How to Add Extra Information to Errors in Go Handling Specific Errors Using Sentinel Errors

One way you could check for a particular type of error might be using the Error method on an error type to get the message from the error and compare that value to the type of error you’re looking for. Imagine that in your program, you want to show a message other than there was an error: uh oh when the error value is uh oh. One approach to handling this case would be to check the value returned from the Error method, like so:

if err.Error() == "uh oh" {
	// Handle 'uh oh' error.
	fmt.Println("oh no!")
}

Now, save your code and use go run to run your program again:

go run main.go

How to Add Extra Information to Errors in Go Handling Specific Errors Using Sentinel Errors

Checking the string value of err.Error() to see if it’s the value uh oh, as in the code above, would work in this case. But the code would not work if the uh oh error string is slightly different elsewhere in the program. Checking errors this way can also lead to significant updates to code if the error’s message itself needs to be updated because every place the error is checked would need to be updated. Take the following code, for example:

func giveMeError() error {
	return fmt.Errorf("uh h")
}

err := giveMeError()
if err.Error() == "uh h" {
	// "uh h" error code
}

How to Add Extra Information to Errors in Go Handling Specific Errors Using Sentinel Errors

Now, let’s say you were using one of these error values in the typo example from before:

var errUhOh = fmt.Errorf("uh h")

func giveMeError() error {
	return errUhOh
}

err := giveMeError()
if err == errUhOh {
	// "uh oh" error code
}

How to Add Extra Information to Errors in Go Handling Specific Errors Using Sentinel Errors

This time the output will show the generic error output for the 1 value, but it uses the custom oh no! message when it sees the errUhOh error returned from validateValue for 2:

Outputvalidating 1... there was an error: that's odd
validating 2... oh no!
validating 3... valid!

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