Accumulate and inspect multiple errors

Accumulating multiple errors in go-multierror allows you to collect failures across several operations and return them as a single result. By using multierror.Append, you can combine individual errors into a structured container that supports standard Go error handling patterns.

Accumulate errors and verify results

When performing a series of tasks, you can use multierror.Append to collect any errors that occur. The function handles nil values gracefully, only adding non-nil errors to the collection. After all operations are complete, ErrorOrNil provides a convenient way to return either the accumulated errors or nil if no errors were found.

package main

import (
	"errors"

	"github.com/hashicorp/go-multierror"
)

func main() {
	first := errors.New("first")
	second := errors.New("second")
	result := multierror.Append(nil, first, second)
	if result.ErrorOrNil() == nil {
		panic("expected accumulated errors")
	}
}

Inspect underlying errors

To programmatically inspect the specific errors contained within a multierror, use the WrappedErrors method. This method returns a slice of all accumulated errors, allowing you to verify the contents without parsing the formatted error string. WrappedErrors is nil-safe and will return a nil slice if called on a nil multierror.

package main

import (
	"errors"

	"github.com/hashicorp/go-multierror"
)

func main() {
	result := multierror.Append(nil, errors.New("first"), errors.New("second"))
	if len(result.WrappedErrors()) != 2 {
		panic("expected two accumulated errors")
	}
}

Key behaviors

• Flattening: multierror.Append automatically flattens nested multierrors. If you append one multierror to another, the individual errors from the source are added directly to the destination's list.
• Nil Safety: You can pass a nil error as the first argument to multierror.Append; it will initialize a new multierror container for you.
• Interface Compatibility: The value returned by multierror.Append implements the standard Go error interface. Using ErrorOrNil ensures that you return a true nil interface value when no errors are present, preventing common Go gotchas where a non-nil interface points to a nil concrete type.