Unlike Java or Python, which use exceptions for handling errors, Go uses a more explicit error-handling model. This post will delve deep into how errors are handled in Go, why the language designers chose this path, and how you can effectively manage errors in your Go programs.

Why Go dislikes Exceptions

The creators of Go decided to avoid exceptions for several reasons. Primarily, they aimed to create a language that encourages clear and predictable error handling. Exceptions can sometimes lead to complex control flows, which are hard to follow. They can be thrown at many points in a program and caught far away from the source of the problem, making the code harder to read and maintain.

In contrast, Go’s error handling model is designed to encourage developers to deal with errors immediately as soon as they occur. This proximity between error occurrence and handling aims to produce more robust and maintainable code.

The Go Way: Errors as Values

In Go, errors are considered values. The error type is a built-in interface similar to fmt.Stringer:

type error interface {
    Error() string
}

An error variable represents any value that can describe itself as a string. Here is how you might typically see an error handled in Go:

func thisDoesSomething() error {
    // Attempt an operation that can fail.
    err := someOperation()
    if err != nil {
        return err
    }
    return nil
}

In this model, functions that can fail return an error as a normal return value, which is checked by the caller. This approach makes error handling a deliberate act: you have to explicitly check whether an error occurred.

Common Patterns for Error Handling in Go

Propagating Errors

When an error occurs, it is common practice in Go to return the error up to the caller of your function. This way, the caller can handle it appropriately, whether by logging the error, retrying the operation, or failing gracefully.

if err := doSomething(); err != nil {
    fmt.Println("An error occurred:", err)
    return err
}

Wrapping Errors

Go 1.13 introduced error wrapping, which allows you to add additional context to an error while preserving the original error. This is particularly useful when you want to maintain a stack trace or add descriptive messages without losing the original cause of the error:

if err := doSomething(); err != nil {
    return fmt.Errorf("doSomething failed: %w", err)
}

Custom Error Types

Sometimes, you may need more control over error handling. In such cases, you can define custom error types. This is particularly useful when you want to distinguish between different error conditions programmatically.

type MyError struct {
    Msg    string
    Code   int
}

func (e *MyError) Error() string {
    return fmt.Sprintf("code %d: %s", e.Code, e.Msg)
}

func doSomething() error {
    return &MyError{"Something bad happened", 1234}
}

Best Practices handling Errors in Go

1. Handle errors where they make sense: Don’t just propagate errors for the sake of it. Sometimes it’s better to handle the error directly where it occurs.
2. Be explicit with error conditions: It’s better to check for specific error conditions than to rely on general error handling.
3. Avoid panics for common errors: Use panics only for truly unexpected conditions that should terminate normal operation of your program. Common errors should be represented as normal error values.
4. Document error conditions: When writing functions that return errors, document what those errors will be, and under what conditions they’ll be returned.