How do I use the errors package to create and wrap errors in Go?
Jun 23, 2025 pm 11:29 PM
In Go, effective error handling uses errors.New or fmt.Errorf for basic errors, wraps errors with %w to preserve context, and inspects errors using errors.Is and errors.As. 1. errors.New and fmt.Errorf create simple errors without context. 2. Wrapping with %w adds context while preserving the original error’s identity. 3. errors.Is checks for a target error in the chain. 4. errors.As finds and assigns a matching error type. 5. Avoid wrapping sentinel or unrelated errors to prevent noise and complexity.
In Go, handling errors effectively is crucial for building robust applications. The errors package, especially with the introduction of error wrapping in Go 1.13, provides a clean and powerful way to create and manage errors across your codebase.
Creating Basic Errors
The most straightforward way to create an error in Go is by using errors.New() or fmt.Errorf(). These functions are perfect when you just need a simple error message without any extra context:
import "errors"
err := errors.New("something went wrong")Or with formatting:
err := fmt.Errorf("invalid value: %v", val)These methods are useful in simple cases like validating inputs or returning known failure conditions from functions.
If all you need is a basic error that doesn’t require inspection later, these are perfectly fine. Just remember that they don’t carry any additional metadata or cause information — they’re flat strings.
Wrapping Errors for Context
When writing layered applications, it's often important to know not just what error occurred, but also where it came from. This is where error wrapping comes in handy.
You can wrap an error using the %w verb in fmt.Errorf():
err := fmt.Errorf("failed to read config: %w", originalErr)This wraps originalErr inside a new error while preserving its identity. Later, you can use errors.Is() or errors.As() to inspect the chain and check if a specific error exists somewhere in the call stack.
Here’s why this matters:
- It allows higher-level functions to add context to low-level errors.
- You can still programmatically check what kind of error occurred deep down.
- Stack traces (if used) remain clean and meaningful.
So, whenever you return an error that originated elsewhere, consider wrapping it instead of just formatting — it makes debugging much easier.
Unwrapping and Inspecting Errors
Once you have wrapped errors, you’ll likely want to inspect them at some point — for example, to handle specific types of errors differently.
Go provides two main tools for this:
errors.Is(err, target)– checks if any error in the chain matches the target.errors.As(err, &target)– finds the first error in the chain that matches a given type and assigns it to the target.
For example:
if errors.Is(err, os.ErrNotExist) {
// handle file not found case
}Or with custom error types:
var syntaxErr *SyntaxError
if errors.As(err, &syntaxErr) {
fmt.Println("Line:", syntaxErr.Line)
}These functions walk through the chain of wrapped errors automatically, so you don't need to manually unwrap each layer yourself.
Just keep in mind:
-
errors.Iscompares based on equality (==) or implementsIs() bool. -
errors.Aschecks for type match and sets the pointer if found. - Only exported fields and types should be used for inspection.
When Not to Wrap
While wrapping errors is helpful, there are times when it’s unnecessary or even counterproductive.
For example:
- If you're creating a sentinel error meant to be matched exactly, don't wrap it.
- If you're generating a completely new error unrelated to the original one, just format it normally.
- If you're logging the error already and only passing it up for visibility, wrapping might add noise.
A good rule of thumb: wrap errors when you want to preserve their identity for inspection later. Otherwise, stick with plain formatting.
Also, avoid wrapping already-wrapped errors multiple times unless necessary — it can make the error chain harder to follow.
That’s basically how you work with the errors package in modern Go. It's not complicated, but it does require a bit of care when deciding whether to wrap or just format.
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