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The syntax of wildcard arguments in types has changed from `_` to `?`. Example:
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The syntax of wildcard arguments in types is changing from `_` to `?`. Example:
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```scala
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List[?]
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Map[?<:AnyRef, ?>:Null]
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We would like to use the underscore syntax `_` to stand for an anonymous type parameter, aligning it with its meaning in
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value parameter lists. So, just as `f(_)` is a shorthand for the lambda `x => f(x)`, in the future `C[_]` will be a shorthand
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for the type lambda `[X] =>> C[X]`. This makes higher-kinded types easier to use. It also removes the wart that, used as a type
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parameter, `F[_]` means `F` is a type constructor whereas used as a type, `F[_]` means it is a wildcard (i.e. existential) type.
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for the type lambda `[X] =>> C[X]`. This will make higher-kinded types easier to use. It will also remove the wart that, used as a type
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parameter, `F[_]` means `F` is a type constructor, whereas used as a type, `F[_]` means it is a wildcard (i.e. existential) type.
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In the future, `F[_]` will mean the same thing, no matter where it is used.
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We pick `?` as a replacement syntax for wildcard types, since it aligns with
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A step-by-step migration is made possible with the following measures:
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1. In Scala 3.0, both `_` and `?` are legal names for wildcards.
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2. In Scala 3.1, `_`is deprecated in favor of `?` as a name for a wildcard. A `-rewrite` option is
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1. In earlier versions of Scala 3, both `_` and `?` are legal names for wildcards.
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2. In Scala 3.4, `_`will be deprecated in favor of `?` as a name for wildcards. A `-rewrite` option is
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available to rewrite one to the other.
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3.In Scala 3.2, the meaning of `_`changes from wildcard to placeholder for type parameter.
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4.The Scala 3.1 behavior is already available today under the `-source future` setting.
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3.At some later point in the future, the meaning of `_`will change from wildcard to placeholder for type parameters.
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4.Some deprecation warnings are already available under the `-source future` setting.
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To smooth the transition for codebases that use kind-projector, we adopt the following measures under the command line
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option `-Ykind-projector`:
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available to rewrite one to the other.
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3. In Scala 3.3, `*` is removed again, and all type parameter placeholders will be expressed with `_`.
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These rules make it possible to crossbuild between Scala 2 using the kind projector plugin and Scala 3.0 - 3.2 using the compiler option `-Ykind-projector`.
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These rules make it possible to cross-build between Scala 2 using the kind projector plugin and Scala 3.0 - 3.2 using the compiler option `-Ykind-projector`.
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There is also a migration path for users that want a one-time transition to syntax with `_` as a type parameter placeholder.
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With option `-Ykind-projector:underscores` Scala 3 will regard `_` as a type parameter placeholder, leaving `?` as the only syntax for wildcards.
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