How to generalize the round methods - scala

Question

I have the following four methods, using BigDecimal to round a number:
private def round(input: Byte, scale: Int): Byte = {
BigDecimal(input).setScale(scale, RoundingMode.HALF_UP).byteValue()
}
private def round(input: Short, scale: Int): Short = {
BigDecimal(input).setScale(scale, RoundingMode.HALF_UP).shortValue()
}
private def round(input: Int, scale: Int): Int = {
BigDecimal(input).setScale(scale, RoundingMode.HALF_UP).intValue()
}
private def round(input: Long, scale: Int): Long = {
BigDecimal(input).setScale(scale, RoundingMode.HALF_UP).longValue()
}
And plan to generalize it into a single round:
private def round[T](input: Any, scale: Int, f: (BigDecimal) => T): T = {
f(BigDecimal(input.asInstanceOf[T]).setScale(scale, RoundingMode.HALF_UP))
}
and use this round like this:
round[Byte](b, scale, _.byteValue)
round[Short](s, scale, _.shortValue)
But the above generalized round didn't work because BigDecimal.apply cannot apply to T, what am I supposed to do?

Answer 1

You could use the Numeric type class
def round[T](input: T, scale: Int, f: BigDecimal => T)(implicit n: Numeric[T]): T = {
f(BigDecimal(n.toDouble(input)).setScale(scale, RoundingMode.HALF_UP))
}
Which can be used as:
round(5.525, 2, _.doubleValue)
res0: Double = 5.53
round(123456789L, -5, _.longValue)
res1: Long = 123500000
Another way might be to create a BigDecimalConverter type class, which is not as concise but solves the issue of converting to Double (which is not a good idea for a generic function, like Régis Jean-Gilles commented below).
Updated with a fromBigDecimal method to clean up the round function (thanks to Régis Jean-Gilles).
trait BigDecimalConverter[T] {
def toBigDecimal(in: T) : BigDecimal
def fromBigDecimal(bd: BigDecimal) : T
}
object BigDecimalConverter {
implicit object IntToBigDecimal extends BigDecimalConverter[Int] {
def toBigDecimal(in: Int) = BigDecimal(in)
def fromBigDecimal(bd: BigDecimal) = bd.toInt
}
implicit object DoubleToBigDecimal extends BigDecimalConverter[Double] {
def toBigDecimal(in: Double) = BigDecimal(in)
def fromBigDecimal(bd: BigDecimal) = bd.toDouble
}
implicit object LongToBigDecimal extends BigDecimalConverter[Long] {
def toBigDecimal(in: Long) = BigDecimal(in)
def fromBigDecimal(bd: BigDecimal) = bd.toLong
}
implicit object BigDecimalToBigDecimal extends BigDecimalConverter[BigDecimal] {
def toBigDecimal(in: BigDecimal) = in
def fromBigDecimal(bd: BigDecimal) = bd
}
}
def round[T](input: T, scale: Int)(implicit bdc: BigDecimalConverter[T]): T =
bdc.fromBigDecimal(
bdc.toBigDecimal(input).setScale(scale, BigDecimal.RoundingMode.HALF_UP)
)
Which can be correctly used with Double, Long, BigDecimal, ... :
round(10, 1)
round(Long.MaxValue - 1000L, -1)
round(BigDecimal("1234"), -2)