Merge pull request #7610 from dotty-staging/change-match-syntax

Change match syntax

Author: odersky

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -951,7 +951,7 @@ object Parsers {
       recur(top)
     }
 
-    /**   operand { infixop operand | ‘given’ (operand | ParArgumentExprs) } [postfixop],
+    /**   operand { infixop operand | MatchClause } [postfixop],
      *
      *  respecting rules of associativity and precedence.
      *  @param isOperator    the current token counts as an operator.
@@ -981,18 +981,14 @@ object Parsers {
           }
           else recur(operand())
         }
-        else reduceStack(base, top, minPrec, leftAssoc = true, in.name, isType)
+        else
+          val t = reduceStack(base, top, minPrec, leftAssoc = true, in.name, isType)
+          if !isType && in.token == MATCH then recur(matchClause(t))
+          else t
 
       recur(first)
     }
 
-    def applyGiven(t: Tree, operand: () => Tree): Tree =
-      atSpan(startOffset(t), in.offset) {
-        in.nextToken()
-        val args = if (in.token == LPAREN) parArgumentExprs()._1 else operand() :: Nil
-        Apply(t, args)
-      }.setGivenApply()
-
 /* -------- IDENTIFIERS AND LITERALS ------------------------------------------- */
 
     /** Accept identifier and return its name as a term name. */
@@ -1028,9 +1024,11 @@ object Parsers {
     def wildcardIdent(): Ident =
       atSpan(accept(USCORE)) { Ident(nme.WILDCARD) }
 
-    /** Accept identifier acting as a selector on given tree `t`. */
+    /** Accept identifier or match clause acting as a selector on given tree `t` */
     def selector(t: Tree): Tree =
-      atSpan(startOffset(t), in.offset) { Select(t, ident()) }
+      atSpan(startOffset(t), in.offset) {
+        if in.token == MATCH then matchClause(t) else Select(t, ident())
+      }
 
     /** Selectors ::= id { `.' id }
      *
@@ -1737,11 +1735,10 @@ object Parsers {
      *                      |  HkTypeParamClause ‘=>’ Expr
      *                      |  [SimpleExpr `.'] id `=' Expr
      *                      |  SimpleExpr1 ArgumentExprs `=' Expr
-     *                      |  Expr2
-     *                      |  [‘inline’] Expr2 `match' (`{' CaseClauses `}' | CaseClause)
+     *                      |  PostfixExpr [Ascription]
+     *                      |  ‘inline’ InfixExpr MatchClause
      *  Bindings          ::=  `(' [Binding {`,' Binding}] `)'
      *  Binding           ::=  (id | `_') [`:' Type]
-     *  Expr2             ::=  PostfixExpr [Ascription]
      *  Ascription        ::=  `:' InfixType
      *                      |  `:' Annotation {Annotation}
      *                      |  `:' `_' `*'
@@ -1780,7 +1777,7 @@ object Parsers {
       }
     }
 
-    def expr1(location: Location.Value = Location.ElseWhere): Tree = in.token match {
+    def expr1(location: Location.Value = Location.ElseWhere): Tree = in.token match
       case IF =>
         in.endMarkerScope(IF) { ifExpr(in.offset, If) }
       case WHILE =>
@@ -1886,31 +1883,28 @@ object Parsers {
             case IF =>
               ifExpr(start, InlineIf)
             case _ =>
-              val t = postfixExpr()
-              if (in.token == MATCH) matchExpr(t, start, InlineMatch)
-              else
-                syntaxErrorOrIncomplete(i"`match` or `if` expected but ${in.token} found")
-                t
+              postfixExpr() match
+                case t @ Match(scrut, cases) =>
+                  InlineMatch(scrut, cases).withSpan(t.span)
+                case t =>
+                  syntaxError(em"`inline` must be followed by an `if` or a `match`", start)
+                  t
         else expr1Rest(postfixExpr(), location)
-    }
+    end expr1
 
-    def expr1Rest(t: Tree, location: Location.Value): Tree = in.token match {
+    def expr1Rest(t: Tree, location: Location.Value): Tree = in.token match
       case EQUALS =>
-         t match {
-           case Ident(_) | Select(_, _) | Apply(_, _) =>
-             atSpan(startOffset(t), in.skipToken()) { Assign(t, subExpr()) }
-           case _ =>
-             t
-         }
+        t match
+          case Ident(_) | Select(_, _) | Apply(_, _) =>
+            atSpan(startOffset(t), in.skipToken()) { Assign(t, subExpr()) }
+          case _ =>
+            t
       case COLON =>
         in.nextToken()
-        val t1 = ascription(t, location)
-        if (in.token == MATCH) expr1Rest(t1, location) else t1
-      case MATCH =>
-        matchExpr(t, startOffset(t), Match)
+        ascription(t, location)
       case _ =>
         t
-    }
+    end expr1Rest
 
     def ascription(t: Tree, location: Location.Value): Tree = atSpan(startOffset(t)) {
       in.token match {
@@ -1951,20 +1945,20 @@ object Parsers {
         mkIf(cond, thenp, elsep)
       }
 
-    /**    `match' (`{' CaseClauses `}' | CaseClause)
+    /**    MatchClause ::= `match' `{' CaseClauses `}'
      */
-    def matchExpr(t: Tree, start: Offset, mkMatch: (Tree, List[CaseDef]) => Match) =
+    def matchClause(t: Tree): Match =
       in.endMarkerScope(MATCH) {
-        atSpan(start, in.skipToken()) {
-          mkMatch(t, casesExpr(caseClause))
+        atSpan(t.span.start, in.skipToken()) {
+          Match(t, inBracesOrIndented(caseClauses(caseClause)))
         }
       }
 
-    /**    `match' (`{' TypeCaseClauses `}' | TypeCaseClause)
+    /**    `match' `{' TypeCaseClauses `}'
      */
     def matchType(t: Tree): MatchTypeTree =
       atSpan(t.span.start, accept(MATCH)) {
-        MatchTypeTree(EmptyTree, t, casesExpr(typeCaseClause))
+        MatchTypeTree(EmptyTree, t, inBracesOrIndented(caseClauses(typeCaseClause)))
       }
 
     /** FunParams         ::=  Bindings
@@ -2032,7 +2026,7 @@ object Parsers {
     /** PostfixExpr   ::= InfixExpr [id [nl]]
      *  InfixExpr     ::= PrefixExpr
      *                  | InfixExpr id [nl] InfixExpr
-     *                  | InfixExpr ‘given’ (InfixExpr | ParArgumentExprs)
+     *                  | InfixExpr MatchClause
      */
     def postfixExpr(): Tree = postfixExprRest(prefixExpr())
 
@@ -2063,6 +2057,7 @@ object Parsers {
      *                 |  Path
      *                 |  `(' [ExprsInParens] `)'
      *                 |  SimpleExpr `.' id
+     *                 |  SimpleExpr `.' MatchClause
      *                 |  SimpleExpr (TypeArgs | NamedTypeArgs)
      *                 |  SimpleExpr1 ArgumentExprs
      *  Quoted        ::= ‘'’ ‘{’ Block ‘}’
@@ -2395,10 +2390,6 @@ object Parsers {
       }
     }
 
-    def casesExpr(clause: () => CaseDef): List[CaseDef] =
-      if in.token == CASE then clause() :: Nil
-      else inBracesOrIndented(caseClauses(clause))
-
     /** CaseClauses         ::= CaseClause {CaseClause}
      *  TypeCaseClauses     ::= TypeCaseClause {TypeCaseClause}
      */

compiler/src/dotty/tools/dotc/parsing/Scanners.scala

@@ -524,7 +524,8 @@ object Scanners {
         else if (lastWidth != nextWidth)
           errorButContinue(spaceTabMismatchMsg(lastWidth, nextWidth))
       currentRegion match {
-        case Indented(curWidth, others, prefix, outer) if curWidth < nextWidth && !others.contains(nextWidth) =>
+        case Indented(curWidth, others, prefix, outer)
+        if curWidth < nextWidth && !others.contains(nextWidth) && nextWidth != lastWidth =>
           if (token == OUTDENT)
             errorButContinue(
               i"""The start of this line does not match any of the previous indentation widths.

docs/docs/internals/syntax.md

@@ -147,8 +147,7 @@ FunArgTypes       ::=  InfixType
                     |  ‘(’ [ ‘[given]’ FunArgType {‘,’ FunArgType } ] ‘)’
                     |  ‘(’ ‘[given]’ TypedFunParam {‘,’ TypedFunParam } ‘)’
 TypedFunParam     ::=  id ‘:’ Type
-MatchType         ::=  InfixType `match`
-                       (‘{’ TypeCaseClauses ‘}’ | TypeCaseClause)
+MatchType         ::=  InfixType `match` ‘{’ TypeCaseClauses ‘}’
 InfixType         ::=  RefinedType {id [nl] RefinedType}                        InfixOp(t1, op, t2)
 RefinedType       ::=  WithType {[nl | ‘with’] Refinement}                      RefinedTypeTree(t, ds)
 WithType          ::=  AnnotType {‘with’ AnnotType}                             (deprecated)
@@ -185,9 +184,8 @@ BlockResult       ::=  [‘implicit’] FunParams ‘=>’ Block
 FunParams         ::=  Bindings
                     |  id
                     |  ‘_’
-Expr1             ::=  ‘if’ ‘(’ Expr ‘)’ {nl}
-                       Expr [[semi] ‘else’ Expr]                                If(Parens(cond), thenp, elsep?)
-                    |  ‘if’ Expr ‘then’ Expr [[semi] ‘else’ Expr]               If(cond, thenp, elsep?)
+Expr1             ::=  [‘inline’] ‘if’ ‘(’ Expr ‘)’ {nl} Expr [[semi] ‘else’ Expr] If(Parens(cond), thenp, elsep?)
+                    |  [‘inline’] ‘if’  Expr ‘then’ Expr [[semi] ‘else’ Expr]    If(cond, thenp, elsep?)
                     |  ‘while’ ‘(’ Expr ‘)’ {nl} Expr                           WhileDo(Parens(cond), body)
                     |  ‘while’ Expr ‘do’ Expr                                   WhileDo(cond, body)
                     |  ‘try’ Expr Catches [‘finally’ Expr]                      Try(expr, catches, expr?)
@@ -198,17 +196,16 @@ Expr1             ::=  ‘if’ ‘(’ Expr ‘)’ {nl}
                     |  HkTypeParamClause ‘=>’ Expr                              PolyFunction(ts, expr)
                     |  [SimpleExpr ‘.’] id ‘=’ Expr                             Assign(expr, expr)
                     |  SimpleExpr1 ArgumentExprs ‘=’ Expr                       Assign(expr, expr)
-                    |  Expr2
-                    |  [‘inline’] Expr2 ‘match’
-                       (‘{’ CaseClauses ‘}’ | CaseClause)                       Match(expr, cases) -- point on match
-Expr2             ::=  PostfixExpr [Ascription]
+                    |  PostfixExpr [Ascription]
+                    |  ‘inline’ InfixExpr MatchClause
 Ascription        ::=  ‘:’ InfixType                                            Typed(expr, tp)
                     |  ‘:’ Annotation {Annotation}                              Typed(expr, Annotated(EmptyTree, annot)*)
 Catches           ::=  ‘catch’ (Expr | CaseClause)
 PostfixExpr       ::=  InfixExpr [id]                                           PostfixOp(expr, op)
 InfixExpr         ::=  PrefixExpr
                     |  InfixExpr id [nl] InfixExpr                              InfixOp(expr, op, expr)
-                    |  InfixExpr ‘given’ (InfixExpr | ParArgumentExprs)
+                    |  InfixExpr MatchClause
+MatchClause       ::=  ‘match’ ‘{’ CaseClauses ‘}’                   Match(expr, cases)
 PrefixExpr        ::=  [‘-’ | ‘+’ | ‘~’ | ‘!’] SimpleExpr                       PrefixOp(expr, op)
 SimpleExpr        ::=  Path
                     |  Literal
@@ -221,6 +218,7 @@ SimpleExpr        ::=  Path
                     |  ‘new’ TemplateBody
                     |  ‘(’ ExprsInParens ‘)’                                    Parens(exprs)
                     |  SimpleExpr ‘.’ id                                        Select(expr, id)
+                    |  SimpleExpr ‘.’ MatchClause
                     |  SimpleExpr TypeArgs                                      TypeApply(expr, args)
                     |  SimpleExpr ArgumentExprs                                 Apply(expr, args)
                     |  SimpleExpr ‘_’                                           PostfixOp(expr, _)

docs/docs/reference/changed-features/match-syntax.md

@@ -0,0 +1,43 @@
+---
+layout: doc-page
+title: Match Expressions
+---
+
+The syntactical precedence of match expressions has been changed.
+`match` is still a keyword, but it is used like an alphabetical operator. This has several consequences:
+
+ 1. `match` expressions can be chained:
+
+    ```scala
+    xs match {
+      case Nil => "empty"
+      case x :: xs1 => "nonempty"
+    } match {
+      case "empty" => 0
+      case "nonempty" => 1
+    }
+
+ 2. `match` may follow a period:
+
+     ```scala
+     if xsDefined
+        && xs.match {
+             case Nil => false
+             case _ => true
+           }
+     then "nonempty"
+     else "empty"
+
+ 3. The scrutinee of a match expression must be an `InfixExpr`. Previously the scrutinee could be followed by a type ascription `: T`, but this is no longer supported. So `x : T match { ... }` now has to be
+ written `(x: T) match { ... }`.
+
+## Syntax
+
+The new syntax of match expressions is as follows.
+```
+InfixExpr    ::=  ...
+               |  InfixExpr MatchClause
+SimpleExpr   ::=  ...
+               |  SimpleExpr ‘.’ MatchClause
+MatchClause  ::=  ‘match’ ‘{’ CaseClauses ‘}’
+```

docs/docs/reference/contextual/typeclasses.md

@@ -10,25 +10,22 @@ with canonical implementations defined by given instances. Here are some example
 ### Semigroups and monoids:
 
 ```scala
-trait SemiGroup[T] {
+trait SemiGroup[T] with
   def (x: T) combine (y: T): T
-}
-trait Monoid[T] extends SemiGroup[T] {
+
+trait Monoid[T] extends SemiGroup[T] with
   def unit: T
-}
-object Monoid {
+
+object Monoid with
   def apply[T](given Monoid[T]) = summon[Monoid[T]]
-}
 
-given Monoid[String] {
+given Monoid[String] with
   def (x: String) combine (y: String): String = x.concat(y)
   def unit: String = ""
-}
 
-given Monoid[Int] {
+given Monoid[Int] with
   def (x: Int) combine (y: Int): Int = x + y
   def unit: Int = 0
-}
 
 def sum[T: Monoid](xs: List[T]): T =
     xs.foldLeft(Monoid[T].unit)(_.combine(_))

docs/sidebar.yml

@@ -127,6 +127,8 @@ sidebar:
               url: docs/reference/changed-features/implicit-conversions.html
             - title: Overload Resolution
               url: docs/reference/changed-features/overload-resolution.html
+            - title: Match Expressions
+              url: docs/reference/changed-features/match-syntax.html
             - title: Vararg Patterns
               url: docs/reference/changed-features/vararg-patterns.html
             - title: Pattern Bindings

tests/neg/cannot-reduce-inline-match.check

@@ -1,11 +1,11 @@
 -- Error: tests/neg/cannot-reduce-inline-match.scala:3:13 --------------------------------------------------------------
 3 |    inline x match { // error
-  |    ^
-  |    cannot reduce inline match with
-  |     scrutinee:  {
-  |      "f"
-  |    } : String("f")
-  |     patterns :  case _:Int
+  |           ^
+  |           cannot reduce inline match with
+  |            scrutinee:  {
+  |             "f"
+  |           } : String("f")
+  |            patterns :  case _:Int
   | This location is in code that was inlined at cannot-reduce-inline-match.scala:9
 4 |      case _: Int =>
 5 |    }

tests/pos-special/fatal-warnings/unchecked-scrutinee.scala

@@ -1,5 +1,5 @@
 object Test {
-  List(1: @unchecked, 2, 3): @unchecked match {
+  (List(1: @unchecked, 2, 3): @unchecked) match {
     case a :: as =>
   }
 }

tests/pos/i6997c.scala

@@ -4,11 +4,11 @@ import scala.quoted.{_, given}, scala.quoted.matching._
 
 inline def mcr(x: => Any): Any = ${mcrImpl('x)}
 
-def mcrImpl(body: Expr[Any])(given ctx: QuoteContext): Expr[Any] = {
-  body match case '{$x: $t} =>
-  '{
-    val tmp: $t = $x
-    println(tmp)
-    tmp
-  }
-}
+def mcrImpl(body: Expr[Any])(given ctx: QuoteContext): Expr[Any] =
+  body match
+    case '{$x: $t} =>
+      '{
+        val tmp: $t = $x
+        println(tmp)
+        tmp
+      }

tests/pos/matches.scala

@@ -0,0 +1,20 @@
+package matches
+object Test with
+  2 min 3 match
+    case 2 => "OK"
+    case 3 => "?"
+  match
+    case "OK" =>
+    case "?" => assertFail()
+
+  val x = 4
+  if 2 < 3
+     || x.match
+          case 4 => true
+          case _ => false
+     || (2 + 2).match
+          case 4 => true
+          case _ => false
+  then
+    println("ok")
+end Test

tests/pos/multi-given.scala

@@ -4,3 +4,5 @@ trait C
 
 def fancy(given a: A, b: B, c: C) = "Fancy!"
 def foo(implicit a: A, b: B, c: C) = "foo"
+
+given A, B {}