Go中的'key，ok：= k。（* dns.A）'是什么意思？

Question

I am pretty new to Go and trying to write a DNS server by using package miekg DNS. According its example, I copy & pasted a simple snippet to perform A record request:

package main

import "fmt"
import "github.com/miekg/dns"

func main() {
    config, _ := dns.ClientConfigFromFile("/etc/resolv.conf")
    c := new(dns.Client)
    m := new(dns.Msg)
    zone := "miek.nl"
    m.SetQuestion(dns.Fqdn(zone), dns.TypeA)
    m.SetEdns0(4096, true)
    r, _, err := c.Exchange(m, config.Servers[0]+":"+config.Port)
    if err != nil {
        return
    }
    if r.Rcode != dns.RcodeSuccess {
        return
    }
    _ = "breakpoint"
    for _, k := range r.Answer {
        if key, ok := k.(*dns.A); ok {
            fmt.Printf("%+v
", key)
        }
    }
}

Loading by godebug, stopped at _ = "breakpoint":

./godebug run ~/aRequest.go  -d /usr/local/go/
-> _ = "breakpoint"
(godebug) p r.Answer
[]dns.RR{(*dns.A)(0xc8200120c0)}
(godebug) n
-> for _, k := range r.Answer {
(godebug) n
-> if key, ok := k.(*dns.A); ok {
(godebug) p k
&dns.A{Hdr:dns.RR_Header{Name:"miek.nl.", Rrtype:0x1, Class:0x1, Ttl:0x708, Rdlength:0x4}, A:net.IP{0x8b, 0xa2, 0xc4, 0x4e}}
(godebug) n
-> fmt.Printf("%+v
", key)
(godebug) p key 
&dns.A{Hdr:dns.RR_Header{Name:"miek.nl.", Rrtype:0x1, Class:0x1, Ttl:0x708, Rdlength:0x4}, A:net.IP{0x8b, 0xa2, 0xc4, 0x4e}}
(godebug) p ok
true

According godebug, ok == true and key seemed same as k. How could k.(*dns.A) return two variables?

Answer 1

The expression k.(*dns.A) is a type assertion.

The code asserts that k contains a *dns.A. The first return value is of type *dns.A. The second return value is a untyped bool indicating whether the assertion holds.