non-keyedlens.scala

/* The Scalaz version would look something like this: */

import scalaz.Lens.lensg  
val bandNameL = lensg[Band, String](b => n => b.copy(name = n), _.name)
val guitarTuningL = lensg[Guitar, String](g => t => g.copy(tuning = t), _.tuning)
  
// KeyedLens #1: Here Person is a key.
def guitarL(p: Person) = lensg[Band, Guitar](
  b => newValue => b.copy(members = b.members + (p -> newValue)),
  b => b.members(p))
  
// KeyedLens #2: Here Int is the key.
def stringGaugeL(s: Int) = lensg[Guitar, Double](
  g => newValue => g.copy(gauges = g.gauges.updated(s - 1, newValue)),
  g => g.gauges(s - 1))
  
// Compose a keyed and non-keyed lens
def guitaristsTuningL(p: Person) = guitarL(p) >=> guitarTuningL
  
// Compose 2 keyed lenses
def guitaristsGaugeL(p: Person, s: Int) = guitarL(p) >=> stringGaugeL(s)
  
/*
  You get these types:
      
  bandNameL: Lens[Band,String]
  guitarTuningL: Lens[Guitar,String]
  guitarL: (p: Person)Lens[Band,Guitar]
  stringGaugeL: (s: Int)Lens[Guitar,Double]
  guitaristsTuningL: (p: Person)LensFamily[Band,String]
  guitaristsGaugeL: (p: Person, s: Int)LensFamily[Band,Double]
  
  The usage looks like:
*/
  
guitaristsTuningL(acle).get(band)
guitaristsGaugeL(acle, 7).get(band)
guitaristsTuningL(acle).set(band, "G#FA#D#FA#D#")
guitaristsGaugeL(acle, 7).set(band, 0.0666666)
  
/* ...which IMO is not as pretty pretty as David's solution,
     since the arguments are split up.
        
   But if you are writing it in a State comprehension, the scalaz 
   version works nicely; but I don't think the KeyedLens version will.
   Here's what the scalaz version could look like: */
  
val changeAcleS = for {
  oldTuning <- guitaristsTuningL(acle).st
  old7Gauge <- guitaristsGaugeL(acle, 7).st
  _ <- guitaristsTuningL(acle) := "G#FA#D#FA#D#"
  _ <- guitaristsGaugeL(acle, 7) := 0.0666666
} yield (oldTuning, old7Gauge)
  
/* alternatively: */
  
val changeAcleS2 = for {
  oldTuning <- guitaristsTuningL(acle) <:= "G#FA#D#FA#D#"
  old7Gauge <- guitaristsGaugeL(acle, 7) <:= 0.0666666
} yield (oldTuning, old7Gauge)
  
/* or */
  
val changeAcleS3 = for {
  _ <- guitaristsTuningL(acle) := "G#FA#D#FA#D#"
  _ <- guitaristsGaugeL(acle, 7) := 0.0666666
} yield ()
  
  
// save all
val (updatedBand, (oldTuning, old7Gauge)) = changeAcleS(band)

// save updated band, discard old values
val bandOnly = changeAcleS3.exec(band)