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Improve AndThen use of Single #3560

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Aug 13, 2020
Merged

Improve AndThen use of Single #3560

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49 changes: 36 additions & 13 deletions core/src/main/scala/cats/data/AndThen.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -70,21 +70,49 @@ sealed abstract class AndThen[-T, +R] extends (T => R) with Product with Seriali
override def andThen[A](g: R => A): AndThen[T, A] =
// Fusing calls up to a certain threshold, using the fusion
// technique implemented for `cats.effect.IO#map`
this match {
case Single(f, index) if index != fusionMaxStackDepth =>
Single(f.andThen(g), index + 1)
g match {
case atg: AndThen[R, A] =>
(this, atg) match {
case (Single(f, indexf), Single(gs, indexg)) if indexf + indexg < fusionMaxStackDepth =>
Single(f.andThen(gs), indexf + indexg + 1)
case (Concat(left, Single(f, indexf)), Single(gs, indexg)) if indexf + indexg < fusionMaxStackDepth =>
Concat(left, Single(f.andThen(gs), indexf + indexg + 1))
case _ =>
Concat(this, atg)
}
case _ =>
andThenF(AndThen(g))
this match {
case Single(f, index) if index != fusionMaxStackDepth =>
Single(f.andThen(g), index + 1)
case Concat(left, Single(f, index)) if index != fusionMaxStackDepth =>
Concat(left, Single(f.andThen(g), index + 1))
case _ =>
Concat(this, Single(g, 0))
}
}

override def compose[A](g: A => T): AndThen[A, R] =
// Fusing calls up to a certain threshold, using the fusion
// technique implemented for `cats.effect.IO#map`
this match {
case Single(f, index) if index != fusionMaxStackDepth =>
Single(f.compose(g), index + 1)
g match {
case atg: AndThen[A, T] =>
(this, atg) match {
case (Single(f, indexf), Single(gs, indexg)) if indexf + indexg < fusionMaxStackDepth =>
Single(f.compose(gs), indexf + indexg + 1)
case (Concat(Single(f, indexf), right), Single(gs, indexg)) if indexf + indexg < fusionMaxStackDepth =>
Concat(Single(f.compose(gs), indexf + indexg + 1), right)
case _ =>
Concat(atg, this)
}
case _ =>
composeF(AndThen(g))
this match {
case Single(f, index) if index != fusionMaxStackDepth =>
Single(f.compose(g), index + 1)
case Concat(Single(f, index), right) if index != fusionMaxStackDepth =>
Concat(Single(f.compose(g), index + 1), right)
case _ =>
Concat(Single(g, 0), this)
}
}

private def runLoop(start: T): R = {
Expand All @@ -103,11 +131,6 @@ sealed abstract class AndThen[-T, +R] extends (T => R) with Product with Seriali
loop(this, start)
}

final private def andThenF[X](right: AndThen[R, X]): AndThen[T, X] =
Concat(this, right)
final private def composeF[X](right: AndThen[X, T]): AndThen[X, R] =
Concat(right, this)

// converts left-leaning to right-leaning
final protected def rotateAccum[E](_right: AndThen[R, E]): AndThen[T, E] = {
@tailrec
Expand Down