How to get error as string from an error interface?
I want to assign ERROR: Fake error! to a variable and then convert it to a slice []byte(error_str)
prefix_err := "ERROR: "
defer func() {
if err := recover(); err != nil {
// get error message from "err" as string
}
}()
panic("Fake error!")
There are two different problems here:
The first problem: The actual error type is actually a built-in interface. To implement it you implement a method on the type func Error() string. So anything with the error type can be converted to a string format by calling its Error function.
The second problem: panic doesn't take in an error, it takes in an interface{}. The recover builtin similarly returns whatever interface{} you passed to panic. In this case, calling Error won't work. Indeed, you didn't pass an error into panic, you passed in a string (which is common).
There are a lot of ways you can get a string from an interface{}, and a big type switch is one. But overall I think the easiest option is
myBytes := []byte(fmt.Sprintf("%s%v", prefix_err, err))
Which will always get a string representation of whatever the interface{} was, calling String() or Error() as necessary, and then convert it into a []byte.
(Minor footnote: it's not considered Go style to use underscores in variable names, use camelCase instead).
Edit:
Since it turns out we had a partial "X Y problem" with the conversion to a []byte to write into an io.Writer. If you want to log this to Stderr, you can instead use fmt.Fprintf(os.Stderr, "%s%v\n", prefix_err, err) or even fmt.Fprintln(os.Stderr, prefix_err, err). Any Fprint will correctly print to any io.Writer, including a logfile.
Panics do not always take the type of an Error. In your case you are passing in a string.
panic("Fake Error")
err := recover() // => string (interface)
To recover on an error interface, you need to pass an error
panic(fmt.Errorf("Fake Error!"))
err := recover() // => error (interface)
e := err.(error) // type check
e.Error() // => string
If you are not quite sure which one you'll get, you can type check when you recover and handle accordingly.
[]byte? My only guess is that you want to write it to a file, andio.Writertakes in a[]byte. If so, there are easier interfaces that take in strings.os.Stderrand a filefmt.Fprint[ln|f](io.Stderr, "my string")In fact, you can substitute my answer withfmt.Fprintf(io.Stderr, "%s%v\n", prefix_err, err)