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Adds Semigroup for Const #718

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Dec 6, 2015
Merged

Adds Semigroup for Const #718

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5 changes: 5 additions & 0 deletions core/src/main/scala/cats/data/Const.scala
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Expand Up @@ -72,6 +72,11 @@ private[data] sealed abstract class ConstInstances extends ConstInstances0 {
}

private[data] sealed abstract class ConstInstances0 extends ConstInstances1 {

implicit def constSemigroup[A: Semigroup, B]: Semigroup[Const[A, B]] = new Semigroup[Const[A, B]] {
def combine(x: Const[A, B], y: Const[A, B]): Const[A, B] = x combine y
}

implicit def constPartialOrder[A: PartialOrder, B]: PartialOrder[Const[A, B]] = new PartialOrder[Const[A, B]]{
def partialCompare(x: Const[A, B], y: Const[A, B]): Double =
x partialCompare y
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2 changes: 2 additions & 0 deletions tests/src/test/scala/cats/tests/ConstTests.scala
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Expand Up @@ -25,6 +25,8 @@ class ConstTests extends CatsSuite {
// Algebra checks for Serializability of instances as part of the laws
checkAll("Monoid[Const[Int, String]]", GroupLaws[Const[Int, String]].monoid)

checkAll("Const[NonEmptyList[Int], String]", GroupLaws[Const[NonEmptyList[Int], String]].semigroup)

// Note while Eq is a superclass of PartialOrder and PartialOrder a superclass
// of Order, you can get different instances with different (more general) constraints.
// For instance, you can get an Order for Const if the first type parameter has an Order,
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