defer, panic, and recover

defer schedules a function call to run right before the surrounding function returns, regardless of how it returns. It's most often used for cleanup — closing a file, unlocking a mutex — written right next to the code that opened it.

package main

import "fmt"

func main() {
	fmt.Println("start")
	defer fmt.Println("cleanup")
	fmt.Println("end")
}

start
end
cleanup

panic and recover

panic immediately stops normal execution and starts unwinding the call stack — Go's closest equivalent to throwing an exception, reserved for truly exceptional situations, not everyday error handling. recover, called inside a deferred function, stops that unwinding and lets the program continue, which is how safeDivide below turns a runtime panic (dividing by zero) into an ordinary returned value.

package main

import "fmt"

func safeDivide(a, b int) (result int) {
	defer func() {
		if r := recover(); r != nil {
			fmt.Println("recovered from:", r)
			result = 0
		}
	}()
	return a / b
}

func main() {
	fmt.Println(safeDivide(10, 2))
	fmt.Println(safeDivide(10, 0))
}

5
recovered from: runtime error: integer divide by zero
0

Next: closures — functions that capture variables from their surrounding scope.