A closure is an anonymous function that references variables outside its body. In Go, that can make an option function convenient—but it also means a deferred call or goroutine may observe a variable later than expected. section 2.11.3 section 6.6.1 section 9.3
Keep configuration with an option
`WithPort(port)` can return a closure that validates `port` and updates an options struct. The returned function retains access to the supplied port; `NewServer` later calls each option to build its configuration. This lets callers supply `WithPort(8080)` or omit options for the default configuration. section 2.11.3
Read the final status in a deferred call
A common mistake is `defer notify(status)` followed by assignments to `status`: the argument is evaluated when `defer` runs, so `notify` receives the earlier value. Use `defer func() { notify(status) }()` when notification needs the value at function return. A deferred closure can still receive an early value through an argument: `defer func(i int) { fmt.Println(i, j) }(i)` evaluates `i` immediately but reads the referenced `j` when the closure runs. Starting both at zero and then incrementing both prints `0 1`. Passing `&status` to a deferred function is another option if its signature can accept a pointer. section 6.6.1
Do not assume a goroutine sees the launch-time loop value
In the shown loop, `for _, i := range []int{1, 2, 3} { go func() { fmt.Print(i) }() }`, the closures refer to the same changing `i`. Execution may print `233` or `333`, rather than the values `123` in some order. Give each goroutine its intended value by creating `val := i` inside the loop and referencing `val`, or by calling `go func(val int) { fmt.Print(val) }(i)`. Neither fix promises an output order. section 9.3
A closure can also give `defer` a shorter surrounding function. When opening files in a loop, deferring `file.Close()` directly leaves each close pending until the outer function returns. Put one iteration in an immediately called closure so its defer runs at that iteration’s end. A separate helper function does the same job and may be clearer and easier to test. section 4.6
