Auto-uncurry n-ary functions.

Implements SIP scala#897.

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

@@ -588,6 +588,25 @@ object desugar {
     Function(params, Match(selector, cases))
   }
 
+  /** Map n-ary function `(p1, ..., pn) => body` where n != 1 to unary function as follows:
+   *
+   *    x$1 => {
+   *      val p1 = x$1._1
+   *      ...
+   *      val pn = x$1._n
+   *      body
+   *    }
+   */
+  def makeUnaryCaseLambda(params: List[ValDef], body: Tree)(implicit ctx: Context): Tree = {
+    val param = makeSyntheticParameter()
+    def selector(n: Int) = Select(refOfDef(param), nme.selectorName(n))
+    val vdefs =
+      params.zipWithIndex.map{
+        case(param, idx) => cpy.ValDef(param)(rhs = selector(idx))
+      }
+    Function(param :: Nil, Block(vdefs, body))
+  }
+
   /** Add annotation with class `cls` to tree:
    *      tree @cls
    */

src/dotty/tools/dotc/typer/Typer.scala

@@ -611,26 +611,44 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
         if (protoFormals.length == params.length) protoFormals(i)
         else errorType(i"wrong number of parameters, expected: ${protoFormals.length}", tree.pos)
 
-      val inferredParams: List[untpd.ValDef] =
-        for ((param, i) <- params.zipWithIndex) yield
-          if (!param.tpt.isEmpty) param
-          else cpy.ValDef(param)(
-            tpt = untpd.TypeTree(
-              inferredParamType(param, protoFormal(i)).underlyingIfRepeated(isJava = false)))
-
-      // Define result type of closure as the expected type, thereby pushing
-      // down any implicit searches. We do this even if the expected type is not fully
-      // defined, which is a bit of a hack. But it's needed to make the following work
-      // (see typers.scala and printers/PlainPrinter.scala for examples).
-      //
-      //     def double(x: Char): String = s"$x$x"
-      //     "abc" flatMap double
-      //
-      val resultTpt = protoResult match {
-        case WildcardType(_) => untpd.TypeTree()
-        case _ => untpd.TypeTree(protoResult)
+      /** Is `formal` a product type which is elementwise compatible with `params`? */
+      def ptIsCorrectProduct(formal: Type) = {
+        val pclass = defn.ProductNClass(params.length)
+        isFullyDefined(formal, ForceDegree.noBottom) &&
+        formal.derivesFrom(pclass) &&
+        formal.baseArgTypes(pclass).corresponds(params) {
+          (argType, param) =>
+            param.tpt.isEmpty || isCompatible(argType, typedAheadType(param.tpt).tpe)
+        }
       }
-      typed(desugar.makeClosure(inferredParams, fnBody, resultTpt), pt)
+
+      val desugared =
+        if (protoFormals.length == 1 && params.length != 1 && ptIsCorrectProduct(protoFormals.head)) {
+          desugar.makeUnaryCaseLambda(params, fnBody)
+        }
+        else {
+          val inferredParams: List[untpd.ValDef] =
+            for ((param, i) <- params.zipWithIndex) yield
+              if (!param.tpt.isEmpty) param
+              else cpy.ValDef(param)(
+                tpt = untpd.TypeTree(
+                  inferredParamType(param, protoFormal(i)).underlyingIfRepeated(isJava = false)))
+
+          // Define result type of closure as the expected type, thereby pushing
+          // down any implicit searches. We do this even if the expected type is not fully
+          // defined, which is a bit of a hack. But it's needed to make the following work
+          // (see typers.scala and printers/PlainPrinter.scala for examples).
+          //
+          //     def double(x: Char): String = s"$x$x"
+          //     "abc" flatMap double
+          //
+          val resultTpt = protoResult match {
+            case WildcardType(_) => untpd.TypeTree()
+            case _ => untpd.TypeTree(protoResult)
+          }
+          desugar.makeClosure(inferredParams, fnBody, resultTpt)
+        }
+      typed(desugared, pt)
     }
   }
 

test/dotc/tests.scala

@@ -111,6 +111,7 @@ class tests extends CompilerTest {
   @Test def neg_abstractOverride() = compileFile(negDir, "abstract-override", xerrors = 2)
   @Test def neg_blockescapes() = compileFile(negDir, "blockescapesNeg", xerrors = 1)
   @Test def neg_bounds() = compileFile(negDir, "bounds", xerrors = 2)
+  @Test def neg_functionArity() = compileFile(negDir, "function-arity", xerrors = 5)
   @Test def neg_typedapply() = compileFile(negDir, "typedapply", xerrors = 3)
   @Test def neg_typedIdents() = compileDir(negDir, "typedIdents", xerrors = 2)
   @Test def neg_assignments() = compileFile(negDir, "assignments", xerrors = 3)

tests/neg/function-arity.scala

@@ -0,0 +1,22 @@
+object Test {
+
+  // From #873:
+
+  trait X extends Function1[Int, String]
+  implicit def f2x(f: Function1[Int, String]): X = ???
+  ({case _ if "".isEmpty => 0} : X) // error: expected String, found Int
+
+  // Tests where parameter list cannot be made into a pattern
+
+  def unary[T](x: T => Unit) = ???
+  unary((x, y) => ())   // error
+
+  unary[(Int, Int)]((x, y) => ())
+
+  unary[(Int, Int)](() => ())   // error
+  unary[(Int, Int)]((x, y, _) => ()) // error
+
+  unary[(Int, Int)]((x: String, y) => ()) // error
+
+
+}

tests/pos/i873.scala → tests/pos/function-arity.scala

@@ -7,4 +7,12 @@ object Test {
   ({case _ if "".isEmpty => ""} : X) // allowed, implicit view used to adapt
 
   // ({case _ if "".isEmpty => 0} : X) // expected String, found Int
+
+  def unary[T](a: T, b: T, f: ((T, T)) => T): T = f((a, b))
+  unary(1, 2, (x, y) => x)
+  unary(1, 2, (x: Int, y) => x)
+  unary(1, 2, (x: Int, y: Float) => x)
+
+  val xs = List(1, 2, 3)
+  xs.zipWithIndex.map(_ + _)
 }