Expose boxed inline value classes in JVM #393
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udalov
reviewed
Sep 12, 2024
| "Exposing" a `value` class is not a single concept. The following are some scenarios that we would like to support: | ||
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| 1. Creating boxed values: for example, creating an actual instance of `PositiveInt`. | ||
| 2. Calling operations that take or return value classes using their boxed representations. |
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Besides calling, there's also overriding, which the current proposal doesn't handle (or does it?), but perhaps it should be mentioned explicitly.
udalov
reviewed
Sep 12, 2024
proposals/jvm-expose-boxed.md
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| The current compilation scheme transforms every operation where a value class is involved into a static function that takes the unboxed variants, and whose name is mangled to prevent clashes. This means those operations are not available for Java consumers. We propose introducing a new `@JvmExposeBoxed` annotation that exposes a variant of the function taking and returning the boxed versions instead (if more than one argument or return type is a value class, the aforementioned variant uses the boxed versions for _all_ of them). We call it the _boxed variant_ of the function. | ||
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| ```kotlin | ||
| annotation class JvmExposedBoxed(val jvmName: String = "") |
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It would be nice to specify retention and targets explicitly here.
udalov
reviewed
Sep 12, 2024
proposals/jvm-expose-boxed.md
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| annotation class JvmExposedBoxed(val jvmName: String = "") | ||
| ``` | ||
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| The `@JvmExposeBoxed` annotation may be applied to a declaration, or a declaration container (classes, files). In the latter case, it should be taken as applied to every single declaration within it. |
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Is it allowed on open members? If yes, it kind of makes KT-31420 partially supported which is a bit suspicious. In particular, we should probably examine all problems keeping us from implementing KT-31420 and see how they affect @JvmExposeBoxed.
udalov
reviewed
Sep 16, 2024
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| > There is a corner case in which `@JvmExposeBoxed` with a name is always needed: when the function has no argument which is a value class, but returns a value class. | ||
| > In that case the compilation scheme performs no mangling. As a result, without the annotation the Java compiler would produce an ambiguity error while resolving the name. |
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This is not fully correct, mangling is still performed for class members.
ilmirus
reviewed
Nov 14, 2024
| CONSTRUCTOR, PROPERTY, FUNCTION, // callables | ||
| CLASS, FILE, // containers | ||
| ) | ||
| annotation class JvmExposedBoxed(val jvmName: String = "", val expose: Boolean = true) |
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Typo - JvmExposedBoxed instead of JvmExposeBoxed.
ilmirus
approved these changes
Nov 19, 2024
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Only comment here about the text itself. The discussion about the feature is done in #394.
We propose modifications to how value classes are exposed in JVM, with the goal of easier consumption from Java. This includes exposing the constructor publicly and giving the ability to expose boxed variants of operations.