package main

import "fmt"

type stack []interface{}

func (k *stack) push(s interface{}) {
    *k = append(*k, s)
}

func (k *stack) pop() (s interface{}, ok bool) {
    if k.empty() {
        return
    }
    last := len(*k) - 1
    s = (*k)[last]
    *k = (*k)[:last]
    return s, true
}

func (k *stack) peek() (s interface{}, ok bool) {
    if k.empty() {
        return
    }
    last := len(*k) - 1
    s = (*k)[last]
    return s, true
}

func (k *stack) empty() bool {
    return len(*k) == 0
}

func main() {
    var s stack
    fmt.Println("new stack:", s)
    fmt.Println("empty?", s.empty())
    s.push(3)
    fmt.Println("push 3. stack:", s)
    fmt.Println("empty?", s.empty())
    s.push("four")
    fmt.Println(`push "four" stack:`, s)
    if top, ok := s.peek(); ok {
        fmt.Println("top value:", top)
    } else {
        fmt.Println("nothing on stack")
    }
    if popped, ok := s.pop(); ok {
        fmt.Println(popped, "popped.  stack:", s)
    } else {
        fmt.Println("nothing to pop")
    }
}