Redesign Tuples with HList-like structure #2199

OlivierBlanvillain · 2017-04-07T07:32:37Z

This PR redesigns Tuples to use an HList-like structure instead of flat case classes (scala.TupleN):

package dotty

trait Tuple
trait TupleCons[+H, +T <: Tuple] extends Tuple
object Unit extends Tuple

object TupleCons {
  def apply[H, T <: Tuple](h: H, t: T): TupleCons[H, T] = ...
  def unapply[H, T <: Tuple](t: TupleCons[H, T]): Pair[H, T] =...
}

These types (and values) are users facing: desugaring turns (a, b, ...) into TupleCons[a, TupleCons[b, ...Unit]] (and TupleCons(a, TupleCons(b, ...()))). At run-time, tuples are represented using case classes for small arities and an Array for large arities.

A new mini-phase named TupleRewrites takes care of optimizing apply and unapply expressions corresponding to creation and pattern matching on tuples. To preserve binary compatibility, Scala2Unpickler is modified to transform tuple types to the new representation, TypeErasure then takes care of erasing (small) tuple types back to scala.TupleN.

TODO:

Restore PatmatExhaustivityTests. The exhaustivity checks need to be updated to properly handle the new representation.
Restore runCompilerFromInterface. This now needs an explicit dotty-library dependency. It used to be inherited "automatically" from the dependsOn(dotty-library) in dotty-compiler.
Change patdef desugaring? The current implementation uses .unapply + direct calls to _i which is still going to be capped at 22. We could lift that by desugaring patdef using vars and pattern matching instead.
Restore pretty printing for tuples.

felixmulder · 2017-04-07T07:57:39Z

No idea - GitHub gets messy with reordered commits in rebases I've noticed...Could be? Otherwise - just change the hash of the latest commit and force push, should get it going again. Also - CI runs a maximum of 20 jobs, currently that means 3 commits concurrently.

…

On Fri, Apr 7, 2017, 09:36 Olivier Blanvillain ***@***.***> wrote: @felixmulder <https://github.com/felixmulder> any idea why the CI decided not to test the latest commit? — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#2199 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ABdYwb3iZL0TIaXcdGCUSUNfO-Ekt0YPks5rtedmgaJpZM4M2jcg> .

sjrd · 2017-04-07T08:17:15Z

GitHub gets messy with reordered commits in rebases I've noticed

It always does that. GitHub has this stupid notion that the commits of a PR should be displayed in timestamp order, instead of in history order. It's only a display thing, though, so I doubt it would affect your CI (it certainly never affected ours).

julienrf · 2017-04-07T08:11:08Z

tests/pos/t0301.scala

@@ -6,7 +6,7 @@ case class Var() extends Expr
 object Analyzer {
  def substitution(expr: Expr, cls: (Var,Var)): Expr =
    expr match {
-      case cls._2 => cls._1  // source of the error
+      case e if e == cls._2 => cls._1  // source of the error


Why this change?

Since _2 is now added thought implicit conversion, the pattern used in this case is no longer stable. (see commit message)

julienrf · 2017-04-07T08:14:49Z

tests/run/double-pattern-type.scala

+    }
+
+    c2 match {
+      case C2(a, b, c) =>


Could you detail what this one expands to?

That's unchanged. (the first commits come from #1938 which this PR is rebased on to be able to test case class with size > 23)

julienrf · 2017-04-07T08:17:35Z

tests/tuples/tuple-accessors.scala

+    v22: Int
+
+    // We can't go above that since patDef is implemented with direct _i calls.
+    // It's should be possible to update the desugaring to use var + pattern


Typo: “it should”

julienrf · 2017-04-07T08:18:03Z

tests/tuples/tuple-appender.scala

+      (1d, 2d, 3d)
+
+    val l3: (String, Boolean, Double, Double, Double) =
+      l1 ++ l2


DanyMariaLee · 2017-04-07T16:02:37Z

Great news!! Will also be available everything from shapeless to operate with HLists the way we used to?

szeiger · 2017-04-07T22:55:14Z

I'd like to point you to https://github.com/szeiger/ErasedTypes which I wrote half a decade ago with the purpose of eventually replacing Scala tuples with HArrays. It contains abstractions over tuple arity that may be useful to add (although some will not work in the same way with Dotty making arbitrary type projections illegal). It also has a clean separation between cons-style HLists and flat HArrays (but I recommend looking at typical use cases to determine whether or not this should be preferred over the implementation here which combines the two).

julienrf · 2017-04-10T16:38:10Z

tests/tuples/tuple-accessors.scala

@@ -120,7 +120,7 @@ object Test {
    v22: Int

    // We can't go above that since patDef is implemented with direct _i calls.
-    // It's should be possible to update the desugaring to use var + pattern
+    // Its should be possible to update the desugaring to use var + pattern


OlivierBlanvillain · 2017-04-25T09:43:32Z

@DanyMariaLee It seems like the current trend is to remove things from the standard library, not add more, so I'm not really sure it shapeless style operations on tuples would have a place in the standard library.

Miles has been talking for a while about a possible shapeless redesign based on Symmetric Monoidal Category (see this branch) that would allow operations like HList#length and Coproduct#length to be factored out into a single implementation over a common structure. The tuple types introduced in this PR would fit perfectly with this design!

OlivierBlanvillain · 2017-04-25T09:43:57Z

@szeiger Thanks for pointing out your HArrays project, it's something I studied briefly before I started working on this. I think the main difference with this PR is that here nothing is intended to be exposed to users besides the TupleCons and Unit types, the fact that tuples are backed by case classes then by arrays is an implementation detail.

The question of array vs linked list representation is easily answered with a binary compatibility requirement: it wouldn't make much sense to us a flat representation up to size 22, then switch to linked afterward. However, if we lower that limit to a more reasonable size, then the question opens again :)

This change is needed for scala/scala3#2199. The idea is to have `type Tuple2[A, B] = (A, B) // Desugars into TupleCons[A, TupleCons[B, Unit]]` in Predef, so that users can refer to "a tuple of size 2" either with `(A, B)` or `Tuple2[A, B]`. It's still possible to refer to the actually underlying class with the explicit prefix, `scala.Tuple2`. Because this code is inside the scala package and mixes the two syntax in breaks with the above PR...

szeiger · 2017-04-26T09:06:42Z

When it comes to such low-level constructs as tuples you may not want to hide too much of the details for performance reasons. I haven't done any benchmarking but exposing the TupleN classes should in theory enable better optimizations because you get monomorphic dispatch.

The motivation for having both HList and HArray in my implementation was not a mixed implementation of tuples depending on size. It doesn't do that (tuples are always flat) and for predictable performance I think it shouldn't do it. But at the type level HArrays are nested, so if you already have a proper HList type you can use that as part of the HArray type, i.e. "here's a flat implementation that corresponds to the following HList". Building up from Nat to HList and finally HArray was a natural thing to do given that my goal was not only to remove the Tuple22 limit but also to allow useful abstractions over arity.

Why would you still have special implementations up to 22 here anyway? I think you should lower that number considerably. 22 was originally chosen as large enough to represent what Scala needed, not because of performance considerations. The latter should be the driving factor for determining a good size for Dotty if you support arbitrary tuple sizes anyway. You are not bound by binary compatibility and you have Scalafix to take care of source compatibility.

DarkDimius · 2017-04-26T09:16:11Z

Why would you still have special implementations up to 22 here anyway? I think you should lower that number considerably. 22 was originally chosen as large enough to represent what Scala needed, not because of performance considerations.

While originally 22 wasn't chosen due to performance considerations, It would be nice to keep performance properties of existing code. I'd prefer to keep 22 limit in place and not lower it.

OlivierBlanvillain · 2017-04-26T12:53:00Z

Why would you still have special implementations up to 22 here anyway?

The current very practical reason is that dotty test suite currently uses the 2.11.x standard library compiled by scalac. Methods like Map.apply and Map.head accept and return scala.Tuple2 instances, which is handled in this PR by preserving binary compatibility.

I think you should lower that number considerably.

I definitely agree. I did a quick search on a large corpus of open-source Scala project (~600K LOC), and every tuple instantiated, pattern-matched on or accessed with _i explicitly was on tuple of size 6 or below. Above that, it's only generic operations (for example, in Play), for which both array and linked list representation should be superior. It also seems that the majority of generic operations are based on head & tail, which obviously works better on a linked representation.

TupleN classes should in theory enable better optimizations because you get monomorphic dispatch.

The tuple interface has no methods, so they shouldn't be dynamic dispatch happening on these tuples. Non-generic operations with statically known length use a cast and a direct call, generic operations must rely on pattern matching. If we imagine an up to size 6 case classes then linked list representation, and always put the TupleCons case first, we should get about the same performance than a linked list for head & tail.

I should probably plug in a few benchmarks I did just on the representations themselves: bench.pdf. Each representation is special cased up to size 4 with case classes. UnrolledHList is a linked list with 3 different cons nodes, holding 1, 2 and 3 heads respectively (actual definitions).

odersky · 2017-04-29T09:18:19Z

Is this ready for review? In this case it would be good to rebase first.

Changes are as follows: - New types for tuples in dotty library: Tuple = Unit | TupleCons Head Tail <: Tuple - Desugaring uses this structure instead of the scala.TupleN for types and expressions - New TupleRewrites phase does rewrites the HList structure into scala.TupleN like case classes (for small arities) or a case class wrapping an Array (for large arities)

To stay binary compatibility with scala 2.11 binaries, we hijack the unpickling of types to fake an HList structure. Later on in erasure, HList types are eraised back to scala.TupleN (for N <= 22). In addition because the actual scala.TupleN classes are neither `<: Tuple` now `<: TupleCons`, these types needs to be systematically araised to Object. In addition to all these carefully compensating hacks, this also imposes a new contain: the dotty-library, which defines several `Tuple/TupleCons` methods, can should now *always* be compiled by dotty itself. Indeed, when compiled with scalac, these types will stay as they are instead of being eraised to Object.

I couldn't get rid of the `dotty-library` project completely, it's still used the first time sbt-briged is compiled with dotty. The dependencies are as follows: - (no dep) compile `dotty-compile` with scalac [0] - (no dep) compile `dotty-library` with scalac [1] - ([0], [1]) → compile `dotty-sbt-bridge` with dotty [2] - ([0], [2]) → compile `dotty-library-bootstraped` with dotty + sbt After that, [1] should never be used again.

@unchecked

- dotc/scala-collections.blacklist: Blacklist Unit and TupleN from scala-collections test - tests/neg/i1653.scala: Having an additional superclass on Unit seams to affect the error here (not sure why). - tests/pos/t0301.scala: Since `_2` is now added thought implicit conversion, the pattern used in this case is no longer stable. - tests/pos/tcpoly_bounds1.scala: This test uses the fact that `(A, B) <: Product2[A, B]`, which is no longer the case. - tests/repl/patdef.check: Having the `_i` via implicit conversions remove the @unchecked here - tests/run/tuple-match.scala: Replace Tuple1 in pattern matching with explicit TupleCons pattern. TupleN can still be used in apply position (defined as defs in DottyPredef), but not in unapply position. - tests/run/unapply.scala: Also uses the fact that `(A, B) <: Product2[A, B]`. - tests/run/withIndex.scala: Might be a limitation in typer?: 14 | case _: Array[Tuple2[_, _]] => true | ^^^^^^^^^^^^ |unreducible application of higher-kinded type [A, B] => dotty.TupleCons[A, dotty.TupleCons[B, Unit]] to wildcard arguments

The LogPendingSubTypesThreshold change is requires for tests on size ~25 tuples that would otherwise trigger -Yno-deep-subtypes

Because all tests depend on dotty-library-bootstrapped publishLocal is always needed to run tests.

OlivierBlanvillain · 2017-05-01T09:10:36Z

@odersky yes! The last missing part is to update pattern matching exhaustivity. The current implementation is special cased for tuples; to obtain the same warnings when using hlists this code would need to be either generalised for GADTs or special cased for the new structure.

mdedetrich · 2017-05-01T09:25:00Z

@szeiger

Why would you still have special implementations up to 22 here anyway? I think you should lower that number considerably. 22 was originally chosen as large enough to represent what Scala needed, not because of performance considerations. The latter should be the driving factor for determining a good size for Dotty if you support arbitrary tuple sizes anyway. You are not bound by binary compatibility and you have Scalafix to take care of source compatibility.

AFAIK in scalac, 22 was chosen actually due to the Scala runtime size. The blowup in jar size for the various Tuple/Product/Function etc classes that get generated is polynomial relative to the arity, and once you start going above 22 you start adding megs onto the Scala runtime

@OlivierBlanvillain

It seems like the current trend is to remove things from the standard library, not add more, so I'm not really sure it shapeless style operations on tuples would have a place in the standard library.

I think that there should be an exception for shapeless style operations because they tend to be central to how the language is used. A lot of the things which shapeless provides, if they were baked into the actual scalac backend, would be a lot more performant and they would also remove a huge amount of boilerplate.

That doesn't mean the entirety of shapeless should be in Scalac, but a lot of the tuple/case class conversion functions, and other methods of abstracting over arity I think would be welcome. Its painful to see languages like Ruby and Python able to do something in one line which Scala doesn't have an answer for without shapeless (or it does but it takes 20 lines)

odersky

Impressive work! I have lots of comments on details but the general approach looks good.

One overall remark: Have a closer look what goes on in Definitions. It seems a lot of the things I criticise really come down to the fact that you don't make optimal use of the things defined there.

odersky · 2017-05-07T16:02:54Z

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

+          case 0 => unitLiteral
+          case _ if ctx.mode is Mode.Type =>
+            // Transforming Tuple types: (T1, T2) → TupleCons[T1, TupleCons[T2, Unit]]
+            val nil: Tree = TypeTree(defn.UnitType)


Use hnilType for consistency with hconsType?

odersky · 2017-05-07T16:03:50Z

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

+            ts.foldRight(nil)(hconsType)
+          case _ =>
+            // Transforming Tuple trees: (T1, T2, ..., TN) → TupleCons(T1, TupleCons(T2, ... (TupleCons(TN, ())))
+            val nil: Tree = unitLiteral


Again for naming consistency, use hnil, hcons?

odersky · 2017-05-07T16:04:34Z

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

@@ -140,7 +140,7 @@ object Config {
  final val LogPendingUnderlyingThreshold = 50

  /** How many recursive calls to isSubType are performed before logging starts. */
-  final val LogPendingSubTypesThreshold = 50
+  final val LogPendingSubTypesThreshold = 70


Why was this increased?

I have some tests with size 24 tuples that triggers this limit, I'm going to revert this change and run these test without -Yno-deep-subtypes.

odersky · 2017-05-07T16:07:15Z

compiler/src/dotty/tools/dotc/core/Definitions.scala

-  lazy val ProductNType = mkArityArray("scala.Product", MaxTupleArity, 0)
+  lazy val TupleNType        = mkArityArray("scala.Tuple", MaxImplementedTupleArity, 1)
+  lazy val TupleNSymbol      = TupleNType.map(t => if (t == null) t else t.classSymbol)
+  lazy val DottyTupleNType   = mkArityArray("dotty.DottyTuple", MaxImplementedTupleArity, 1)


Defined like this TupleNSymbol is an Array[AnyRef]. It's better to use if (t == null) NoSymbol which makes it an Array[Symbol].

odersky · 2017-05-07T16:07:27Z

compiler/src/dotty/tools/dotc/core/Definitions.scala

+  lazy val TupleNType        = mkArityArray("scala.Tuple", MaxImplementedTupleArity, 1)
+  lazy val TupleNSymbol      = TupleNType.map(t => if (t == null) t else t.classSymbol)
+  lazy val DottyTupleNType   = mkArityArray("dotty.DottyTuple", MaxImplementedTupleArity, 1)
+  lazy val DottyTupleNModule = DottyTupleNType.map(t => if (t == null) t else t.classSymbol.companionModule.symbol)


odersky · 2017-05-07T17:38:35Z

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

@@ -758,10 +758,18 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
        else errorType(i"wrong number of parameters, expected: ${protoFormals.length}", tree.pos)

      /** Is `formal` a product type which is elementwise compatible with `params`? */
-      def ptIsCorrectProduct(formal: Type) = {
+      def ptIsCorrectProduct(formal: Type): Boolean = {
+        val cons = defn.TupleConsType.symbol


defn.TupleConsClass (once it is introduced, see other comment)

odersky · 2017-05-07T17:40:25Z

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

+        val cons = defn.TupleConsType.symbol
+
+        // Flatten types nested in TupleCons as a List[Type].
+        def tupleType(t: Type, acc: List[Type] = Nil): List[Type] =


Rename to tupleTypeArgs

compiler/src/dotty/tools/dotc/transform/TupleRewrites.scala

odersky · 2017-05-07T17:45:44Z

library/src_dotc/dotty/DottyPredef.scala

@@ -0,0 +1,471 @@
+package dotty


I am not sure about the src_dotc scheme or the other aspects of the build. Let's discuss this aspect with the dotty team as a whole.

odersky · 2017-05-07T17:46:32Z

library/src_dotc/dotty/DottyPredef.scala

+
+  def Tuple1[A](a: A): TC[A, Unit] = TC(a, ())
+
+  implicit class Tuple2Assessors[A, B](l: TC[A, TC[B, Unit]]) {


Assessors? Should it not be Accessors?

julienrf · 2017-06-30T11:00:14Z

What’s the status of this PR?

Also, this might be out of the scope of this PR but I think you should not stop in the mid-way and do the same for sealed traits ;) (using Either instead of TupleCons).

OlivierBlanvillain · 2017-06-30T11:09:27Z

@julienrf Lots of the complexity of this PR is due to have dotty as cross complied scalac/dotty project. Since going for a bootstrap only compiler is very close on the road-map, I'm going to wait for that before coming back to this PR.

Sum types will hopefully be the follow up :)

antonkulaga · 2018-03-29T23:13:32Z

Why would you still have special implementations up to 22 here anyway? I think you should lower that number considerably. 22 was originally chosen as large enough to represent what Scala needed

Actually, I hitted 22 fields limit in some real-case scenarios (when I had to write large case classes to parse large cvs-s)

joan38 · 2018-04-26T10:52:24Z

Hi guys,
Any news on this feature? Is it still just paused?
Cheers

ctongfei · 2018-05-15T19:08:51Z

I wonder if it is possible to create a typesafe apply method on this HList-like Tuple type.

Something like

trait Tuple { self: A =>
  def apply[I <: Int & Singleton, X](i: I)
    (implicit ev: TupleIndexer.Aux[A, I, X]): X = ev(self)
}

So you could do the following at compile time:

val t: (Int, String, Double)
t(0) // type: Int
t(1) // type: String
t(2) // type: Double
t(3) // ERROR: does not compile

OlivierBlanvillain · 2018-05-25T11:33:06Z

@joan38 It's postponed until scalac can emit TASTY and we can recompile everything from TASTY, given than the stdlib and thus the entire world has a binary dependency on Tuple2. Until then, the hacks needed in this PR to retain binary compatibility are a bit too much IMO.

joan38 · 2018-05-25T12:02:26Z

Thanks @OlivierBlanvillain for the update. So I guess until 2.14 millstone appears? This feature is really promising.

Glavo · 2018-05-25T20:12:22Z

I am worried that this change will slow down the compilation speed.

felixmulder · 2018-05-25T20:14:30Z

@Glavo - don't be :)

Also, dotty has a benchmarking suite that is being used for a lot of pull requests. So now the compiler team can actually know if something would slow down the compiler before merging.

nicolasstucki · 2018-10-16T15:08:38Z

@OlivierBlanvillain now we have another implementation of the same idea, can we close this PR? Or is there something useful that was not covered by the current implementation?

allanrenucci · 2018-10-16T15:11:19Z

Maybe we should extract the test cases

joan38 · 2018-10-16T15:46:29Z

@nicolasstucki @allanrenucci Could you point out to the other implementation?

allanrenucci · 2018-10-16T15:59:22Z

@joan38 #4938

4 of these tests are failing, I put them in pending a trace. Intrestingly they all fail in different ways: Compiler crash tests/pending/tuple-patmat-extract.scala Compilation error tests/pending/tuple-for-comprehension.scala Runtime error tests/pending/tuple-erased.scala Doesn't pass Ycheck tests/pending/tuple-cons.scala

4 of these tests are failing, I put them in pending for now. Interestingly each of them fails in different ways: Compiler crash tests/pending/tuple-patmat-extract.scala Compilation error tests/pending/tuple-for-comprehension.scala Runtime error tests/pending/tuple-erased.scala Doesn't pass Ycheck tests/pending/tuple-cons.scala

nicolasstucki · 2018-10-17T05:43:54Z

There is also #5182, where the macro implementation changed completely.

Add tuple >22 test cases from #2199

lJoublanc · 2019-01-23T12:04:29Z

How will this interact with the standard collections zip operator, which is left-associative (rather than right-associative, like the implementation suggested here)?
It would be nice to be able to say e.g. List(a) zip List(b) zip List(c) === List((a,(b,c))) === List((a,b,c)), but with the existing associativity of zip, you end up with List(((a,b),c)) <> List((a,b,c)).

AnirudhVyas · 2020-03-30T15:49:10Z

has this been added to Dotty already?

nicolasstucki · 2020-03-30T16:22:23Z

Yes. Under scala.Tuple. (a,b, c) is equivalent to a *: b *: c *: ()

julienrf reviewed Apr 7, 2017

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julienrf reviewed Apr 10, 2017

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OlivierBlanvillain force-pushed the rip-22 branch from a792788 to 5b48d46 Compare April 10, 2017 16:59

OlivierBlanvillain force-pushed the rip-22 branch from 967bd9d to 6214895 Compare April 25, 2017 09:42

OlivierBlanvillain force-pushed the rip-22 branch 2 times, most recently from b11a2f7 to 303a7d3 Compare April 27, 2017 09:05

OlivierBlanvillain added 11 commits May 1, 2017 09:53

Minor formating changes

e1ef43d

Remove dependency between dotty-compiler and dotty-library

9f4fca6

Add TupleN defs, types and implicit classes for source compat

69a402a

Add tests for new the tuples

22ffef2

The LogPendingSubTypesThreshold change is requires for tests on size ~25 tuples that would otherwise trigger -Yno-deep-subtypes

Broke PatmatExhaustivityTest and runCompilerFromInterface

0e477a0

Always publishLocal before tests

491947a

Because all tests depend on dotty-library-bootstrapped publishLocal is always needed to run tests.

Explicitly type implicit in a doc-tool patdef

6c92389

OlivierBlanvillain force-pushed the rip-22 branch from 303a7d3 to 6c92389 Compare May 1, 2017 08:50

odersky self-requested a review May 5, 2017 12:59

odersky requested changes May 7, 2017

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eparejatobes mentioned this pull request Aug 30, 2017

Investigate alternative encodings for Scala products ohnosequences/stuff#48

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Blaisorblade mentioned this pull request Jan 17, 2018

Sketch of possible typelevel extension of principled metaprogramming #3844

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OlivierBlanvillain self-assigned this Jan 18, 2018

odersky mentioned this pull request Apr 1, 2018

Add opaque types #4028

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OlivierBlanvillain closed this Oct 16, 2018

nicolasstucki added a commit that referenced this pull request Oct 17, 2018

Merge pull request #5266 from dotty-staging/tuple-tests

39e6a49

Add tuple >22 test cases from #2199

tribbloid mentioned this pull request Jul 19, 2020

Scala DSL breandan/kotlingrad#11

Open


		def Tuple1[A](a: A): TC[A, Unit] = TC(a, ())

		implicit class Tuple2Assessors[A, B](l: TC[A, TC[B, Unit]]) {

Redesign Tuples with HList-like structure #2199

Redesign Tuples with HList-like structure #2199

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OlivierBlanvillain commented Jun 30, 2017 •

edited

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joan38 commented Oct 16, 2018 •

edited

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AnirudhVyas commented Mar 30, 2020 •

edited

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