add tests (and coverage) and decoupling

Suppose you had to test this code, how would you reset the counter after each test? Suppose you wanted to run those tests in parallel, could you do it? Now suppose that you wanted to count more than one thing per program, could you do it? No, of course not. Clearly the answer is to encapsulate the count variable in a type.

package counter
type Counter struct {
count int
}
func (c *Counter) Increment(n int) int {
c.count += n
return c.count
}

Now imagine that this problem isn’t restricted to just counters, but your applications main business logic. Can you test it in isolation? Can you test it in parallel? Can you use more than one instance at a time? If the answer those question is no, the reason is package level state. Avoid package level state. Reduce coupling and spooky action at a distance by providing the dependencies a type needs as fields on that type rather than using package variables.