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Don't combine lefts on Xor and XorT combine #1034

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May 19, 2016
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Don't combine lefts on Xor and XorT combine #1034

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faf05f9
Don't combine lefts on Xor and XorT combine
ceedubs May 14, 2016
2f8e0fe
Use more descriptive binding name in Xor.combine
ceedubs May 17, 2016
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46 changes: 38 additions & 8 deletions core/src/main/scala/cats/data/Xor.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -133,13 +133,43 @@ sealed abstract class Xor[+A, +B] extends Product with Serializable {

def merge[AA >: A](implicit ev: B <:< AA): AA = fold(identity, ev.apply)

final def combine[AA >: A, BB >: B](that: AA Xor BB)(implicit AA: Semigroup[AA], BB: Semigroup[BB]): AA Xor BB = this match {
case Xor.Left(a1) => that match {
case Xor.Left(a2) => Xor.Left(AA.combine(a1, a2))
case Xor.Right(b2) => Xor.Left(a1)
}
/**
* Combine with another `Xor` value.
*
* If this `Xor` is a `Left` then it will be returned as-is.
* If this `Xor` is a `Right` and `that` `Xor` is a left, then `that` will be
* returned.
* If both `Xor`s are `Right`s, then the `Semigroup[BB]` instance will be used
* to combine both values and return them as a `Right`.
* Note: If both `Xor`s are `Left`s then their values are not combined. Use
* `Validated` if you prefer to combine `Left` values.
*
* Examples:
* {{{
* scala> import cats.data.Xor
* scala> import cats.implicits._
* scala> val l1: Xor[String, Int] = Xor.left("error 1")
* scala> val l2: Xor[String, Int] = Xor.left("error 2")
* scala> val r3: Xor[String, Int] = Xor.right(3)
* scala> val r4: Xor[String, Int] = Xor.right(4)
*
* scala> l1 combine l2
* res0: Xor[String, Int] = Left(error 1)
*
* scala> l1 combine r3
* res1: Xor[String, Int] = Left(error 1)
*
* scala> r3 combine l1
* res2: Xor[String, Int] = Left(error 1)
*
* scala> r3 combine r4
* res3: Xor[String, Int] = Right(7)
* }}}
*/
final def combine[AA >: A, BB >: B](that: AA Xor BB)(implicit BB: Semigroup[BB]): AA Xor BB = this match {
case left @ Xor.Left(_) => left
case Xor.Right(b1) => that match {
case Xor.Left(a2) => Xor.Left(a2)
case left @ Xor.Left(_) => left
case Xor.Right(b2) => Xor.Right(BB.combine(b1, b2))
}
}
Expand Down Expand Up @@ -168,7 +198,7 @@ private[data] sealed abstract class XorInstances extends XorInstances1 {
def show(f: A Xor B): String = f.show
}

implicit def xorMonoid[A, B](implicit A: Semigroup[A], B: Monoid[B]): Monoid[A Xor B] =
implicit def xorMonoid[A, B](implicit B: Monoid[B]): Monoid[A Xor B] =
new Monoid[A Xor B] {
def empty: A Xor B = Xor.Right(B.empty)
def combine(x: A Xor B, y: A Xor B): A Xor B = x combine y
Expand Down Expand Up @@ -229,7 +259,7 @@ private[data] sealed abstract class XorInstances extends XorInstances1 {

private[data] sealed abstract class XorInstances1 extends XorInstances2 {

implicit def xorSemigroup[A, B](implicit A: Semigroup[A], B: Semigroup[B]): Semigroup[A Xor B] =
implicit def xorSemigroup[A, B](implicit B: Semigroup[B]): Semigroup[A Xor B] =
new Semigroup[A Xor B] {
def combine(x: A Xor B, y: A Xor B): A Xor B = x combine y
}
Expand Down
40 changes: 39 additions & 1 deletion core/src/main/scala/cats/data/XorT.scala
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Expand Up @@ -110,7 +110,45 @@ final case class XorT[F[_], A, B](value: F[A Xor B]) {

def merge[AA >: A](implicit ev: B <:< AA, F: Functor[F]): F[AA] = F.map(value)(_.fold(identity, ev.apply))

def combine(that: XorT[F, A, B])(implicit F: Apply[F], A: Semigroup[A], B: Semigroup[B]): XorT[F, A, B] =
/**
* Similar to [[Xor.combine]] but mapped over an `F` context.
*
* Examples:
* {{{
* scala> import cats.data.XorT
* scala> import cats.implicits._
* scala> val l1: XorT[Option, String, Int] = XorT.left(Some("error 1"))
* scala> val l2: XorT[Option, String, Int] = XorT.left(Some("error 2"))
* scala> val r3: XorT[Option, String, Int] = XorT.right(Some(3))
* scala> val r4: XorT[Option, String, Int] = XorT.right(Some(4))
* scala> val noneXorT: XorT[Option, String, Int] = XorT.left(None)
*
* scala> l1 combine l2
* res0: XorT[Option, String, Int] = XorT(Some(Left(error 1)))
*
* scala> l1 combine r3
* res1: XorT[Option, String, Int] = XorT(Some(Left(error 1)))
*
* scala> r3 combine l1
* res2: XorT[Option, String, Int] = XorT(Some(Left(error 1)))
*
* scala> r3 combine r4
* res3: XorT[Option, String, Int] = XorT(Some(Right(7)))
*
* scala> l1 combine noneXorT
* res4: XorT[Option, String, Int] = XorT(None)
*
* scala> noneXorT combine l1
* res5: XorT[Option, String, Int] = XorT(None)
*
* scala> r3 combine noneXorT
* res6: XorT[Option, String, Int] = XorT(None)
*
* scala> noneXorT combine r4
* res7: XorT[Option, String, Int] = XorT(None)
* }}}
*/
def combine(that: XorT[F, A, B])(implicit F: Apply[F], B: Semigroup[B]): XorT[F, A, B] =
XorT(F.map2(this.value, that.value)(_ combine _))

def toValidated(implicit F: Functor[F]): F[Validated[A, B]] =
Expand Down