The maybe function in Kotlin

Maybe.kt

// The same behavior as the Haskell version
fun <T, U> T?.maybe(default: U, f: (T) -> U) =
    when (this) {
        null -> default
        else -> f(this)
    }
    
// When `f` returns null you get the default
fun <T, U> T?.maybeBad(default: U, f: (T) -> U) =
    this?.let { f(it) } ?: default

// A less generic version where U is forced to be a non-null type.
fun <T, U : Any> T?.maybeOk(default: U, f: (T) -> U) =
    this?.let { f(it) } ?: default