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ZIO Stream: Implement ZStream#changes #4697

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Merged
merged 2 commits into from
Mar 17, 2021
Merged

ZIO Stream: Implement ZStream#changes #4697

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2fb74c8
implement ZStream#changes
adamgfraser Feb 15, 2021
9808e60
remove unused pattern variable
adamgfraser Feb 15, 2021
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12 changes: 12 additions & 0 deletions streams-tests/shared/src/test/scala/zio/stream/ZStreamSpec.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -719,6 +719,18 @@ object ZStreamSpec extends ZIOBaseSpec {
.map(assert(_)(isLeft(equalTo("Boom"))))
}
),
testM("changes") {
checkM(pureStreamOfInts) { stream =>
for {
actual <- stream.changes.runCollect.map(_.toList)
expected <- stream.runCollect.map { as =>
as.foldLeft[List[Int]](Nil) { (s, n) =>
if (s.isEmpty || s.head != n) n :: s else s
}.reverse
}
} yield assert(actual)(equalTo(expected))
}
},
testM("collect") {
assertM(ZStream(Left(1), Right(2), Left(3)).collect { case Right(n) =>
n
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30 changes: 30 additions & 0 deletions streams/shared/src/main/scala/zio/stream/ZStream.scala
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Expand Up @@ -554,6 +554,36 @@ abstract class ZStream[-R, +E, +O](val process: ZManaged[R, Nothing, ZIO[R, Opti
): ZStream[R1, E1, O1] =
catchAllCause(pf.applyOrElse[Cause[E], ZStream[R1, E1, O1]](_, ZStream.halt(_)))

/**
* Returns a new stream that only emits elements that are not equal to the
* previous element emitted, using natural equality to determine whether two
* elements are equal.
*/
def changes: ZStream[R, E, O] =
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Would dedup be a better name? (Not sure about that.)

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@adamgfraser adamgfraser Mar 16, 2021 •
edited
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Yes I wondered about that too. This is distinctUntilChanged in reactive streams and removeDuplicates in some other frameworks so deduplicate is definitely reasonable. The slightly tricky thing is it doesn't necessary remove all duplicates in the sense that if the stream emits A, A, B, A the resulting stream will be A, B, A so you will still have two A values just not two consecutive ones.

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I'm ok with changes especially if there is no clear pattern.

changesWith(_ == _)

/**
* Returns a new stream that only emits elements that are not equal to the
* previous element emitted, using the specified function to determine
* whether two elements are equal.
*/
def changesWith(f: (O, O) => Boolean): ZStream[R, E, O] =
ZStream {
for {
ref <- Ref.makeManaged[Option[O]](None)
p <- self.process
pull = for {
z0 <- ref.get
c <- p
(z1, chunk) = c.foldLeft[(Option[O], Chunk[O])]((z0, Chunk.empty)) {
case ((Some(o), os), o1) if (f(o, o1)) => (Some(o1), os)
case ((_, os), o1) => (Some(o1), os :+ o1)
}
_ <- ref.set(z1)
} yield chunk
} yield pull
}

/**
* Re-chunks the elements of the stream into chunks of
* `n` elements each.
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