Allow single-line lambdas after : (#23821)

Previously, we need to indent after the arrow, e.g.
```scala
xs.map: x =>
  x + 1
```
We now also allow to write the lambda on a single line:
```scala
xs.map: x => x + 1
```
The lambda extends to the end of the line.

This is a trial balloon to see whether anything breaks. If we want to
pursue this it needs a SIP.

Author: odersky

compiler/src/dotty/tools/dotc/config/Feature.scala

@@ -40,6 +40,7 @@ object Feature:
   val packageObjectValues = experimental("packageObjectValues")
   val multiSpreads = experimental("multiSpreads")
   val subCases = experimental("subCases")
+  val relaxedLambdaSyntax = experimental("relaxedLambdaSyntax")
 
   def experimentalAutoEnableFeatures(using Context): List[TermName] =
     defn.languageExperimentalFeatures

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

@@ -1090,27 +1090,56 @@ object Parsers {
       }
 
     /** Is the token sequence following the current `:` token classified as a lambda?
-     *  This is the case if the input starts with an identifier, a wildcard, or
-     *  something enclosed in (...) or [...], and this is followed by a `=>` or `?=>`
-     *  and an INDENT.
-     */
-    def followingIsLambdaAfterColon(): Boolean =
+     *  If yes return a defined parsing function to parse the lambda body, if not
+     *  return None. The case is triggered in two situations:
+     *   1. If the input starts with an identifier, a wildcard, or something
+     *      enclosed in (...) or [...], this is followed by a `=>` or `?=>`,
+     *      and one of the following two subcases applies:
+     *     1a. The next token is an indent. In this case the return parsing function parses
+     *         an Expr in location Location.InColonArg.
+     *     1b. Under relaxedLambdaSyntax: the next token is on the same line and the enclosing region is not `(...)`.
+     *         In this case the parsing function parses an Expr in location Location.InColonArg
+     *         enclosed in a SingleLineLambda region, and then eats the ENDlambda token
+     *         generated by the Scanner at the end of that region.
+     *     The reason for excluding (1b) in regions enclosed in parentheses is to avoid
+     *     an ambiguity with type ascription `(x: A => B)`, where function types are only
+     *     allowed inside parentheses.
+     *   2. Under relaxedLambdaSyntax: the input starts with a `case`.
+     */
+    def followingIsLambdaAfterColon(): Option[() => Tree] =
       val lookahead = in.LookaheadScanner(allowIndent = true)
                       .tap(_.currentRegion.knownWidth = in.currentRegion.indentWidth)
-      def isArrowIndent() =
-        lookahead.isArrow
-        && {
+      def isArrowIndent(): Option[() => Tree] =
+        if lookahead.isArrow then
           lookahead.observeArrowIndented()
-          lookahead.token == INDENT || lookahead.token == EOF
-        }
-      lookahead.nextToken()
-      if lookahead.isIdent || lookahead.token == USCORE then
+          if lookahead.token == INDENT || lookahead.token == EOF then
+            Some(() => expr(Location.InColonArg))
+          else if in.featureEnabled(Feature.relaxedLambdaSyntax) then
+            isParamsAndArrow() match
+              case success @ Some(_) => success
+              case _ if !in.currentRegion.isInstanceOf[InParens] =>
+                Some: () =>
+                  val t = inSepRegion(SingleLineLambda(_)):
+                    expr(Location.InColonArg)
+                  accept(ENDlambda)
+                  t
+              case _ => None
+          else None
+        else None
+      def isParamsAndArrow(): Option[() => Tree] =
         lookahead.nextToken()
-        isArrowIndent()
-      else if lookahead.token == LPAREN || lookahead.token == LBRACKET then
-        lookahead.skipParens()
-        isArrowIndent()
-      else false
+        if lookahead.isIdent || lookahead.token == USCORE then
+          lookahead.nextToken()
+          isArrowIndent()
+        else if lookahead.token == LPAREN || lookahead.token == LBRACKET then
+          lookahead.skipParens()
+          isArrowIndent()
+        else if lookahead.token == CASE && in.featureEnabled(Feature.relaxedLambdaSyntax) then
+          Some(() => singleCaseMatch())
+        else
+          None
+      isParamsAndArrow()
+    end followingIsLambdaAfterColon
 
     /** Can the next lookahead token start an operand as defined by
      *  leadingOperandTokens, or is postfix ops enabled?
@@ -1175,12 +1204,19 @@ object Parsers {
         case _ => infixOp
     }
 
-    /** True if we are seeing a lambda argument after a colon of the form:
+    /** Optionally, if we are seeing a lambda argument after a colon of the form
      *    : (params) =>
      *      body
+     *  or a single-line lambda (under relaxedLambdaSyntax)
+     *    : (params) => body
+     *  or a case clause (under relaxedLambdaSyntax)
+     *    : case pat guard => rhs
+     *  then return the function used to parse `body` or the case clause.
      */
-    def isColonLambda =
-      sourceVersion.enablesFewerBraces && in.token == COLONfollow && followingIsLambdaAfterColon()
+    def detectColonLambda: Option[() => Tree] =
+      if sourceVersion.enablesFewerBraces && in.token == COLONfollow
+      then followingIsLambdaAfterColon()
+      else None
 
     /**   operand { infixop operand | MatchClause } [postfixop],
      *
@@ -1204,17 +1240,19 @@ object Parsers {
           opStack = OpInfo(top1, op, in.offset) :: opStack
           colonAtEOLOpt()
           newLineOptWhenFollowing(canStartOperand)
-          if isColonLambda then
-            in.nextToken()
-            recur(expr(Location.InColonArg))
-          else if maybePostfix && !canStartOperand(in.token) then
-            val topInfo = opStack.head
-            opStack = opStack.tail
-            val od = reduceStack(base, topInfo.operand, 0, true, in.name, isType)
-            atSpan(startOffset(od), topInfo.offset) {
-              PostfixOp(od, topInfo.operator)
-            }
-          else recur(operand(location))
+          detectColonLambda match
+            case Some(parseExpr) =>
+              in.nextToken()
+              recur(parseExpr())
+            case _ =>
+              if maybePostfix && !canStartOperand(in.token) then
+                val topInfo = opStack.head
+                opStack = opStack.tail
+                val od = reduceStack(base, topInfo.operand, 0, true, in.name, isType)
+                atSpan(startOffset(od), topInfo.offset) {
+                  PostfixOp(od, topInfo.operator)
+                }
+              else recur(operand(location))
         else
           val t = reduceStack(base, top, minPrec, leftAssoc = true, in.name, isType)
           if !isType && in.token == MATCH then recurAtMinPrec(matchClause(t))
@@ -2358,6 +2396,7 @@ object Parsers {
 
     /** Expr              ::=  [`implicit'] FunParams (‘=>’ | ‘?=>’) Expr
      *                      |  TypTypeParamClause ‘=>’ Expr
+     *                      |  ExprCaseClause                              -- under experimental.relaxedLambdaSyntax
      *                      |  Expr1
      *  FunParams         ::=  Bindings
      *                      |  id
@@ -2409,6 +2448,8 @@ object Parsers {
           val arrowOffset = accept(ARROW)
           val body = expr(location)
           makePolyFunction(tparams, body, "literal", errorTermTree(arrowOffset), start, arrowOffset)
+        case CASE if in.featureEnabled(Feature.relaxedLambdaSyntax) =>
+          singleCaseMatch()
         case _ =>
           val saved = placeholderParams
           placeholderParams = Nil
@@ -2472,9 +2513,8 @@ object Parsers {
             if in.token == CATCH then
               val span = in.offset
               in.nextToken()
-              (if in.token == CASE then Match(EmptyTree, caseClause(exprOnly = true) :: Nil)
-                else subExpr(),
-                span)
+              (if in.token == CASE then singleCaseMatch() else subExpr(),
+               span)
             else (EmptyTree, -1)
 
           handler match {
@@ -2769,8 +2809,10 @@ object Parsers {
      *                 |  SimpleExpr (TypeArgs | NamedTypeArgs)
      *                 |  SimpleExpr1 ArgumentExprs
      *                 |  SimpleExpr1 ColonArgument
-     *  ColonArgument ::= colon [LambdaStart]
+     *  ColonArgument ::= colon {LambdaStart}
      *                    indent (CaseClauses | Block) outdent
+     *                 |  colon LambdaStart {LambdaStart} expr ENDlambda -- under experimental.relaxedLambdaSyntax
+     *                 |  colon ExprCaseClause                    -- under experimental.relaxedLambdaSyntax
      *  LambdaStart   ::= FunParams (‘=>’ | ‘?=>’)
      *                 |  TypTypeParamClause ‘=>’
      *  ColonArgBody  ::= indent (CaseClauses | Block) outdent
@@ -2853,12 +2895,14 @@ object Parsers {
                   makeParameter(name.asTermName, typedOpt(), Modifiers(), isBackquoted = isBackquoted(id))
                 }
               case _ => t
-          else if isColonLambda then
-            val app = atSpan(startOffset(t), in.skipToken()) {
-              Apply(t, expr(Location.InColonArg) :: Nil)
-            }
-            simpleExprRest(app, location, canApply = true)
-          else t
+          else detectColonLambda match
+            case Some(parseExpr) =>
+              val app =
+                atSpan(startOffset(t), in.skipToken()):
+                  Apply(t, parseExpr() :: Nil)
+              simpleExprRest(app, location, canApply = true)
+            case None =>
+              t
     end simpleExprRest
 
     /** SimpleExpr    ::=  ‘new’ ConstrApp {`with` ConstrApp} [TemplateBody]
@@ -3165,9 +3209,9 @@ object Parsers {
         case ARROW => atSpan(in.skipToken()):
           if exprOnly then
             if in.indentSyntax && in.isAfterLineEnd && in.token != INDENT then
-              warning(em"""Misleading indentation: this expression forms part of the preceding catch case.
+              warning(em"""Misleading indentation: this expression forms part of the preceding case.
                           |If this is intended, it should be indented for clarity.
-                          |Otherwise, if the handler is intended to be empty, use a multi-line catch with
+                          |Otherwise, if the handler is intended to be empty, use a multi-line match or catch with
                           |an indented case.""")
             expr()
           else block()
@@ -3184,6 +3228,9 @@ object Parsers {
       CaseDef(pat, grd1, body)
     }
 
+    def singleCaseMatch() =
+      Match(EmptyTree, caseClause(exprOnly = true) :: Nil)
+
     /** TypeCaseClause     ::= ‘case’ (InfixType | ‘_’) ‘=>’ Type [semi]
      */
     def typeCaseClause(): CaseDef = atSpan(in.offset) {

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

@@ -617,7 +617,9 @@ object Scanners {
         && !statCtdTokens.contains(lastToken)
         && !isTrailingBlankLine
 
-      if newlineIsSeparating
+      if currentRegion.closedBy == ENDlambda then
+        insert(ENDlambda, lineOffset)
+      else if newlineIsSeparating
          && canEndStatTokens.contains(lastToken)
          && canStartStatTokens.contains(token)
          && !isLeadingInfixOperator(nextWidth)
@@ -1599,6 +1601,8 @@ object Scanners {
    *   InParens    a pair of parentheses (...) or brackets [...]
    *   InBraces    a pair of braces { ... }
    *   Indented    a pair of <indent> ... <outdent> tokens
+   *   InCase      a case of a match
+   *   SingleLineLambda  the rest of a line following a `:`
    */
   abstract class Region(val closedBy: Token):
 
@@ -1667,6 +1671,7 @@ object Scanners {
       case _: InBraces => "}"
       case _: InCase => "=>"
       case _: Indented => "UNDENT"
+      case _: SingleLineLambda => "end of single-line lambda"
 
     /** Show open regions as list of lines with decreasing indentations */
     def visualize: String =
@@ -1680,6 +1685,7 @@ object Scanners {
   case class InParens(prefix: Token, outer: Region) extends Region(prefix + 1)
   case class InBraces(outer: Region) extends Region(RBRACE)
   case class InCase(outer: Region) extends Region(OUTDENT)
+  case class SingleLineLambda(outer: Region) extends Region(ENDlambda)
 
   /** A class describing an indentation region.
    *  @param width   The principal indentation width

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

@@ -203,8 +203,10 @@ object Tokens extends TokensCommon {
     // A `:` recognized as starting an indentation block
   inline val SELFARROW = 90;        enter(SELFARROW, "=>") // reclassified ARROW following self-type
 
+  inline val ENDlambda = 99;        enter(ENDlambda, "end of single-line lambda")
+
   /** XML mode */
-  inline val XMLSTART = 99;         enter(XMLSTART, "$XMLSTART$<") // TODO: deprecate
+  inline val XMLSTART = 100;         enter(XMLSTART, "$XMLSTART$<") // TODO: deprecate
 
   final val alphaKeywords: TokenSet = tokenRange(IF, END)
   final val symbolicKeywords: TokenSet = tokenRange(USCORE, CTXARROW)
@@ -267,7 +269,7 @@ object Tokens extends TokensCommon {
   final val canStartStatTokens3: TokenSet = canStartExprTokens3 | mustStartStatTokens | BitSet(
     AT, CASE, END)
 
-  final val canEndStatTokens: TokenSet = atomicExprTokens | BitSet(TYPE, GIVEN, RPAREN, RBRACE, RBRACKET, OUTDENT)
+  final val canEndStatTokens: TokenSet = atomicExprTokens | BitSet(TYPE, GIVEN, RPAREN, RBRACE, RBRACKET, OUTDENT, ENDlambda)
 
   /** Tokens that stop a lookahead scan search for a `<-`, `then`, or `do`.
    *  Used for disambiguating between old and new syntax.

docs/_docs/internals/syntax.md

@@ -243,6 +243,7 @@ CapFilter         ::=  ‘.’ ‘as’ ‘[’ QualId ’]’
 ```ebnf
 Expr              ::=  FunParams (‘=>’ | ‘?=>’) Expr                            Function(args, expr), Function(ValDef([implicit], id, TypeTree(), EmptyTree), expr)
                     |  TypTypeParamClause ‘=>’ Expr                             PolyFunction(ts, expr)
+                    |  ExprCaseClause
                     |  Expr1
 BlockResult       ::=  FunParams (‘=>’ | ‘?=>’) Block
                     |  TypTypeParamClause ‘=>’ Block
@@ -293,8 +294,11 @@ SimpleExpr        ::=  SimpleRef
                     |  SimpleExpr ColonArgument                                 -- under language.experimental.fewerBraces
                     |  SimpleExpr ‘_’                                           PostfixOp(expr, _) (to be dropped)
                     |  XmlExpr							-- to be dropped
-ColonArgument     ::=  colon [LambdaStart]
+ColonArgument     ::=  colon {LambdaStart}
                        indent (CaseClauses | Block) outdent
+                    |  colon LambdaStart {LambdaStart} expr ENDlambda                         -- ENDlambda is inserted for each production at next EOL
+                                                                                -- does not apply if enclosed in parens
+                    |  colon ExprCaseClause
 LambdaStart       ::=  FunParams (‘=>’ | ‘?=>’)
                     |  TypTypeParamClause ‘=>’
 Quoted            ::=  ‘'’ ‘{’ Block ‘}’