SIP-XX Allow Partial Function Literals to be defined with Parentheses

content/partial-function-parens.md

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+---
+layout: sip
+permalink: /sips/:title.html
+stage: implementation
+status: under-review
+title: SIP-XX Allow Partial Function Literals to be defined with Parentheses
+---
+
+**By: Li Haoyi**
+
+## History
+
+| Date          | Version            |
+|---------------|--------------------|
+| Aug 22nd 2025 | Initial Draft      |
+
+## Summary
+
+This proposal is to allow parens `(...)` to be used instead of curly braces `{...}`
+when defining partial functions which have only one `case`:
+
+
+```scala
+Seq((1, 2), (3, 4)).collect(case (a, b) if b > 2 => a) // 3
+```
+
+Currently this syntax is disallowed:
+
+```scala
+scala> Seq((1, 2), (3, 4)).collect(case (a, b) if b > 2 => a)
+-- [E018] Syntax Error: --------------------------------------------------------
+1 |Seq((1, 2), (3, 4)).collect(case (a, b) if b > 2 => a)
+  |                            ^^^^
+  |                            expression expected but case found
+  |
+  | longer explanation available when compiling with `-explain`
+```
+
+Partial functions with multiple `case` blocks should also be allowed to use parentheses
+:
+
+
+```scala
+Seq((1, 2), (3, 4)).collect(
+  case (a, b) if b > 2 => a
+  case _ => ???
+)
+```
+
+Multi-line `case` blocks should work with parens as well, just like multi-line function
+literals without `case` already work:
+
+```scala
+Seq((1, 2), (3, 4)).collect(
+  case (a, b) =>
+    println(b)
+    a
+)
+
+// This already works today
+Seq((1, 2), (3, 4)).collect(
+  (a, b) =>
+    println(b)  
+    a
+)
+```
+
+For consistency, we also allow parentheses to be used in `match` statements:
+
+```scala
+(1, 2) match (
+  case (a, b) if b > 2 => a
+  case _ => ???
+)
+```
+
+And we should allow multi-`case` single-line partial functions, just like you can already do
+with `match` today:
+
+```scala
+Seq((1, 2), (3, 4)).collect{ case (a, b) if b > 2 => a case (a, b) if a > 2 => b }
+Seq((1, 2), (3, 4)).collect(case (a, b) if b > 2 => a case (a, b) if a > 2 => b)
+
+// This already works today
+(1, 2) match { case (a, b) if b > 2 => a case _ => ??? }
+```
+
+
+Partial function literals should also be allowed to be defined without parentheses for
+single-line scenarios such as:
+
+```scala
+val partial: PartialFunction[(Int, Int), Int] = case (a, b) if b > 2 => a
+```
+
+This delimiter-less syntax is similar to what is already allowed today in `catch` blocks:
+
+```scala
+try ???
+catch case e: Exception => ???
+```
+
+
+## Motivation
+
+With Scala 3's [Optional Braces](https://docs.scala-lang.org/scala3/reference/other-new-features/indentation.html),
+and [SIP-44's Fewer Braces](https://docs.scala-lang.org/sips/fewer-braces.html), single-line
+partial functions are one of the only remaining places where curly braces are mandatory in Scala
+syntax.
+
+```scala
+// No way to write this without curlies
+Seq((1, 2), (3, 4)).collect{ case (a, b) if b > 2 => a } 
+```
+Needing to swap between parens and curlies also adds friction of converting a function to 
+a partial function, which happens frequently:
+
+```scala
+Seq((1, 2), (3, 4)).map((a, b) => a) // OK
+Seq((1, 2), (3, 4)).collect(case (a, b) if b > 2 => a) // BAD
+Seq((1, 2), (3, 4)).collect{ case (a, b) if b > 2 => a } // OK
+```
+
+This also currently causes visual messiness in method call chains where some single-line 
+methods use parens and others use curlies:
+
+```scala
+Seq((1, 2), (3, 4), (5, 6))
+  .filter(_._1 < 5) // PARENS
+  .collect{ case (a, b) if b > 2 => a } // CURLIES
+  .reduce(_ + _) // PARENS
+```
+
+In the syntax of other expressions, curly braces are only
+necessary to define "blocks" with multiple statements: `if`-`else`, `try`-`catch`-`finally`,
+`for`-`yield`, `do`-`while`, method calls like `foo()` etc. all allow you to replace curly braces
+with parentheses or elide them altogether using indentation.
+This is unlike other languages like Java that mandate curly braces in these syntactic constructs.
+Furthermore, in most expressions, Optional Braces means you do not have to write the curlies
+if you do not want to.
+
+This proposal brings partial functions in-line with the rest of Scala syntax, with the curly
+braces only being mandatory for multi-statement blocks, and made optional with Scala 3's
+Optional Braces. With this proposal, curly braces are _only_ for opening multi-statement blocks,
+_always_ in places that can be replaced by indentation-based blocks if the user wants to do so.
+They are no longer also incidentally tied to partial-function syntax as they were before.
+
+With this change, all the snippets below are now valid: we can see how the syntax of `()`, `{}`,
+or `.collect:` followed by indentation is now fully orthogonal to the partial function `case` 
+expression within them, resulting in a much more regular syntax than before when parentheses
+were prohibited but the other syntaxes worked.
+
+```scala
+Seq((1, 2), (3, 4)).collect(case (a, b) if b > 2 => a)
+Seq((1, 2), (3, 4)).collect { case (a, b) if b > 2 => a }
+Seq((1, 2), (3, 4)).collect: 
+  case (a, b) if b > 2 => a
+
+Seq((1, 2), (3, 4)).collect( 
+  case (a, b) if b > 2 => a
+  case (a, b) if a > 2 => b
+)
+
+Seq((1, 2), (3, 4)).collect {
+  case (a, b) if b > 2 => a
+  case (a, b) if a > 2 => b
+}
+
+Seq((1, 2), (3, 4)).collect: 
+  case (a, b) if b > 2 => a
+  case (a, b) if a > 2 => b
+
+Seq((1, 2), (3, 4)).collect(
+  case (a, b) =>
+    println(b)
+    a
+)
+
+Seq((1, 2), (3, 4)).map((a, b) => a)
+Seq((1, 2), (3, 4)).map { (a, b) => a }
+Seq((1, 2), (3, 4)).map:
+  (a, b) => a
+Seq((1, 2), (3, 4)).map:
+  (a, b) =>
+    println(b)
+    a
+
+Seq((1, 2), (3, 4)).collect(
+  case (a, b) if b > 2 =>
+    println(b)
+    a
+)
+```