fix in Kotlin https://replit.com/@ramirez7/fixkt#main.kt

fix.kt

// Kotlin implementation of `fix`
// https://hackage.haskell.org/package/base-4.15.0.0/docs/Data-Function.html#v:fix
//
// Note the use of `Lazy`. It is the key to this working at all - without it, we
// spin until we get a stack overflow.
fun <A> fix(f: (Lazy<A>) -> A): A =
  object {
    val x: A by lazy {
      f(lazy { this.x })
    }
  }.x

fun main(args: Array<String>) {
  // Port of the `fix`-based factorial from the above haddocks
  //
  // >>> fix (\rec n -> if n <= 1 then 1 else n * rec (n-1)) 5
  // 120
  val fac = fix<(Int) -> Int> { rec -> { n -> if (n <= 1) 1 else n * rec.value(n - 1) } }
  
  println(fac(5))
}

repl.it says it works 💯

> kotlinc -d main.jar main.kt
> kotlin -classpath main.jar MainKt
120