Variable scoping issue in struct

Getting Help
1

Hi,

I find myself in an weird situation whereby I have no colleagues (in between contracts) to rubber duck a problem I’m having. I’m hoping someone can point out the stupid mistake I am making.

I’m making a “server” type struct with the “loop” function running as a separate goroutine, a nice pattern I’ve used a few times.


import (
	"time"

	"backtag/app/policy"
)

type policyServer struct {
	update chan policy.Mandate
}

func (p *policyServer) loop(config Config) bool {
	// declare state for the loop

	next := time.Now().Add(config.PolicyDelay)

	// the mandate to begin with

	mandate := policy.Mandate{}

	// act like a server

	for {
		// determine how long we must wait before fetching the latest mandate

		var wait time.Duration
		if now := time.Now(); next.After(now) {
			wait = next.Sub(now)
		}

		select {
		// an update is ready
		case update := <-p.update:
			mandate = update

			// time to request an update
		case <-time.After(wait):
			go func(ch chan policy.Mandate) {
				// will pretend to fetch new mandate for now..
				time.Sleep(2 * time.Second)
				ch <- policy.Mandate{}
			}(p.update)
			next = next.Add(config.PolicyInterval)
		}
	}
}

func (p *policyServer) BlockTag(number uint) bool {
	panic("implement me")
}

func (p *policyServer) BlockIP(ip string) bool {
	panic("implement me")
}

func NewPolicyServer(config Config) policy.Tester {
	srv := policyServer{}
	go srv.loop(config)
	return &srv
}

My issue is that this won’t compile because the “mandate” var is unused. However, “next” doesn’t have this problem, so it can’t be a scoping issue, which was my first thought.

As you can see, I am (re)assigning “mandate” in a case block, just like “next” in the case block below it.

Can anyone help me out?

Thanks in advance,
Jonathan

2

The problem is that you declared mandate here

mandate := policy.Mandate{}

And you set it here

		case update := <-p.update:
			mandate = update

But you didn’t use it anywhere. As far as the compiler is concerned you could delete the declaration and setting of mandate and your program would work fine.

3

Ah, of course.

The “next” var was distracting me, but I see not only did I assign next, i used it “next.Add(…)”

… had I continued to implement rather than compile half way through, I might not of noticed.

Thanks for the spot!

Jonathan

1 Like
4

I don’t suppose you’re the brother of Kate who ran a hosting company called Memset?

5

I am yes!

6

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