T option` is always a single reference (atomic on any width); `'T voption` is a struct that grows with `'T` and can span more than one word – for a `'T` already close to a full word (like `Position`'s `[<Struct>]` `int64`, where the voption tag pads it past one), a torn write is possible. Use `'T option` for such a field specifically, and only for it – a local binding built from it, never touched by another thread, stays `voption` as always.
- The mirror case, an API that *takes* `'T option` (an optional argument `?caret = …`, a field typed `IDisposable option`): stay in `ValueOption` through the whole chain and convert once, last – `x |> ValueOption.bind _.Position |> ValueOption.toOption`, never `x |> ValueOption.toOption |> Option.bind _.Position`.
- `vsintegration` has `voption`-returning counterparts of the FSharp.Core collection functions, suffixed `V`, in `FSharp.Editor/Common/Extensions.fs`: `Seq.tryHeadV`/`tryFindV`/`tryFindIndexV`/`tryPickV`/`chooseV`, `Array.tryHeadV`/`tryFindV`/`tryPickV`/`chooseV`, `List.tryFindV`, `ImmutableArray.tryHeadV`. Reach for those rather than the `option`-returning original. The module is `[<AutoOpen>]` and compiles before the rest of `FSharp.Editor`, so a file in the `Microsoft.VisualStudio.FSharp.Editor` namespace needs no `open` for them. `src/Compiler` has no equivalents.
- `struct ('T1 * 'T2)` tuples and `[<Struct>]` types on allocation-sensitive paths.
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