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Use unlimited MathContext for BigDecimal arithmetic #3303

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Feb 24, 2020
Merged

Use unlimited MathContext for BigDecimal arithmetic #3303

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fe342bb
Use MathContext.UNLIMITED for BigDecimal arithmetic
travisbrown Feb 18, 2020
a1aff96
Use default Arbitrary for BigDecimal
travisbrown Feb 18, 2020
cbe2247
Add note about precision of BigDecimal operations
travisbrown Feb 18, 2020
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3 changes: 0 additions & 3 deletions kernel-laws/shared/src/test/scala/cats/kernel/laws/LawTests.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -314,9 +314,6 @@ class Tests extends TestsConfig with AnyFunSuiteLike with FunSuiteDiscipline wit
checkAll("Hash[Queue[Int]", HashTests[Queue[Int]].hash)

{
// default Arbitrary[BigDecimal] is a bit too intense :/
implicit val arbBigDecimal: Arbitrary[BigDecimal] =
Arbitrary(arbitrary[Double].map(n => BigDecimal(n.toString)))
checkAll("Order[BigDecimal]", OrderTests[BigDecimal].order)
checkAll("CommutativeGroup[BigDecimal]", CommutativeGroupTests[BigDecimal].commutativeGroup)
checkAll("CommutativeGroup[BigDecimal]", SerializableTests.serializable(CommutativeGroup[BigDecimal]))
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15 changes: 12 additions & 3 deletions kernel/src/main/scala/cats/kernel/instances/BigDecimalInstances.scala
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Expand Up @@ -8,11 +8,20 @@ trait BigDecimalInstances {
new BigDecimalGroup
}

/**
* Note that combining, removing, and inverting `BigDecimal` values will use unlimited precision
* operations.
*
* This matches the behavior of Scala 2.12 and earlier versions, but not Scala 2.13, which means
* that `+` and `|+|` (or `sum` and `combineAll`) may not agree if you are working with values with
* limited-precision `MathContext`s.
*/
class BigDecimalGroup extends CommutativeGroup[BigDecimal] {
val empty: BigDecimal = BigDecimal(0)
def combine(x: BigDecimal, y: BigDecimal): BigDecimal = x + y
def inverse(x: BigDecimal): BigDecimal = -x
override def remove(x: BigDecimal, y: BigDecimal): BigDecimal = x - y
def combine(x: BigDecimal, y: BigDecimal): BigDecimal = new BigDecimal(x.bigDecimal.add(y.bigDecimal), x.mc)
def inverse(x: BigDecimal): BigDecimal = new BigDecimal(x.bigDecimal.negate(), x.mc)
override def remove(x: BigDecimal, y: BigDecimal): BigDecimal =
new BigDecimal(x.bigDecimal.subtract(y.bigDecimal), x.mc)
}

class BigDecimalOrder extends Order[BigDecimal] with Hash[BigDecimal] {
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