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fix(rxquery)!: stop seeding sumBy with Unchecked.defaultof #26

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fix(rxquery)!: stop seeding sumBy with Unchecked.defaultof #26

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2 changes: 2 additions & 0 deletions CHANGELOG.md
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Original file line number Diff line number Diff line change
Expand Up @@ -10,11 +10,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
### Changed

- **Breaking:** `AwaitOperationConfiguration` cases are prefixed with `Await` (`AwaitSequential`, `AwaitParallel 4`, ...) so they no longer collide with `System.Threading.Tasks.Parallel`
- **Breaking:** `rxquery` `sumBy` adds values of the projected type: it requires `(+) : 'Value * 'Value -> 'Value`
- `mapAsync` and `iterAsync` validate the concurrency limit of the options when they are called

### Fixed

- `rxquery` emitted its elements on the thread pool, out of order and after the source had moved on; `yield` and `zero` are now synchronous
- `rxquery` `sumBy` passed `null` to the `(+)` of reference types

## [0.3.1] - 2026-01-28

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14 changes: 12 additions & 2 deletions src/FSharp.Control.R3/Observable.fs
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Expand Up @@ -163,10 +163,20 @@ module Builders =
member _.MinBy (s : Observable<'a>, [<ProjectionParameter>] valueSelector : 'a -> 'b) = s.MinByAsync (new Func<'a, 'b> (valueSelector))

[<CustomOperation("sumBy")>]
member inline _.SumBy (s : Observable<_>, [<ProjectionParameter>] valueSelector : _ -> _) =
member inline _.SumBy (s : Observable<_>, [<ProjectionParameter>] valueSelector : _ -> 'Value) =
// The first element seeds the sum: seeding with Unchecked.defaultof passed null to the (+) of reference types,
// while requiring a Zero member would reject types such as TimeSpan whose zero is a field
s
|> _.Select(valueSelector)
|> _.AggregateAsync(Unchecked.defaultof<_>, new Func<_, _, _> (fun a b -> a + b))
|> _.AggregateAsync(
ValueNone,
new Func<_, _, _> (fun sum value ->
match sum with
| ValueNone -> ValueSome value
| ValueSome sum -> ValueSome (sum + value)
),
new Func<_, _> (ValueOption.defaultValue Unchecked.defaultof<'Value>)
)

[<CustomOperation("zip", IsLikeZip = true)>]
member _.Zip (s1 : Observable<_>, s2 : Observable<_>, [<ProjectionParameter>] resultSelector : _ -> _) =
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