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Increase laziness of LazyList.cons.apply #9095

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merged 1 commit into from Jul 2, 2020
Merged

Increase laziness of LazyList.cons.apply #9095

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2 changes: 1 addition & 1 deletion src/library/scala/collection/immutable/LazyList.scala
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Expand Up @@ -1134,7 +1134,7 @@ object LazyList extends SeqFactory[LazyList] {
* @param hd The first element of the result lazy list
* @param tl The remaining elements of the result lazy list
*/
def apply[A](hd: => A, tl: => LazyList[A]): LazyList[A] = newLL(sCons(hd, tl))
def apply[A](hd: => A, tl: => LazyList[A]): LazyList[A] = newLL(sCons(hd, newLL(tl.state)))

/** Maps a lazy list to its head and tail */
def unapply[A](xs: LazyList[A]): Option[(A, LazyList[A])] = #::.unapply(xs)
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33 changes: 28 additions & 5 deletions test/junit/scala/collection/immutable/LazyListLazinessTest.scala
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Expand Up @@ -693,6 +693,20 @@ class LazyListLazinessTest {
assertLazyAllSkipping(op, 4)
}

private def genericCons_unapply_properlyLazy(unapply: LazyList[Int] => Option[(Int, LazyList[Int])]): Unit = {
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I noticed there were no tests for these while I was in the area, so I added tests

assertLazyAllSkipping(unapply, 1)
}

@Test
def cons_unapply_properlyLazy(): Unit = {
genericCons_unapply_properlyLazy(LazyList.cons.unapply)
}

@Test
def `#::_unapply_properlyLazy`(): Unit = {
genericCons_unapply_properlyLazy(LazyList.#::.unapply)
}

/* factory laziness tests */

@Test
Expand Down Expand Up @@ -746,13 +760,12 @@ class LazyListLazinessTest {
assertRepeatedlyLazy(factory)
}

@Test
def `#:: properlyLazy`(): Unit = {
private def genericCons_properlyLazy(cons: (=> Int, => LazyList[Int]) => LazyList[Int]): Unit = {
val headInitFactory = lazyListFactory { init =>
def gen(index: Int): LazyList[Int] = {
def elem(): Int = { init.evaluate(index); index }
if (index >= LazinessChecker.count) LazyList.empty
else elem() #:: gen(index + 1)
else cons(elem(), gen(index + 1))
}

gen(0)
Expand All @@ -764,7 +777,7 @@ class LazyListLazinessTest {
if (index >= LazinessChecker.count) LazyList.empty
else {
init.evaluate(index)
index #:: gen(index + 1)
cons(index, gen(index + 1))
}
}

Expand All @@ -774,7 +787,17 @@ class LazyListLazinessTest {
}

@Test
def `#::: properlyLazy`(): Unit = {
def cons_properlyLazy(): Unit = {
genericCons_properlyLazy(LazyList.cons(_, _))
}

@Test
def `#::_properlyLazy`(): Unit = {
genericCons_properlyLazy(_ #:: _)
}

@Test
def `#:::_properlyLazy`(): Unit = {
val headInitFactory = lazyListFactory { init =>
def gen(index: Int): LazyList[Int] = {
def elem(): LazyList[Int] = LazyList.fill(1) { init.evaluate(index); index }
Expand Down