[SE-NNNN] Metatype Keypaths #2420
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rjmccall
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Apr 29, 2024
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| # Metatype Keypaths | |||
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| * Proposal: [SE-NNNN](NNNN-filename.md) | |||
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@amritpan Could you set a name here to NNNN-metatype-keypaths.md?
| let kpWithLiteral = \Bee.Type.name // key path literal \Bee.Type | ||
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| Attempting to write the above metatype keypath without including `.Type will trigger an error diagnostic: |
| Attempting to write the above metatype keypath without including `.Type will trigger an error diagnostic: | ||
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| ``` | ||
| let kpWithLiteral = \Bee.name // error: static member 'name' cannot be used on instance of type 'Bee' |
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I think it's okay to just have error: ... here without the code.
| ``` | ||
| ### Access semantics | ||
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| Immutable static properties will form the read-only keypaths just like immutable instance properties. |
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Maybe it should be something like - Metatype key paths share almost all access semantics with instance member key paths with a notable exception to mutable static properties - such properties always form ReferenceWritableKeyPath because their base type (a metatype type) always has reference semantics.
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There are 2 notable updates to this proposal (compared to the original pitch) after working through the implementation.
Metatype keypaths are back deployable. Modules/libraries using older compilers will not need to re-compile to see correct operator comparison results. (The entire Implications on adoption section in the pitch is no longer an issue). For reference: Static properties in older modules/libraries will be missing property descriptors when they are referenced via keypath in a module using a compiler with this feature implementation. However, we discovered that the missing property descriptors fortunately did not affect Equatable/Hashable conformance in the Standard Library.
Diagnostics will show an error message if
.Typeis left out of a metatype keypath reference (where the first component is a static property), but a fix-it is not possible at this time.I have left the pitch text intact but the above discoveries are reflected in the proposal text.