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Add Order instance for scala.Symbol #1395

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Sep 28, 2016
Merged

Add Order instance for scala.Symbol #1395

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d75186a
Add Order instance for scala.Symbol
durban Sep 24, 2016
318e456
Add comment that Symbols are interned
durban Sep 28, 2016
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1 change: 1 addition & 0 deletions AUTHORS.md
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Original file line number Diff line number Diff line change
Expand Up @@ -41,6 +41,7 @@ possible:
* Csongor Kiss
* Dale Wijnand
* Daniel Spiewak
* Daniel Urban
* Dave Gurnell
* Dave Rostron
* David Allsopp
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7 changes: 7 additions & 0 deletions kernel-laws/src/test/scala/cats/kernel/laws/LawTests.scala
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Expand Up @@ -30,6 +30,9 @@ class LawTests extends FunSuite with Discipline {
implicit val arbitraryBitSet: Arbitrary[BitSet] =
Arbitrary(arbitrary[List[Short]].map(ns => BitSet(ns.map(_ & 0xffff): _*)))

implicit val arbitrarySymbol: Arbitrary[Symbol] =
Arbitrary(arbitrary[String].map(s => Symbol(s)))

// this instance is not available in scalacheck 1.13.2.
// remove this once a newer version is available.
implicit val cogenBigInt: Cogen[BigInt] =
Expand All @@ -40,6 +43,9 @@ class LawTests extends FunSuite with Discipline {
implicit val cogenBigDecimal: Cogen[BigDecimal] =
Cogen[Double].contramap(_.toDouble)

implicit val cogenSymbol: Cogen[Symbol] =
Cogen[String].contramap(_.name)

{
// needed for Cogen[Map[...]]
implicit val ohe: Ordering[HasEq[Int]] = Ordering[Int].on(_.a)
Expand All @@ -64,6 +70,7 @@ class LawTests extends FunSuite with Discipline {
laws[OrderLaws, Unit].check(_.order)
laws[OrderLaws, Boolean].check(_.order)
laws[OrderLaws, String].check(_.order)
laws[OrderLaws, Symbol].check(_.order)
laws[OrderLaws, Byte].check(_.order)
laws[OrderLaws, Short].check(_.order)
laws[OrderLaws, Char].check(_.order)
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1 change: 1 addition & 0 deletions kernel/src/main/scala/cats/kernel/instances/all.scala
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Expand Up @@ -22,6 +22,7 @@ trait AllInstances
with ShortInstances
with StreamInstances
with StringInstances
with SymbolInstances
with TupleInstances
with UnitInstances
with VectorInstances
15 changes: 15 additions & 0 deletions kernel/src/main/scala/cats/kernel/instances/symbol.scala
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@@ -0,0 +1,15 @@
package cats.kernel
package instances

package object symbol extends SymbolInstances

trait SymbolInstances {
implicit val catsKernelStdOrderForSymbol: Order[Symbol] = new SymbolOrder
}

class SymbolOrder extends Order[Symbol] {
override def eqv(x: Symbol, y: Symbol): Boolean =
x eq y
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Any reason not to use == here? I assume Symbol's equal method is correct and we don't need to worry here that eq is correct.

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Symbols are interned ... might as well take advantage of that fact.

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Yes, that was my intention.

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yes, I know that eq is how it is implemented due to the interning, but equals already does this:
/~https://github.com/scala/scala/blob/2.12.x/src/library/scala/Symbol.scala#L31

so, this, to me, assumes the jit may not do the job, yet each reviewer of the code has to recall this fact about how Symbol instances are created.

That said, I'm happy to simply add a comment here to explain this so that the code is clearer to someone unfamiliar (I think Symbol is actually rather unused as a scala feature these days).

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Ok, comment added in 318e456.

def compare(x: Symbol, y: Symbol): Int =
if (x eq y) 0 else x.name compareTo y.name
}