Allow collectve parameters for extension methods

Author: odersky

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -824,12 +824,37 @@ object desugar {
    *    object name extends parents { self => body }
    *
    *  to:
+   *
    *    <module> val name: name$ = New(name$)
    *    <module> final class name$ extends parents { self: name.type => body }
+   *
+   *  Special case for extension methods with collective parameters. Expand:
+   *
+   *     given object name[tparams](x: T) extends parents { self => bpdy }
+   *
+   *  to:
+   *
+   *     given object name extends parents { self => body' }
+   *
+   *  where every definition in `body` is expanded to an extension method
+   *  taking type parameters `tparams` and a leading paramter `(x: T)`.
+   *  See: makeExtensionDef
    */
   def moduleDef(mdef: ModuleDef)(implicit ctx: Context): Tree = {
     val impl = mdef.impl
     val mods = mdef.mods
+    impl.constr match {
+      case DefDef(_, tparams, (vparams @ (vparam :: Nil)) :: _, _, _) =>
+        assert(mods.is(Given))
+        return moduleDef(
+          cpy.ModuleDef(mdef)(
+            mdef.name,
+            cpy.Template(impl)(
+              constr = emptyConstructor,
+              body = impl.body.map(makeExtensionDef(_, tparams, vparams)))))
+      case _ =>
+    }
+
     val moduleName = normalizeName(mdef, impl).asTermName
     def isEnumCase = mods.isEnumCase
 
@@ -869,6 +894,36 @@ object desugar {
     }
   }
 
+  /** Given tpe parameters `Ts` (possibly empty) and a leading value parameter `(x: T)`,
+   *  map a method definition
+   *
+   *     def foo [Us] paramss ...
+   *
+   *  to
+   *
+   *     <extension> def foo[Ts ++ Us](x: T) parammss ...
+   *
+   *  If the given member `mdef` is not of this form, flag it as an error.
+   */
+
+  def makeExtensionDef(mdef: Tree, tparams: List[TypeDef], leadingParams: List[ValDef]) given (ctx: Context): Tree = {
+    val allowed = "allowed here, since collective parameters are given"
+    mdef match {
+      case mdef: DefDef =>
+        if (mdef.mods.is(Extension)) {
+          ctx.error(em"No extension method $allowed", mdef.sourcePos)
+          mdef
+        }
+        else cpy.DefDef(mdef)(tparams = tparams ++ mdef.tparams, vparamss = leadingParams :: Nil)
+          .withFlags(Extension)
+      case mdef: Import =>
+        mdef
+      case mdef =>
+        ctx.error(em"Only methods $allowed", mdef.sourcePos)
+        mdef
+    }
+  }
+
   /** Transforms
    *
    *    <mods> type $T >: Low <: Hi

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

@@ -2860,11 +2860,20 @@ object Parsers {
     /** GivenDef          ::=  [id] [DefTypeParamClause] GivenBody
      *  GivenBody         ::=  [‘as ConstrApp {‘,’ ConstrApp }] {GivenParamClause} [TemplateBody]
      *                      |  ‘as’ Type {GivenParamClause} ‘=’ Expr
+     *                      |  ‘(’ DefParam ‘)’ TemplateBody
      */
     def instanceDef(newStyle: Boolean, start: Offset, mods: Modifiers, instanceMod: Mod) = atSpan(start, nameStart) {
       var mods1 = addMod(mods, instanceMod)
       val name = if (isIdent && (!newStyle || in.name != nme.as)) ident() else EmptyTermName
       val tparams = typeParamClauseOpt(ParamOwner.Def)
+      var leadingParamss =
+        if (in.token == LPAREN)
+          try paramClause(prefix = true) :: Nil
+          finally {
+            newLineOptWhenFollowedBy(LBRACE)
+            if (in.token != LBRACE) syntaxErrorOrIncomplete("`{' expected")
+          }
+        else Nil
       val parents =
         if (!newStyle && in.token == FOR || isIdent(nme.as)) { // for the moment, accept both `given for` and `given as`
           in.nextToken()
@@ -2880,11 +2889,15 @@ object Parsers {
         }
         else {
           newLineOptWhenFollowedBy(LBRACE)
-          val tparams1 = tparams.map(tparam => tparam.withMods(tparam.mods | PrivateLocal))
-          val vparamss1 = vparamss.map(_.map(vparam =>
-            vparam.withMods(vparam.mods &~ Param | ParamAccessor | PrivateLocal)))
+          val (tparams1, vparamss1) =
+            if (leadingParamss.nonEmpty)
+              (tparams, leadingParamss)
+            else
+              (tparams.map(tparam => tparam.withMods(tparam.mods | PrivateLocal)),
+               vparamss.map(_.map(vparam =>
+                 vparam.withMods(vparam.mods &~ Param | ParamAccessor | PrivateLocal))))
           val templ = templateBodyOpt(makeConstructor(tparams1, vparamss1), parents, Nil)
-          if (tparams.isEmpty && vparamss.isEmpty) ModuleDef(name, templ)
+          if (tparams.isEmpty && vparamss1.isEmpty || leadingParamss.nonEmpty) ModuleDef(name, templ)
           else TypeDef(name.toTypeName, templ)
         }
       finalizeDef(instDef, mods1, start)

docs/docs/internals/syntax.md

@@ -388,6 +388,7 @@ EnumDef           ::=  id ClassConstr InheritClauses EnumBody
 GivenDef          ::=  [id] [DefTypeParamClause] GivenBody
 GivenBody         ::=  [‘as ConstrApp {‘,’ ConstrApp }] {GivenParamClause} [TemplateBody]
                     |  ‘as’ Type {GivenParamClause} ‘=’ Expr
+                    |  ‘(’ DefParam ‘)’ TemplateBody
 Template          ::=  InheritClauses [TemplateBody]                            Template(constr, parents, self, stats)
 InheritClauses    ::=  [‘extends’ ConstrApps] [‘derives’ QualId {‘,’ QualId}]
 ConstrApps        ::=  ConstrApp {‘with’ ConstrApp}

docs/docs/reference/contextual/extension-methods.md

@@ -80,7 +80,7 @@ So `circle.circumference` translates to `CircleOps.circumference(circle)`, provi
 
 ### Given Instances for Extension Methods
 
-Given instances that define extension methods can also be defined without a `for` clause. E.g.,
+Given instances that define extension methods can also be defined without an `as` clause. E.g.,
 
 ```scala
 given StringOps {
@@ -94,8 +94,33 @@ given {
   def (xs: List[T]) second[T] = xs.tail.head
 }
 ```
-If such given instances are anonymous (as in the second clause), their name is synthesized from the name
-of the first defined extension method.
+If such given instances are anonymous (as in the second clause), their name is synthesized from the name of the first defined extension method.
+
+### Given Instances with Collective Parameters
+
+If a given instance has several extension methods one can pull out the left parameter section
+as well as any type parameters of these extension methods into the given instance itself.
+For instance, here is a given instance with two extension methods.
+```scala
+given ListOps {
+  def (xs: List[T]) second[T]: T = xs.tail.head
+  def (xs: List[T]) third[T]: T = xs.tail.tail.head
+}
+```
+The repetition in the parameters can be avoided by moving the parameters into the given instance itself. The following version is a shorthand for the code above.
+```scala
+given ListOps[T](xs: List[T]) {
+  def second: T = xs.tail.head
+  def third: T = xs.tail.tail.head
+}
+```
+This syntax just adds convenience at the definition site. Applications of such extension methods are exactly the same as if their parameters were repeated in each extension method.
+Examples:
+```scala
+val xs = List(1, 2, 3)
+xs.second[Int]
+ListOps.third[T](xs)
+```
 
 ### Operators
 
@@ -143,4 +168,6 @@ to the [current syntax](../../internals/syntax.md).
 ```
 DefSig            ::=  ...
                     |  ‘(’ DefParam ‘)’ [nl] id [DefTypeParamClause] DefParamClauses
+GivenBody         ::=  ...
+                    |  ‘(’ DefParam ‘)’ TemplateBody
 ```

tests/run/extmethods2.scala

@@ -14,4 +14,25 @@ object Test extends App {
   }
 
   test given TC()
-}
+
+  object A {
+    given ListOps[T](xs: List[T]) {
+      def second: T = xs.tail.head
+      def third: T = xs.tail.tail.head
+    }
+    given (xs: List[Int]) {
+      def prod = (1 /: xs)(_ * _)
+    }
+  }
+
+  object B {
+    import given A._
+    val xs = List(1, 2, 3)
+    assert(xs.second[Int] == 2)
+    assert(xs.third == 3)
+    assert(A.ListOps.second[Int](xs) == 2)
+    assert(A.ListOps.third(xs) == 3)
+    assert(xs.prod == 6)
+  }
+}
+