How to get error message in a string in golang?

Question

I referred this question - os.Error - string value (Golang), but that is not working in my case.

func (t Trans) TicketQty() (intQty int, err string) {
  defer func() {    
    str := recover()
    if(str != nil){     
      err = "an error"
    }
  }()   
  Qty := t.TransObject["qty"].(map[string] interface{})["ticket fv"].(float64)  
  intQty = 10
  return 
}

In that err I need the error message that is thrown, e.g. if the parsing logic fails, it is throwing an error automatically. That is the error I need to catch. I am showing this only for example - whatever the runtime exception is, I need to catch it and pass as err in the return.

How to achieve that?

Answer 1

Use the errors package to create new errors.

err = errors.New("an error")

The returned error can be treated as a string by either accessing err.Error(), or using the fmt package functions (for example fmt.Println(err)).

Don't use recover unless you really know what you're doing. It's idiomatic to return all errors, and to deal with them when they arise.

See Error handling and Go, and Defer, Panic and Recover on the Go blog for more info.

EDIT:

Re-reading your question, it looks like you're trying to recover from possible failed type assertions. In this instance it's recommended to use the "comma, ok" idiom (mentioned in the previously linked section of the docs), which (slightly paraphrased to be more general) means:

"If the type assertion fails, [the receiver variable] will still exist and be of type [whatever was asserted], but it will have the zero value..."

Simple example to test if an interface{} is actually a float64 through type assertion, and produce a custom error if it fails (instead of panicking):

package main

import (
    "errors"
    "fmt"
)

// assertFloat64 attempts a type assertion to float64.
// It returns a custom error if it fails.
func assertFloat64(n interface{}) error {
    // Type assertion. Is n float64?
    f, ok := n.(float64)
    // If yes,
    if ok {
        // print the result
        fmt.Printf("%f is float64\n", f)
        // and return nil error.
        return nil
    }
    // Else return our custom error
    return errors.New(fmt.Sprintf("could not assert that \"%v\" is float64.\n", n))
}

func main() {
    // Successful
    // 1024.0 is float64
    err := assertFloat64(1024.0)
    if err != nil {
        fmt.Println(err)
    }

    // Failure
    // "foo" isn't float64
    err = assertFloat64("foo")
    if err != nil {
        fmt.Println(err)
    }
}

Will print:

1024.000000 is float64
could not assert that "foo" is float64.

Playground

Answer 2

I think this would do what you want:

func (t Trans) TicketQty() (intQty int, err error) {
  defer func() {    
    ex := recover()
    if(ex != nil){     
      // "%v" prints the value of ex
      // for strings, it is the string, for errors .Error() method, for Stringer the .String() etc
      // Errorf returns an error instead of a string
      err = fmt.Errorf("%v", ex)
    }
  }()   
  ... // do your thing
  return
}

Answer 3

Adding to @intermernet's answer, if you are creating custom error messages using errors.New(), you might want to keep a check out for the specific error message created. Additionally any other kind of error messages too can be checked in the same manner. Just use the fmt.Sprintf() function with strings.Contains().

You can do this very simply:

if fmt.Sprint(err) == "My Custom Error" {
    // do something
}

Or better use strings.Contains():

if strings.Contains(fmt.Sprint(err), "Custom Error") {
    // do something
}