BandLenses.scala

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.
val guitarL = KeyedLens[Band, Person, Guitar](
    b => p => newValue => b.copy(members = b.members + (p -> newValue)),
    b => p => b.members(p))

// KeyedLens #2: Here Int is the key.
val stringGaugeL = KeyedLens[Guitar, Int, Double](
    g => s => newValue => g.copy(gauges = g.gauges.updated(s-1, newValue)),
    g => s => g.gauges(s-1))

// Compose a keyed and non-keyed lens
val guitaristsTuningL = guitarL >@=> guitarTuningL

// Compose 2 keyed lenses
val guitaristsGaugeL = guitarL >@=@> stringGaugeL