Go: returning from defer

Question

I want to return an error from a function if it panics (in Go):
func getReport(filename string) (rep report, err error) {
    rep.data = make(map[string]float64)

    defer func() {
        if r := recover(); r != nil {
            fmt.Println("Recovered in f", r)
            err, _ = r.(error)
            return nil, err
        }
    }()
    panic("Report format not recognized.")
    // rest of the getReport function, which can try to out-of-bound-access a slice
    ...
} 

I appear to have misunderstood the very concept of panic and defer. Can anybody enlighten me?

Chris Gregori · Accepted Answer

In a deferred function you can alter the returned parameters, but you can't return a new set. So a simple change to what you have will make it work. There is another problem with what you wrote, namely that the you've paniced with a string but are expecting an error in your type assertion. Here is a fix for both of those (Play) package main import ( "errors" "fmt" ) type report *string func getReport(filename string) (rep report, err error) { defer func() { if r := recover(); r != nil { fmt.Println("Recovered in f", r) // find out exactly what the error was and set err switch x := r.(type) { case string: err = errors.New(x) case error: err = x default: err = errors.New("Unknown panic") } // invalidate rep rep = nil // return the modified err and rep } }() panic("Report format not recognized.") // rest of the getReport function, which can try to out-of-bound-access a slice ///... } func main() { rep, err := getReport("hello") fmt.Println("rep", rep) fmt.Println("err", err) } https://play.golang.org/p/U-r2n0EsU1S

Go Language

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