Showcasing the usefulness of inline receivers in Kotlin

InlineReceiver.kt

inline fun <T, U> (() -> T?).map (crossinline f: (T) -> U): (() -> U?)
{
    return { this()?.let(f) }
}

inline fun <T> (() -> T?).filter (crossinline pred: (T) -> Boolean): (() -> T?)
{
    return {
        var out = this()
        while (out != null && !pred(out)) out = this()
        out
    }
}

inline fun <T> (() -> T?).toList(): List<T>
{
    val out = arrayListOf<T>()
    while (true) {
        val item = this()
        if (item == null) break
        out.add(item)
    }
    return out
}

inline fun <T> Sequence<T>.supplier(): (() -> T?)
{
    val iter = iterator()
    return { if (iter.hasNext()) iter.next() else null }
}

fun test()
{
    val seq = sequenceOf(1, 2, 3)
    println(seq.supplier().map { it + 1 }.filter { it % 2 == 0 }.toList())

    /* desugars to:

    val out1 = ArrayList<Int>()
    while (true) {
        val tmp1 = if (seq.iter.hasNext()) iter.next() else null
        var out2 = if (tmp1 != null) tmp1 + 1 else null
        while (out != null && (out % 2 != 0)) {
            val tmp2 = if (seq.iter.hasNext()) iter.next() else null
            out2 = if (tmp2 != null) tmp2 + 1 else null
        }
        if (out2 == null) break
        out1.add(item)
    }
    println(out1)
     */
}