How Can I Achieve True Recursive Error Wrapping and Unwrapping in Go?

Error Wrapping with Recursive Is() and As() in Go

Many developers recommend wrapping errors in Go using fmt.Errorf with the %w verb, but this method does not provide true recursive wrapping. To recursively check errors using Is() and As(), custom error types can be used.

Custom Error Types for Recursion

Here's a custom error type errorChain that supports error wrapping and recursive checks:

type errorChain struct {
    err  error
    next *errorChain
}

func Wrap(errs ...error) error {
    out := errorChain{err: errs[0]}

    n := &out
    for _, err := range errs[1:] {
        n.next = &errorChain{err: err}
        n = n.next
    }
    return out
}

Adding Is() and As() to the Custom Type

The key to enabling recursive checks is to implement both Is() and As() methods on the custom type. These methods allow error comparisons at the error contained within the chain, rather than the chain itself:

func (c errorChain) Is(err error) bool { return errors.Is(c.err, err) }

func (c errorChain) As(target any) bool { return errors.As(c.err, target) }

Using errorChain

With these methods in place, you can wrap errors and perform recursive checks:

errs := Wrap(errors.New("error 0"), errors.New("error 1"), errors.New("error 2"))

fmt.Println(errors.Is(errs, errors.New("error 0")))  // true
fmt.Println(errors.Is(errs, errors.New("error 1")))  // true
fmt.Println(errors.Is(errs, errors.New("error 2")))  // true

Using Unwrap() to Iterate the Chain

The Unwrap() method in errorChain allows you to traverse the wrapped errors in a chain:

var currentError error = errs
for {
    currentError = errors.Unwrap(currentError)
    if currentError == nil {
        break
    }
    fmt.Println(currentError)
}

This example prints all the errors in the chain:

error 0
error 1
error 2

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