diff --git a/docs/docs/release-notes/syntax-changes-0.22.md b/docs/docs/release-notes/syntax-changes-0.22.md new file mode 100644 index 000000000000..8037a2f0706b --- /dev/null +++ b/docs/docs/release-notes/syntax-changes-0.22.md @@ -0,0 +1,110 @@ +--- +layout: doc-page +title: Syntax Changes in Dotty 0.22 +--- + +In 2019, we experimented with several syntax changes in Dotty, most notably in the area of +implicits replacements. In Dotty 0.22, released on Feb 5, 2020, we have settled on what +we believe will be the definite syntax for Scala 3. Dotty 0.23 will support only this +syntax. Previous variants will no longer be supported. + +Here is a rundown of how previous variants need to be rewritten in the +new syntax. This will be useful as a migration and learning help for people who have already +written code in one of the previous versions of Dotty. + +## Given Instances + +Given instances are always written with `given` and `as`. The previous use of `:` instead of `as` is no longer supported. Examples: +```scala +given intOrd as Ordering[Int] { ... } +given [T] as Ordering[List[T]] { ... } +given ctx as ExecutionContext = ... +given Ordering[String] { ... } +``` +`as` can be omitted if the instance is anonymous and does not have parameters, as in the last definition above. + +## Context Parameters + +Context parameters are the replacement of Scala 2's implicit parameters. Context parameters and arguments both start with `using`. Examples: +```scala +def max[T](x: T, y: T)(using Ordering[T]): T = ... +given [T](using Ordering[T]) as Ordering[List[T]] { ... } + +max(a, b)(using intOrd) +``` +The previous syntax that uses `given` also for context parameters and arguments is no longer supported. + +Context bounds remain supported as a shorthand for one-parameter typeclass constraints. So the two definitions above could also be written as +```scala +def max[T: Ordering](x: T, y: T): T = ... +given [T: Ordering] as Ordering[List[T]] { ... } +``` + +## Context Functions + +Implicit function types `implicit A => B` have been replaced with context function types, which are written `A ?=> B`. The syntax `(given A) => B` that was used in earlier Dotty versions is no longer supported. + +The same change applies to context function values. It's now +```scala +(x: A) ?=> E +``` +instead of `(implicit x: A) => E` or `(given x: A) => E`. + +## Given Imports + +The syntax of wildcard given import selectors is now `given _`. Examples +```scala +import a.{given _} +import b.{_, given _} +``` +The previous syntax, which used just `given` without an underscore is no longer supported. The change was made to better align with typed given import selectors such as `given Ordering[T]`, +which are unchanged. + +## Collective Extensions + +Collective extensions are now a separate construct. Example: +```scala +extension [T] on List[T] { + def second: T ... + def takeRightWhile(p: T => Boolean): List[T] = ... +} +``` +Collective extensions still _expand_ to given instances with regular extension methods, but the previous syntaxes that expressed them as some syntactic variant of given instances are no longer supported. + +## Extension Methods + +There have been two syntax changes for regular extension methods. First, +any type parameters are now written in front, following the `def`. Second, +a "`.`" in front of the method name is now allowed (but not required). Example: +```scala +def [T](xs: List[T]).second: T +``` +The previous syntax which used type parameters after the method name is no longer supported. + +## Optional Braces For Definitions + +Braces around the definitions of a class, object or similar construct can now be omitted +if the leading signature of the definition is followed by a `:` at the end a line. Examples: +```scala +trait Text: + def toString: String + +class Str(str: String) extends Text: + def toString = str + +class Append(txt1: Text, txt2: Text) extends Text: + def toString = txt1 ++ txt2 + +object Empty extends Text: + def toString = "" + +extension on (t: Text): + def length = toString.length + +given Ordering[Text]: + def compare(txt1: Text, txt2: Text): Int = + summon[Ordering[String]].compare(txt1.toString, txt2.toString) +``` +Previous variants required a `with` instead of the `:` or inserted braces around indented code after class, object, ... without any leading token. These are no longer supported. + +Note that this interpretation of `:` as an alternative to braces only applies to class-level definitions. The use of `:` at the end of a line to imply braces around a following _function argument_ is not affected by this change. It still requires the `Yindent-colons` option.