Qupla program

import Qupla

// seq23 calculates and prints sequence x(n+1) = isEven(x(n)) ? x(n)/2 : 3*x(n)+1
// It is a proven fact (by experiment) that with any positive integer x(0) < 100.000.000
// this sequence always reaches 1 after finite number of steps.
// Maybe someone already proved it for any positive integer :)
// E.g. starting with x(0) = 27 sequence comes to value 1 after 110 steps.
// Run with -eval
// (c) lunfardo

template seq23<T> {
    func T seq23<T>(T n){
        dummy = print<T>(n)
        is2 = equal<T>(n, as<T>(1))
        return is2 ? as<T>(2) : seq23do<T>(n)
    }
    func T seq23do<T>(T n){
        qr = divMod<T>(n, as<T>(2))
        return isZero[sign<T>(qr.remainder)]
                 ? seq23<T>(qr.quotient)
                 : seq23<T>(mul3add1(n))
    }
    func T mul3add1(T n){
        mul3 = rshift<T>(n)
        return add<T>(mul3, as<T>(1))
    }
}

// 110 steps
eval seq23<Int>(27)