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Go Control Flow: Defer, Panic, and Recover

Go has a few unique control flow mechanisms for handling cleanup and unexpected errors.

1. Defer

defer schedules a function call to be run immediately before the surrounding function returns. It is most commonly used for cleanup (like closing a file or a database connection).

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package main
import "fmt"
import "os"

func main() {
    f := createFile("/tmp/defer.txt")
    defer closeFile(f) // This runs AT THE END of main()
    writeFile(f)
}

func createFile(p string) *os.File {
    fmt.Println("creating")
    f, err := os.Create(p)
    if err != nil {
        panic(err)
    }
    return f
}

func writeFile(f *os.File) {
    fmt.Println("writing")
    fmt.Fprintln(f, "data")
}

func closeFile(f *os.File) {
    fmt.Println("closing")
    f.Close()
}

2. Panic

A panic typically means something went unexpectedly wrong. We mostly use it to fail fast on errors that shouldn't happen during normal operation.

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panic("a problem occurred")

When a function panics, its execution stops, and any deferred functions are executed.

3. Recover

recover is a built-in function that regains control of a panicking goroutine. recover is only useful inside deferred functions.

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func main() {
    defer func() {
        if r := recover(); r != nil {
            fmt.Println("Recovered from panic:", r)
        }
    }()

    panic("CRITICAL ERROR")
    fmt.Println("This will never run")
}

Summary Table

Keyword Purpose
defer Delay execution until the function returns. Used for cleanup.
panic Stop normal execution immediately. Used for unrecoverable errors.
recover Catch a panic and stop the program from crashing.