Best way to score current extremum in collection type

Question

I’m currently a little tired so I might be missing the obvious.

I have a var _minVal: Option[Double], which shall hold the minimal value contained in a collection of Doubles (or None, if the collection is empty)

When adding a new item to the collection, I have too check if _minVal is either None or greater than the new item (=candidate for new mimimum).

I’ve gone from

_minVal = Some(_minVal match {
    case Some(oldMin) => if (candidate < oldMin) candidate
                         else                    oldMin
    case None         =>                         candidate
})

(not very DRY) to

_minVal = Some(min(_minVal getOrElse candidate, candidate))

but still think I might be missing something…

Answer 1

Without Scalaz, you are going to pay some RY. But I'd write it as:

_minVal = _minVal map (candidate min) orElse Some(candidate)

EDIT

Eric Torreborre, of Specs/Specs2 fame, was kind enough to pursue the Scalaz solution that has eluded me. Being a testing framework guy, he wrote the answer in a testing format, instead of the imperative, side-effecting original. :-)

Here's the version using _minVal, Double instead of Int, side-effects, and some twists of mine now that Eric has done the hard work.

// From the question (candidate provided for testing purposes)
var _minVal: Option[Double] = None
def candidate = scala.util.Random.nextDouble

// A function "min"
def min = (_: Double) min (_: Double)

// A function "orElse"
def orElse = (_: Option[Double]) orElse (_: Option[Double])

// Extract function to decrease noise
def updateMin = _minVal map min.curried(_: Double)

// This is the Scalaz vesion for the above -- type inference is not kind to it
// def updateMin = (_minVal map min.curried).sequence[({type lambda[a] = (Double => a)})#lambda, Double]

// Say the magic words
import scalaz._
import Scalaz._   

def orElseSome = (Option(_: Double)) andThen orElse.flip.curried
def updateMinOrSome = updateMin <*> orElseSome

// TAH-DAH!
 _minVal = updateMinOrSome(candidate)