Return and handle an error

Note: This topic is part of a multi-part tutorial that begins with Create a Go module.

Handling errors is an essential feature of solid code. In this section, you'll add a bit of code to return an error from the greetings module, then handle it in the caller.

Steps

1. Modify greetings/greetings.go to return an error

In greetings/greetings.go, add the code highlighted below.

There's no sense sending a greeting back if you don't know who to greet. Return an error to the caller if the name is empty. Copy the following code into greetings.go and save the file.

package greetings

import (
    "errors"
    "fmt"
)

// Hello returns a greeting for the named person.
func Hello(name string) (string, error) {
    // If no name was given, return an error with a message.
    if name == "" {
        return "", errors.New("empty name")
    }

    // If a name was received, return a value that embeds the name
    // in a greeting message.
    message := fmt.Sprintf("Hi, %v. Welcome!", name)
    return message, nil
}

In this code, you:

• Change the function so that it returns two values: a string and an error. Your caller will check the second value to see if an error occurred. (Any Go function can return multiple values. For more, see Effective Go.)
• Import the Go standard library errors package so you can use its errors.New function.
• Add an if statement to check for an invalid request (an empty string where the name should be) and return an error if the request is invalid. The errors.New function returns an error with your message inside.
• Add nil (meaning no error) as a second value in the successful return. That way, the caller can see that the function succeeded.

2. Handle the error in hello/hello.go

In your hello/hello.go file, handle the error now returned by the Hello function, along with the non-error value.

Paste the following code into hello.go.

package main

import (
    "fmt"
    "log"

    "example.com/greetings"
)

func main() {
    // Set properties of the predefined Logger, including
    // the log entry prefix and a flag to disable printing
    // the time, source file, and line number.
    log.SetPrefix("greetings: ")
    log.SetFlags(0)

    // Request a greeting message.
    message, err := greetings.Hello("")
    // If an error was returned, print it to the console and
    // exit the program.
    if err != nil {
        log.Fatal(err)
    }

    // If no error was returned, print the returned message
    // to the console.
    fmt.Println(message)
}

In this code, you:

• Configure the log package to print the command name ("greetings: ") at the start of its log messages, without a time stamp or source file information.
• Assign both of the Hello return values, including the error, to variables.
• Change the Hello argument from Gladys’s name to an empty string, so you can try out your error-handling code.
• Look for a non-nil error value. There's no sense continuing in this case.
• Use the functions in the standard library's log package to output error information. If you get an error, you use the log package's Fatal function to print the error and stop the program.

3. Run hello.go to confirm that the code works

At the command line in the hello directory, run hello.go to confirm that the code works.

Now that you're passing in an empty name, you'll get an error.

$ go run .
greetings: empty name
exit status 1

Conclusion

That's common error handling in Go: Return an error as a value so the caller can check for it.

Next, you'll use a Go slice to return a randomly-selected greeting.