Better conversion: prefer ReadOnlySpan<T> over Span<T> #8779
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jjonescz
approved these changes
Dec 6, 2024
jjonescz
reviewed
Dec 10, 2024
proposals/first-class-span-types.md
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| > Given two types `T₁` and `T₂`, `T₁` is a ***better conversion target*** than `T₂` if one of the following holds: | ||
| > | ||
| > - **`T₁` is `System.ReadOnlySpan<E₁>`, `T₂` is `System.Span<E₂>`, and an identity conversion from `E₁` to `E₂` exists** | ||
| > - **`T₁` is `System.ReadOnlySpan<E₁>` and `T₂` is `System.ReadOnlySpan<E₂>`, or at least one of `T₁` or `T₂` is not a *span type*, and** an implicit conversion from `T₁` to `T₂` exists and no implicit conversion from `T₂` to `T₁` exists |
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span type
Should we be explicit this means Span / ReadOnlySpan? I imagine there might be more span types like TensorSpan.
proposals/first-class-span-types.md
Outdated
| > Given two types `T₁` and `T₂`, `T₁` is a ***better conversion target*** than `T₂` if one of the following holds: | ||
| > | ||
| > - **`T₁` is `System.ReadOnlySpan<E₁>`, `T₂` is `System.Span<E₂>`, and an identity conversion from `E₁` to `E₂` exists** | ||
| > - **`T₁` is `System.ReadOnlySpan<E₁>` and `T₂` is `System.ReadOnlySpan<E₂>`, or at least one of `T₁` or `T₂` is not a *span type*, and** an implicit conversion from `T₁` to `T₂` exists and no implicit conversion from `T₂` to `T₁` exists |
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I think this could be simplified, i.e., this would have the same effect, right? But maybe it's worse because the intention is less clear.
Suggested change
| > - **`T₁` is `System.ReadOnlySpan<E₁>` and `T₂` is `System.ReadOnlySpan<E₂>`, or at least one of `T₁` or `T₂` is not a *span type*, and** an implicit conversion from `T₁` to `T₂` exists and no implicit conversion from `T₂` to `T₁` exists | |
| > - **`T₁` is not `System.Span<E₁>` and** an implicit conversion from `T₁` to `T₂` exists and no implicit conversion from `T₂` to `T₁` exists |
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We'll need to allow the case where one type is Span<E1> and the other is not a span type. See similar comment.
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Update better conversion target to prefer
ReadOnlySpan<T>overSpan<T>.Follow-up from LDM-2024-12-04 and LDM-2024-12-09.