Go: returning from defer

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?

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)

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
    }
}()

have a look at this

package main

import "fmt"

func iWillPanic() {
    panic("ops, panic")
}
func runner() (rtnValue string) {
    rtnValue := ""
    defer func() {
        if r := recover(); r != nil {
            // and your logs or something here, log nothing with panic is not a good idea
            rtnValue = "don't panic" // modify the return value, and it will return
        }
    }()
    iWillPanic()
    return rtnValue
}

func main() {
    fmt.Println("Return Value:", runner())
}

func TestReturnFromPanic(t *testing.T) {
   fn := func(filename string) (rep string, err error) {
       defer func() {
           if r := recover(); r != nil {
               err = fmt.Errorf("panic in getReport %s", r)
           }
       }()
       return filename[100:], nil
   }
   t.Log(fn(``))
}

The named return parameter err is the trick.

https://play.golang.org/p/jpaCa9j2iAf