value(rs: RandomSource): Sample<A>

arb(...)

arb { n }

Arb.create

[1,2,3]
['a', 'b', 'c']
[]

-> yields []

Arb.choose

value(rs)

edgecases(): Exhaustive<A>

(RandomSource) -> List<A>

sealed class Edgecases<out A> {

   abstract fun values(rs: RandomSource): List<A>

   class Static(val values: List<A>) : Edgecases<A> {
      override fun values(rs: RandomSource): List<A> = values
   }
   class Dynamic(val fn: (RandomSource) -> List<A>) : Edgecases<A> {
      override fun values(rs: RandomSource): List<A> = fn(rs)
   }
}

_ -> List<A>

abstract class Arb<out A> : Gen<A>() {

   abstract fun edges(): Edgecases<A> = Edgecases.Static(edgecases())

   abstract fun edgecases(): List<A>

   abstract fun values(rs: RandomSource): Sequence<Sample<A>>

   companion object
}

users who spent the time converting from 3.x to 4.x

data class Edgecases<A>(edges: (RandomSource) -> List<A>)

Sample<A>

Edgecases<A>

fun generate(rs: RandomSource, edgesSamplingMode: Edgecases.SamplingMode = Edecases.SamplingMode.Exhaustive) = // implementation detail

// i'm making stuff up here
sealed class SamplingMode {
    object Exhaustive : SamplingMode() // all exhaustive permutations

    // if we want to dirty our generated distribution with edgecases with fixed probability
    case class FixedProbabilitySampling(val samplingProbability: Double = 0.1) : SamplingMode()

    // if we want more control over the ratio of the distribution
    case class DynamicProbabilitySampling(val startProbability: Double = 1.0, val targetProbability = 0.1, val decayFunction: (previousProbability: Double, iterations: Int) -> Double) : SamplingMode()
}

An interesting function to add would be 

Arb.randomOnly()

  which returns a copy of the arb but with the edge cases removed (that randomOnly name is a bit lame though) and Arb.withEdgeCases to copy the arb with differnt edge cases. (edited)