Error Wrapping and Unwrapping

Snippet

Error Wrapping and Unwrapping

Go 1.13+ introduced error wrapping using the %w verb in fmt.Errorf. This allows you to add context to an error while still being able to check for the original sentinel error using errors.Is.

snippet.go
go
package main
 
import (
    "errors"
    "fmt"
)
 
var ErrNotFound = errors.New("not found")
 
func fetchData() error {
    return fmt.Errorf("database error: %w", ErrNotFound)
}
 
func main() {
    err := fetchData()
    if err != nil {
        if errors.Is(err, ErrNotFound) {
            fmt.Println("Resource was not found")
        }
        fmt.Println("Original error:", err)
        fmt.Println("Unwrapped error:", errors.Unwrap(err))
    }
}

Breakdown

fmt.Errorf("... %w", ErrNotFound)

Wraps the original error, preserving its identity for future checks.

errors.Is(err, ErrNotFound)

Recursively checks if any error in the chain matches the target error.

errors.Unwrap(err)

Returns the underlying error that was wrapped.

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Error Wrapping and Unwrapping

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