for-comprehensions without `_ <- x` and without `yield` #21

puffnfresh · 2014-09-05T04:21:47Z

This would be cool:

for {
  IO.putStrLn("Hello")
  line <- IO.readLine
  IO.putStrLn(line)
} yield ()

This would be even cooler:

for {
  IO.putStrLn("Hello")
  line <- IO.readLine
  IO.putStrLn(line)
}

copumpkin · 2014-09-05T04:22:17Z

👍, somewhat unsurprisingly

lihaoyi · 2014-09-05T05:48:22Z

This would also be cool

for {
  txt = "Hello"
  IO.putStrLn(txt)
  line <- IO.readLine
  IO.putStrLn(line)
}

but +1 for everything that's been suggested here!

paulp · 2014-09-05T08:47:12Z

You can already drop the yield, unless you're doing something with your bucket of units.

NightRa · 2014-09-05T10:51:21Z

Without the yield, it shouldn't necessarily map to unit, it could just do the flatMap's without the final map.

milessabin · 2014-09-07T14:32:28Z

👍

This corrects an error in the change to the trait encoding in scala#5003: getters in traits should have empty bodies and be emitted as abstract. ``` % ~/scala/2.12.0-M4/bin/scalac sandbox/test.scala && javap -c T Compiled from "test.scala" public interface T { public abstract void T$_setter_$x_$eq(int); public int x(); Code: 0: aload_0 1: invokeinterface #15, 1 // InterfaceMethod x:()I 6: ireturn public int y(); Code: 0: aload_0 1: invokeinterface #20, 1 // InterfaceMethod y:()I 6: ireturn public void y_$eq(int); Code: 0: aload_0 1: iload_1 2: invokeinterface #24, 2 // InterfaceMethod y_$eq:(I)V 7: return public void $init$(); Code: 0: aload_0 1: bipush 42 3: invokeinterface #29, 2 // InterfaceMethod T$_setter_$x_$eq:(I)V 8: aload_0 9: bipush 24 11: invokeinterface #24, 2 // InterfaceMethod y_$eq:(I)V 16: return } % qscalac sandbox/test.scala && javap -c T Compiled from "test.scala" public interface T { public abstract void T$_setter_$x_$eq(int); public abstract int x(); public abstract int y(); public abstract void y_$eq(int); public static void $init$(T); Code: 0: aload_0 1: bipush 42 3: invokeinterface #21, 2 // InterfaceMethod T$_setter_$x_$eq:(I)V 8: aload_0 9: bipush 24 11: invokeinterface #23, 2 // InterfaceMethod y_$eq:(I)V 16: return public void $init$(); Code: 0: aload_0 1: invokestatic #27 // Method $init$:(LT;)V 4: return } ```

milessabin · 2016-08-12T11:37:48Z

To resurrect this issue, please rework it as an issue/PR against Lightbend Scala (ie. scala/scala).

Top level modules in Scala currently desugar as: ``` class C; object O extends C { toString } ``` ``` public final class O$ extends C { public static final O$ MODULE$; public static {}; Code: 0: new #2 // class O$ 3: invokespecial #12 // Method "<init>":()V 6: return private O$(); Code: 0: aload_0 1: invokespecial #13 // Method C."<init>":()V 4: aload_0 5: putstatic #15 // Field MODULE$:LO$; 8: aload_0 9: invokevirtual #21 // Method java/lang/Object.toString:()Ljava/lang/String; 12: pop 13: return } ``` The static initalizer `<clinit>` calls the constructor `<init>`, which invokes superclass constructor, assigns `MODULE$= this`, and then runs the remainder of the object's constructor (`toString` in the example above.) It turns out that this relies on a bug in the JVM's verifier: assignment to a static final must occur lexically within the <clinit>, not from within `<init>` (even if the latter is happens to be called by the former). I'd like to move the assignment to <clinit> but that would change behaviour of "benign" cyclic references between modules. Example: ``` package p1; class CC { def foo = O.bar}; object O {new CC().foo; def bar = println(1)}; // Exiting paste mode, now interpreting. scala> p1.O 1 ``` This relies on the way that we assign MODULE$ field after the super class constructors are finished, but before the rest of the module constructor is called. Instead, this commit removes the ACC_FINAL bit from the field. It actually wasn't behaving as final at all, precisely the issue that the stricter verifier now alerts us to. ``` scala> :paste -raw // Entering paste mode (ctrl-D to finish) package p1; object O // Exiting paste mode, now interpreting. scala> val O1 = p1.O O1: p1.O.type = p1.O$@ee7d9f1 scala> scala.reflect.ensureAccessible(p1.O.getClass.getDeclaredConstructor()).newInstance() res0: p1.O.type = p1.O$@64cee07 scala> O1 eq p1.O res1: Boolean = false ``` We will still achieve safe publication of the assignment to other threads by virtue of the fact that `<clinit>` is executed within the scope of an initlization lock, as specified by: https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-5.html#jvms-5.5 Fixes scala/scala-dev#SD-194

@inline

Non local returns aren't eliminated after inlined in 2.11 or 2.12 ``` ⚡ scala Welcome to Scala 2.12.1 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_112). Type in expressions for evaluation. Or try :help. scala> @inlune def foo(a: => Any) = if ("".isEmpty) a else "" <console>:11: error: not found: type inlune @inlune def foo(a: => Any) = if ("".isEmpty) a else "" ^ scala> @inline def foo(a: => Any) = if ("".isEmpty) a else "" foo: (a: => Any)Any scala> class InlineReturn { def test: Any = foo(return "") } defined class InlineReturn scala> :javap -c InlineReturn#test public java.lang.Object test(); Code: 0: new #4 // class java/lang/Object 3: dup 4: invokespecial #32 // Method java/lang/Object."<init>":()V 7: astore_1 8: getstatic #36 // Field $line4/$read$$iw$$iw$.MODULE$:L$line4/$read$$iw$$iw$; 11: aload_1 12: invokedynamic #59, 0 // InvokeDynamic #0:apply:(Ljava/lang/Object;)Lscala/Function0; 17: invokevirtual #63 // Method $line4/$read$$iw$$iw$.foo:(Lscala/Function0;)Ljava/lang/Object; 20: goto 44 23: astore_2 24: aload_2 25: invokevirtual #66 // Method scala/runtime/NonLocalReturnControl.key:()Ljava/lang/Object; 28: aload_1 29: if_acmpne 39 32: aload_2 33: invokevirtual #69 // Method scala/runtime/NonLocalReturnControl.value:()Ljava/lang/Object; 36: goto 41 39: aload_2 40: athrow 41: goto 44 44: areturn Exception table: from to target type 8 20 23 Class scala/runtime/NonLocalReturnControl ``` ``` ⚡ ~/scala/2.11.8/bin/scala Welcome to Scala 2.11.8 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_112). Type in expressions for evaluation. Or try :help. scala> @inline def foo(a: => Any) = if ("".isEmpty) a else "" foo: (a: => Any)Any scala> class InlineReturn { def test: Any = foo(return "") } defined class InlineReturn scala> :javap -c InlineReturn#test public java.lang.Object test(); Code: 0: new #4 // class java/lang/Object 3: dup 4: invokespecial #13 // Method java/lang/Object."<init>":()V 7: astore_1 8: getstatic #19 // Field .MODULE$:L; 11: new #21 // class InlineReturn$$anonfun$test$1 14: dup 15: aload_0 16: aload_1 17: invokespecial #24 // Method InlineReturn$$anonfun$test$1."<init>":(LInlineReturn;Ljava/lang/Object;)V 20: invokevirtual #28 // Method .foo:(Lscala/Function0;)Ljava/lang/Object; 23: goto 39 26: astore_2 27: aload_2 28: invokevirtual #31 // Method scala/runtime/NonLocalReturnControl.key:()Ljava/lang/Object; 31: aload_1 32: if_acmpne 40 35: aload_2 36: invokevirtual #34 // Method scala/runtime/NonLocalReturnControl.value:()Ljava/lang/Object; 39: areturn 40: aload_2 41: athrow Exception table: from to target type 8 26 26 Class scala/runtime/NonLocalReturnControl scala> :quit ```

puffnfresh added the enhancement label Sep 5, 2014

milessabin added the defunct label Aug 12, 2016

milessabin added this to the Parked milestone Aug 12, 2016

milessabin closed this as completed Aug 26, 2016

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for-comprehensions without `_ <- x` and without `yield` #21

for-comprehensions without `_ <- x` and without `yield` #21

puffnfresh commented Sep 5, 2014

copumpkin commented Sep 5, 2014

lihaoyi commented Sep 5, 2014

paulp commented Sep 5, 2014

NightRa commented Sep 5, 2014

milessabin commented Sep 7, 2014

milessabin commented Aug 12, 2016

for-comprehensions without _ <- x and without yield #21

for-comprehensions without _ <- x and without yield #21

Comments

puffnfresh commented Sep 5, 2014

copumpkin commented Sep 5, 2014

lihaoyi commented Sep 5, 2014

paulp commented Sep 5, 2014

NightRa commented Sep 5, 2014

milessabin commented Sep 7, 2014

milessabin commented Aug 12, 2016

for-comprehensions without `_ <- x` and without `yield` #21

for-comprehensions without `_ <- x` and without `yield` #21