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Defer, Panic, Recover

Defer

  • defer delays statement execution until function exits
  • LIFO ordered
  • use cases; closing resources
  • group open and close functions together
  • evaluates arguments at the time defer is called 
    package main

import (
    "fmt"
)

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

Panic

  • return an error to user when program cannot continue at all
  • occur after defer statements 
    package main

import (
    "fmt"
)

func main() {
        fmt.Println("start")
        panic("something bad is happening")
        fmt.Println("end")
}

Recover

  • Used to recover from panics
  • Only useful in deferred functions
  • Current function will not continue, but higher functions in call stack will
package main

import (
    "fmt"
    "log"
)

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

func panicker() {
    fmt.Println("About to panic")
    defer func() {
        if err := recover(); err != nil {
            log.Println("Error: ", err)
            // repanic error if it can't be handled
            // panic(err)
        }
    }()
    panic("something bad happened")
}