Error Handling

Errors as Values

Go does not have exceptions. Instead, functions that can fail return an error value alongside their result. This is arguably Go's most important design decision.

The error Interface

The built-in error type is an interface with a single method:

type error interface {
    Error() string
}

Any type that has an Error() string method is an error. This is the simplest possible contract.

Creating Errors

The standard library provides two ways to create simple errors:

import "errors"

err := errors.New("something went wrong")

import "fmt"

err := fmt.Errorf("user %s not found", username)

fmt.Errorf works like fmt.Sprintf but returns an error. Use it when you need formatted messages.

The Error-Checking Pattern

The canonical Go pattern: call a function, check the error immediately, handle it or return it:

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

The %w verb wraps the original error, preserving the chain for debugging. This pattern appears hundreds of times in any real Go codebase.

"He's dead, Jim." Dr. McCoy never threw an exception --- he returned a clear error status. Go handles errors the same way: no drama, just values.

Custom Error Types

For richer error information, define your own error type:

type ValidationError struct {
    Field   string
    Message string
}

func (e *ValidationError) Error() string {
    return fmt.Sprintf("%s: %s", e.Field, e.Message)
}

When to Return Errors

A function should return an error when:

• An operation can fail (file I/O, network, parsing)
• The failure is recoverable (the caller can do something about it)
• The failure is expected in normal operation

Do not return errors for programming mistakes (like passing a nil pointer where one is never expected). Use panics for those.

Your Task

Write a function validateAge that takes an int and returns an error:

• If age is negative, return an error with the message "age cannot be negative"
• If age is greater than 150, return an error with the message "age is unrealistic"
• Otherwise, return nil (no error)