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remove iteratorFoldM fixes #1716 #1740

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Jun 26, 2017
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remove iteratorFoldM fixes #1716 #1740

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33352a2
remove iteratorFoldM fixes #1716
kailuowang Jun 23, 2017
ba99572
fix foldM to not evaluate one extra uneccessary element
kailuowang Jun 23, 2017
9705b47
update comment
kailuowang Jun 23, 2017
e2f94dc
more efficient implementations
kailuowang Jun 23, 2017
e955595
keep the original sequence
kailuowang Jun 23, 2017
a8b54a9
updated NonEmptyVector as well
kailuowang Jun 23, 2017
e2f53ae
use default foldM implemenation to test short-circuit
kailuowang Jun 23, 2017
312ee35
style correction / remove redundant overrides
kailuowang Jun 23, 2017
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41 changes: 3 additions & 38 deletions core/src/main/scala/cats/Foldable.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -201,7 +201,7 @@ import simulacrum.typeclass
def foldM[G[_], A, B](fa: F[A], z: B)(f: (B, A) => G[B])(implicit G: Monad[G]): G[B] = {
val src = Foldable.Source.fromFoldable(fa)(self)
G.tailRecM((z, src)) { case (b, src) => src.uncons match {
case Some((a, src)) => G.map(f(b, a))(b => Left((b, src)))
case Some((a, src)) => G.map(f(b, a))(b => Left((b, src.value)))
case None => G.pure(Right(b))
}}
}
Expand Down Expand Up @@ -414,41 +414,6 @@ object Foldable {
loop()
}

/**
* Implementation of [[Foldable.foldM]] which can short-circuit for
* structures with an `Iterator`.
*
* For example we can sum a `Stream` of integers and stop if
* the sum reaches 100 (if we reach the end of the `Stream`
* before getting to 100 we return the total sum) :
*
* {{{
* scala> import cats.implicits._
* scala> type LongOr[A] = Either[Long, A]
* scala> def sumStream(s: Stream[Int]): Long =
* | Foldable.iteratorFoldM[LongOr, Int, Long](s.toIterator, 0L){ (acc, n) =>
* | val sum = acc + n
* | if (sum < 100L) Right(sum) else Left(sum)
* | }.merge
*
* scala> sumStream(Stream.continually(1))
* res0: Long = 100
*
* scala> sumStream(Stream(1,2,3,4))
* res1: Long = 10
* }}}
*
* Note that `Foldable[Stream].foldM` uses this method underneath, so
* you wouldn't call this method explicitly like in the example above.
*/
def iteratorFoldM[M[_], A, B](it: Iterator[A], z: B)(f: (B, A) => M[B])(implicit M: Monad[M]): M[B] = {
val go: B => M[Either[B, B]] = { b =>
if (it.hasNext) M.map(f(b, it.next))(Left(_))
else M.pure(Right(b))
}
M.tailRecM(z)(go)
}


/**
* Isomorphic to
Expand All @@ -461,7 +426,7 @@ object Foldable {
* https://github.com/scala/bug/issues/9600 is resolved.
*/
private sealed abstract class Source[+A] {
def uncons: Option[(A, Source[A])]
def uncons: Option[(A, Eval[Source[A]])]
}

private object Source {
Expand All @@ -470,7 +435,7 @@ object Foldable {
}

def cons[A](a: A, src: Eval[Source[A]]): Source[A] = new Source[A] {
def uncons = Some((a, src.value))
def uncons = Some((a, src))
}

def fromFoldable[F[_], A](fa: F[A])(implicit F: Foldable[F]): Source[A] =
Expand Down
3 changes: 0 additions & 3 deletions core/src/main/scala/cats/data/NonEmptyList.scala
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Expand Up @@ -431,9 +431,6 @@ private[data] sealed trait NonEmptyListInstances extends NonEmptyListInstances0
override def fold[A](fa: NonEmptyList[A])(implicit A: Monoid[A]): A =
fa.reduce

override def foldM[G[_], A, B](fa: NonEmptyList[A], z: B)(f: (B, A) => G[B])(implicit G: Monad[G]): G[B] =
Foldable.iteratorFoldM(fa.toList.toIterator, z)(f)

override def find[A](fa: NonEmptyList[A])(f: A => Boolean): Option[A] =
fa find f

Expand Down
3 changes: 0 additions & 3 deletions core/src/main/scala/cats/data/NonEmptyVector.scala
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Expand Up @@ -264,9 +264,6 @@ private[data] sealed trait NonEmptyVectorInstances {
override def fold[A](fa: NonEmptyVector[A])(implicit A: Monoid[A]): A =
fa.reduce

override def foldM[G[_], A, B](fa: NonEmptyVector[A], z: B)(f: (B, A) => G[B])(implicit G: Monad[G]): G[B] =
Foldable.iteratorFoldM(fa.toVector.toIterator, z)(f)

override def find[A](fa: NonEmptyVector[A])(f: A => Boolean): Option[A] =
fa.find(f)

Expand Down
3 changes: 0 additions & 3 deletions core/src/main/scala/cats/instances/list.scala
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Expand Up @@ -90,9 +90,6 @@ trait ListInstances extends cats.kernel.instances.ListInstances {

override def filter[A](fa: List[A])(f: A => Boolean): List[A] = fa.filter(f)

override def foldM[G[_], A, B](fa: List[A], z: B)(f: (B, A) => G[B])(implicit G: Monad[G]): G[B] =
Foldable.iteratorFoldM(fa.toIterator, z)(f)
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can't we maybe do this one?

def step(in: (List[A], B])): G[Either[(List[A], B), A]] = in match {
  case (Nil, b) => G.pure(Right(b))
  case (a :: tail, b) => G.map(f(a, b)) { bnext => Left((tail, bnext)) }
} 

G.tailRecM((fa, z))(step)

right? If we had this, we could cover the common case for List and NonEmptyList, no?


override def fold[A](fa: List[A])(implicit A: Monoid[A]): A = A.combineAll(fa)

override def toList[A](fa: List[A]): List[A] = fa
Expand Down
3 changes: 0 additions & 3 deletions core/src/main/scala/cats/instances/map.scala
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Expand Up @@ -85,9 +85,6 @@ trait MapInstances extends cats.kernel.instances.MapInstances {

override def isEmpty[A](fa: Map[K, A]): Boolean = fa.isEmpty

override def foldM[G[_], A, B](fa: Map[K, A], z: B)(f: (B, A) => G[B])(implicit G: Monad[G]): G[B] =
Foldable.iteratorFoldM(fa.valuesIterator, z)(f)

override def fold[A](fa: Map[K, A])(implicit A: Monoid[A]): A =
A.combineAll(fa.values)

Expand Down
3 changes: 0 additions & 3 deletions core/src/main/scala/cats/instances/set.scala
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Expand Up @@ -43,9 +43,6 @@ trait SetInstances extends cats.kernel.instances.SetInstances {

override def isEmpty[A](fa: Set[A]): Boolean = fa.isEmpty

override def foldM[G[_], A, B](fa: Set[A], z: B)(f: (B, A) => G[B])(implicit G: Monad[G]): G[B] =
Foldable.iteratorFoldM(fa.toIterator, z)(f)

override def fold[A](fa: Set[A])(implicit A: Monoid[A]): A = A.combineAll(fa)

override def toList[A](fa: Set[A]): List[A] = fa.toList
Expand Down
3 changes: 0 additions & 3 deletions core/src/main/scala/cats/instances/stream.scala
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Expand Up @@ -118,9 +118,6 @@ trait StreamInstances extends cats.kernel.instances.StreamInstances {

override def collect[A, B](fa: Stream[A])(f: PartialFunction[A, B]): Stream[B] = fa.collect(f)

override def foldM[G[_], A, B](fa: Stream[A], z: B)(f: (B, A) => G[B])(implicit G: Monad[G]): G[B] =
Foldable.iteratorFoldM(fa.toIterator, z)(f)
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we could copy the List approach above here:

def step(in: (Stream[A], B])): G[Either[(Stream[A], B), A]] = in match {
  case (Stream.empty, b) => G.pure(Right(b))
  case (a #:: tail, b) => G.map(f(a, b)) { bnext => Left((tail, bnext)) }
} 

G.tailRecM((fa, z))(step)

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Patten match Stream into a #:: tail will force an evaluation of the head of the tail, so I used vanilla if


override def fold[A](fa: Stream[A])(implicit A: Monoid[A]): A = A.combineAll(fa)

override def toList[A](fa: Stream[A]): List[A] = fa.toList
Expand Down
3 changes: 0 additions & 3 deletions core/src/main/scala/cats/instances/vector.scala
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Expand Up @@ -90,9 +90,6 @@ trait VectorInstances extends cats.kernel.instances.VectorInstances {

override def collect[A, B](fa: Vector[A])(f: PartialFunction[A, B]): Vector[B] = fa.collect(f)

override def foldM[G[_], A, B](fa: Vector[A], z: B)(f: (B, A) => G[B])(implicit G: Monad[G]): G[B] =
Foldable.iteratorFoldM(fa.toIterator, z)(f)
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Vector.tail is so slow, we are probably better off doing fa.toList.foldM here if we do the above.


override def fold[A](fa: Vector[A])(implicit A: Monoid[A]): A = A.combineAll(fa)

override def toList[A](fa: Vector[A]): List[A] = fa.toList
Expand Down
11 changes: 6 additions & 5 deletions tests/src/test/scala/cats/tests/FoldableTests.scala
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Expand Up @@ -197,12 +197,7 @@ class FoldableTestsAdditional extends CatsSuite {
}
assert(result.value)

// test trampolining
val large = Stream((1 to 10000): _*)
assert(contains(large, 10000).value)

// test laziness of foldM
dangerous.foldM(0)((acc, a) => if (a < 2) Some(acc + a) else None) should === (None)
}

test(".foldLeftM short-circuiting") {
Expand All @@ -226,6 +221,12 @@ class FoldableTestsAdditional extends CatsSuite {
assert(concatUntil("Zero" #:: "STOP" #:: boom, "STOP") == Left("Zero"))
assert(concatUntil("Zero" #:: "One" #:: "STOP" #:: boom, "STOP") == Left("ZeroOne"))
}

test("Foldable[List] doesn't break substitution") {
val result = List.range(0,10).foldM(List.empty[Int])((accum, elt) => Eval.always(elt :: accum))

assert(result.value == result.value)
}
}

class FoldableListCheck extends FoldableCheck[List]("list") {
Expand Down