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Require an Order instance for NonEmptyList's groupBy function #1964

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merged 5 commits into from
Oct 13, 2017
Merged

Require an Order instance for NonEmptyList's groupBy function #1964

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685e949
Require an Order instance for NonEmptyList's groupBy function
igstan Oct 11, 2017
24ff926
There's no mutable TreeMap before Scala 2.12; use only immutable
igstan Oct 12, 2017
9f116d6
Don't use `mapValues`, as per code review comments
igstan Oct 12, 2017
cace36c
Have NonEmptyList.groupBy return a SortedMap instead of just a Map
igstan Oct 12, 2017
2f0a165
Have List.groupByNel return a SortedMap as well
igstan Oct 12, 2017
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36 changes: 21 additions & 15 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 @@ -3,11 +3,10 @@ package data

import cats.instances.list._
import cats.syntax.order._

import scala.annotation.tailrec
import scala.collection.immutable.TreeSet
import scala.collection.immutable.{ SortedMap, TreeMap, TreeSet }
import scala.collection.mutable
import scala.collection.mutable.ListBuffer
import scala.collection.{immutable, mutable}

/**
* A data type which represents a non empty list of A, with
Expand Down Expand Up @@ -321,28 +320,35 @@ final case class NonEmptyList[+A](head: A, tail: List[A]) {
}

/**
* Groups elements inside of this `NonEmptyList` using a mapping function
* Groups elements inside this `NonEmptyList` according to the `Order`
* of the keys produced by the given mapping function.
*
* {{{
* scala> import scala.collection.immutable.SortedMap
* scala> import cats.data.NonEmptyList
* scala> import cats.instances.boolean._
* scala> val nel = NonEmptyList.of(12, -2, 3, -5)
* scala> nel.groupBy(_ >= 0)
* res0: Map[Boolean, cats.data.NonEmptyList[Int]] = Map(false -> NonEmptyList(-2, -5), true -> NonEmptyList(12, 3))
* res0: SortedMap[Boolean, cats.data.NonEmptyList[Int]] = Map(false -> NonEmptyList(-2, -5), true -> NonEmptyList(12, 3))
* }}}
*/
def groupBy[B](f: A => B): Map[B, NonEmptyList[A]] = {
val m = mutable.Map.empty[B, mutable.Builder[A, List[A]]]
def groupBy[B](f: A => B)(implicit B: Order[B]): SortedMap[B, NonEmptyList[A]] = {
implicit val ordering: Ordering[B] = B.toOrdering
var m = TreeMap.empty[B, mutable.Builder[A, List[A]]]

for { elem <- toList } {
m.getOrElseUpdate(f(elem), List.newBuilder[A]) += elem
}
val b = immutable.Map.newBuilder[B, NonEmptyList[A]]
for { (k, v) <- m } {
val head :: tail = v.result // we only create non empty list inside of the map `m`
b += ((k, NonEmptyList(head, tail)))
val k = f(elem)

m.get(k) match {
case None => m += ((k, List.newBuilder[A] += elem))
case Some(builder) => builder += elem
}
}
b.result
}

m.map {
case (k, v) => (k, NonEmptyList.fromListUnsafe(v.result))
} : TreeMap[B, NonEmptyList[A]]
}
}

object NonEmptyList extends NonEmptyListInstances {
Expand Down
2 changes: 1 addition & 1 deletion core/src/main/scala/cats/syntax/list.scala
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Expand Up @@ -27,6 +27,6 @@ final class ListOps[A](val la: List[A]) extends AnyVal {
* }}}
*/
def toNel: Option[NonEmptyList[A]] = NonEmptyList.fromList(la)
def groupByNel[B](f: A => B): Map[B, NonEmptyList[A]] =
def groupByNel[B : Order](f: A => B): Map[B, NonEmptyList[A]] =
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shouldn't the type here also be SortedMap?

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Yes, it should. Working on it.

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Well... current status: yak-shaving inside NonEmptyTraverse doctests, where SortedMap needs an Apply instance.

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I've pushed something that addresses this. I've fixed the NonEmptyTraverse doctests by upcasting to a vanilla Map for which there's an Apply instance. I'm not completely satisfied with this approach, but it's the best I've got right now.

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I think making an Apply and Traversable for SortedMap is probably out of scope, so I think that's okay.

toNel.fold(Map.empty[B, NonEmptyList[A]])(_.groupBy(f))
}