func flatten(a) {
    var flat = []
    a.each { |item|
        flat += (item.kind_of(Array) ? flatten(item) : [item])
    }
    return flat
}

var arr = [[1], 2, [[3,4], 5], [[[]]], [[[6]]], 7, 8, []]
say flatten(arr)      # used-defined function
say arr.flatten       # built-in Array method