Errors and deferred cleanup

The idea

Enter with functions and multiple return values understood. Go commonly returns a result together with an error. A nil error indicates success by convention; a non-nil error should be handled before treating the accompanying result as usable. Expected invalid input should have a clear error path rather than being silently replaced with a plausible answer. The caller decides whether to recover, report, or propagate the problem.

defer schedules a function call for the surrounding function’s return. Its arguments are evaluated when the defer statement executes, while the call itself runs later. Multiple deferred calls run in reverse registration order. This supports cleanup after a resource has been successfully acquired. Be careful about whether the deferred operation captures a value immediately or reads a variable later through a closure.

package main
import "fmt"
func main(){
 n:=3
 defer fmt.Println(n)
 n=8
 fmt.Println(n)
}

8
3

The normal print reads eight. The deferred call retained its already evaluated argument three and runs as main returns.