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add init and size methods to NonEmptyList #1628

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merged 4 commits into from
Apr 26, 2017
Merged

add init and size methods to NonEmptyList #1628

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74f619a
add init and size methods to NonEmptyList
jtjeferreira Apr 20, 2017
fac3c49
add tests
jtjeferreira Apr 24, 2017
b14530c
change NonEmptyList.last implementation
jtjeferreira Apr 24, 2017
63f74f1
Revert "change NonEmptyList.last implementation"
jtjeferreira Apr 25, 2017
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42 changes: 42 additions & 0 deletions core/src/main/scala/cats/data/NonEmptyList.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -17,14 +17,56 @@ final case class NonEmptyList[+A](head: A, tail: List[A]) {

/**
* Return the head and tail into a single list
* {{{
* scala> import cats.data.NonEmptyList
* scala> val nel = NonEmptyList.of(1, 2, 3, 4, 5)
* scala> nel.toList
* res0: scala.collection.immutable.List[Int] = List(1, 2, 3, 4, 5)
* }}}
*/
def toList: List[A] = head :: tail

/**
* Selects the last element
* {{{
* scala> import cats.data.NonEmptyList
* scala> val nel = NonEmptyList.of(1, 2, 3, 4, 5)
* scala> nel.last
* res0: Int = 5
* }}}
*/
def last: A = tail.lastOption match {
case None => head
case Some(a) => a
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tail.lastOption.getOrElse(head) might be more performant.

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he got the exact opposite advice from me a few days ago. Why do you say this? My understanding is that case class matching is very efficient and this also avoids the closure allocation (since getOrElse is by-name).

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@kailuowang kailuowang Apr 26, 2017 •
edited
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@johnynek yeah, I forgot that getOrElse is by-name, and corrected myself in the other comment #1628 (comment)

}

/**
* Selects all elements except the last
*
* {{{
* scala> import cats.data.NonEmptyList
* scala> val nel = NonEmptyList.of(1, 2, 3, 4, 5)
* scala> nel.init
* res0: scala.collection.immutable.List[Int] = List(1, 2, 3, 4)
* }}}
*/
def init: List[A] = tail match {
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@jtjeferreira jtjeferreira Apr 24, 2017 •
edited
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@kailuowang any preference regarding init performance? toList.init is also an alternative

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I am going to stop pretend that I know anything about performance. I don't know if much difference can be made here.

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It (should) be more performant this way I think, because there's one less branch. But only very slightly.

case Nil => List.empty
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Shouldn't this be head :: Nil?

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If the tail is empty the last element is the head, so the result is the empty list

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@peterneyens I added tests for init and size

case t => head :: t.init
}

/**
* The size of this NonEmptyList
*
* {{{
* scala> import cats.data.NonEmptyList
* scala> val nel = NonEmptyList.of(1, 2, 3, 4, 5)
* scala> nel.size
* res0: Int = 5
* }}}
*/
def size: Int = 1 + tail.size

/**
* Applies f to all the elements of the structure
*/
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12 changes: 12 additions & 0 deletions tests/src/test/scala/cats/tests/NonEmptyListTests.scala
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Expand Up @@ -238,6 +238,18 @@ class NonEmptyListTests extends CatsSuite {
}
}

test("NonEmptyList#init is consistent with List#init") {
forAll { nel: NonEmptyList[Int] =>
nel.init should === (nel.toList.init)
}
}

test("NonEmptyList#size is consistent with List#size") {
forAll { nel: NonEmptyList[Int] =>
nel.size should === (nel.toList.size)
}
}

test("NonEmptyList#sorted is consistent with List#sorted") {
forAll { nel: NonEmptyList[Int] =>
nel.sorted.toList should === (nel.toList.sorted)
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