Support ? as wildcard marker under -Xsource:3
#9560
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Like in Scala 3.0, this allows `?` to be used as a type argument in all situations where `_` could be used as a wildcard previously. This should allow us to deprecate the use of `_` as a wildcard in Scala 3 to be able to eventually repurpose it as explained in http://dotty.epfl.ch/docs/reference/changed-features/wildcards.html This is a source-breaking change since a type named `?` is legal in Scala 2 (but not in Scala 3 unless -source 3.0-migration is used). `?` also has a special meaning when the kind-projector plugin is used, but that usage has been deprecated in favor of `*` for a while now.
Unfortunately it looks like it was deprecated in the docs since Sep 2019 (typelevel/kind-projector#109) but only emits a deprecation warnings since the end of November 2020 (i.e. https://github.com/typelevel/kind-projector/releases/tag/v0.11.2) 😕 |
smarter
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Apr 30, 2021
Scala 2.13.6 and 2.12.14 will interpret `?` as a wildcard when using the `-Xsource:3` flag (cf scala/scala#9560)). This means that the old kind-projector syntax will no longer work, so it seems like a good time to remove it. This will also allow us to compile more of the community-build with `-Xsource:3` enabled (cf scala/scala-dev#769). Sincet this is a breaking change, we also bump the version to 0.12.0-SNAPSHOT.
neko-kai
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May 7, 2021
…ymous type lambdas The syntax roughly follows the [proposed new syntax for wildcards and placeholders](https://dotty.epfl.ch/docs/reference/changed-features/wildcards.html#migration-strategy) for Scala 3.2+ and is designed to allow cross-compilation of libraries between Scala 2 and Scala 3 while using the new Scala 3 syntax for both versions. To enable this mode, add `-P:kind-projector:underscore-placeholders` to your scalac command-line. In sbt you may do this as follows: ```scala ThisBuild / scalacOptions += "-P:kind-projector:underscore-placeholders" ``` This mode is designed to be used with scalac versions `2.12.14`+ and `2.13.6`+, these versions add an the ability to use `?` as the existential type wildcard ([scala/scala#9560](scala/scala#9560)), allowing to repurpose the underscore without losing the ability to write existential types. It is not advised that you use this mode with older versions of scalac or without `-Xsource:3` flag, since you will lose the underscore syntax entirely. Here are a few examples: ```scala Tuple2[_, Double] // equivalent to: type R[A] = Tuple2[A, Double] Either[Int, +_] // equivalent to: type R[+A] = Either[Int, A] Function2[-_, Long, +_] // equivalent to: type R[-A, +B] = Function2[A, Long, B] EitherT[_[_], Int, _] // equivalent to: type R[F[_], B] = EitherT[F, Int, B] ``` Examples with `-Xsource:3`'s `?`-wildcard: ```scala Tuple2[_, ?] // equivalent to: type R[A] = Tuple2[A, x] forSome { type x } Either[?, +_] // equivalent to: type R[+A] = Either[x, A] forSome { type x } Function2[-_, ?, +_] // equivalent to: type R[-A, +B] = Function2[A, x, B] forSome { type x } EitherT[_[_], ?, _] // equivalent to: type R[F[_], B] = EitherT[F, x, B] forSome { type x } ```
neko-kai
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May 7, 2021
…ymous type lambdas The syntax roughly follows the [proposed new syntax for wildcards and placeholders](https://dotty.epfl.ch/docs/reference/changed-features/wildcards.html#migration-strategy) for Scala 3.2+ and is designed to allow cross-compilation of libraries between Scala 2 and Scala 3 while using the new Scala 3 syntax for both versions. To enable this mode, add `-P:kind-projector:underscore-placeholders` to your scalac command-line. In sbt you may do this as follows: ```scala ThisBuild / scalacOptions += "-P:kind-projector:underscore-placeholders" ``` This mode is designed to be used with scalac versions `2.12.14`+ and `2.13.6`+, these versions add an the ability to use `?` as the existential type wildcard ([scala/scala#9560](scala/scala#9560)), allowing to repurpose the underscore without losing the ability to write existential types. It is not advised that you use this mode with older versions of scalac or without `-Xsource:3` flag, since you will lose the underscore syntax entirely. Here are a few examples: ```scala Tuple2[_, Double] // equivalent to: type R[A] = Tuple2[A, Double] Either[Int, +_] // equivalent to: type R[+A] = Either[Int, A] Function2[-_, Long, +_] // equivalent to: type R[-A, +B] = Function2[A, Long, B] EitherT[_[_], Int, _] // equivalent to: type R[F[_], B] = EitherT[F, Int, B] ``` Examples with `-Xsource:3`'s `?`-wildcard: ```scala Tuple2[_, ?] // equivalent to: type R[A] = Tuple2[A, x] forSome { type x } Either[?, +_] // equivalent to: type R[+A] = Either[x, A] forSome { type x } Function2[-_, ?, +_] // equivalent to: type R[-A, +B] = Function2[A, x, B] forSome { type x } EitherT[_[_], ?, _] // equivalent to: type R[F[_], B] = EitherT[F, x, B] forSome { type x } ```
smarter
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May 10, 2021
scala#9560 introduced a new meaning for `?` under `-Xsource:3`, but to smooth out the migration it'd be nice if we could also enable this meaning by default. Before doing so, let's deprecate any current usage of `?` as a type that isn't wrapped in backticks.
smarter
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May 10, 2021
scala#9560 introduced a new meaning for `?` under `-Xsource:3`, but to smooth out the migration it'd be nice if we could also enable this meaning by default. Before doing so, let's deprecate any current usage of `?` as a type that isn't wrapped in backticks.
smarter
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May 10, 2021
scala#9560 introduced a new meaning for `?` under `-Xsource:3`, but to smooth out the migration it'd be nice if we could also enable this meaning by default. Before doing so, let's deprecate any current usage of `?` as a type that isn't wrapped in backticks.
lrytz
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May 11, 2021
scala#9560 introduced a new meaning for `?` under `-Xsource:3`, but to smooth out the migration it'd be nice if we could also enable this meaning by default. Before doing so, let's deprecate any current usage of `?` as a type that isn't wrapped in backticks.
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Like in Scala 3.0, this allows
?to be used as a type argument in allsituations where
_could be used as a wildcard previously. This shouldallow us to deprecate the use of
_as a wildcard in Scala 3 to be ableto eventually repurpose it as explained in
http://dotty.epfl.ch/docs/reference/changed-features/wildcards.html
This is a source-breaking change since a type named
?is legal inScala 2 (but not in Scala 3 unless -source 3.0-migration is used).
?also has a special meaning when the kind-projector plugin is used,but that usage has been deprecated in favor of
*for a while now.