Scala 2.13 Scalafix: ExplicitResultTypes for implicit definitions
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rtyley
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Scala 2.13
This is a precursor to: * #536 Scala 2.13.11 added a warning that implicit definitions [should]( https://nrinaudo.github.io/scala-best-practices/tricky_behaviours/type_implicits.html) have an explicit type (because implicit resolution is already complicated enough for the compiler, and things like file-order can actually make a difference to whether implicit scopes are correctly searched: scala/bug#8697 (comment)): * scala/scala#10083 The warning looks like this: ``` [error] ~/code/presence-indicator/app/autoscaling/Notification.scala:7:16: Implicit definition should have explicit type (inferred play.api.libs.json.Reads[autoscaling.Notification]) [quickfixable] [error] implicit val jsonReads = Json.reads[Notification] [error] ^ ``` ...this prepares us for Scala 3, where the explicit type is _required_. More widely, beyond implicit definitions, in **library** code, the best practice is to always add an explicit type to all your **public** members, even when you're happy with what's being inferred - otherwise you can unintentionally break binary-compatibility just by changing the _implementation_ of a field: * https://docs.scala-lang.org/scala3/guides/migration/incompat-type-inference.html * https://nrinaudo.github.io/scala-best-practices/binary_compat/explicit_type_annotations.html * https://scalacenter.github.io/scalafix/docs/rules/ExplicitResultTypes.html ## Automatically fixing this code issue Scalafix actually does a better job than `-quickfix` for this particular task, because it adds imports if it needs to, so that you end up with this in your code: ``` implicit val jsonReads Reads[Notification] = Json.reads[Notification] ``` ...rather than something like: ``` implicit val jsonReads Reads[com.gu.blah.foo.bar.Notification] = Json.reads[Notification] ``` ### Fixing while still on Scala 2.12 - use Scalafix In this commit, we're only trying to fix the implicit definitions, so I've added this in a new `.scalafix.conf` config file: ``` ExplicitResultTypes.onlyImplicits = true ``` The Scalafix rule needs to be run while the project is still on Scala 2.12, not Scala 2.13 (otherwise sbt will say: "Error downloading ch.epfl.scala:sbt-scalafix;sbtVersion=1.0;scalaVersion=2.13:0.13.0"). Once the Scalafix plugin is made available to sbt (by adding `addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % "0.13.0")` to either `project/plugins.sbt` or `~/.sbt/1.0/plugins.sbt`), you can run these commands on the sbt prompt to automatically generate the changes in this PR: ``` scalafixEnable scalafixAll ExplicitResultTypes ```
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JamieB-gu
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This is a precursor to:
Scala 2.13.11 added a warning that implicit definitions should have an explicit type (because implicit resolution is already complicated enough for the compiler, and things like file-order can actually make a difference to whether implicit scopes are correctly searched: scala/bug#8697 (comment)):
The warning looks like this:
...this prepares us for Scala 3, where the explicit type is required.
More widely, beyond implicit definitions, in library code, the best practice is to always add an explicit type to all your public members, even when you're happy with what's being inferred - otherwise you can unintentionally break binary-compatibility just by changing the implementation of a field:
Automatically fixing this code issue
Scalafix actually does a better job than
-quickfixfor this particular task, because it adds imports if it needs to, so that you end up with this in your code:...rather than something like:
Fixing while still on Scala 2.12 - use Scalafix
In this commit, we're only trying to fix the implicit definitions, so I've added this in a new
.scalafix.confconfig file:The Scalafix rule needs to be run while the project is still on Scala 2.12, not Scala 2.13 (otherwise sbt will say: "Error downloading ch.epfl.scala:sbt-scalafix;sbtVersion=1.0;scalaVersion=2.13:0.13.0").
Once the Scalafix plugin is made available to sbt (by adding
addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % "0.13.0")to eitherproject/plugins.sbtor~/.sbt/1.0/plugins.sbt), you can run these commands on the sbt prompt to automatically generate the changes in this PR: