errors.go

/*
Copyright 2015 Google Inc. All rights reserved.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package errors

import "fmt"

// Aggregate represents an object that contains multiple errors, but does not
// necessarily have singular semantic meaning.
type Aggregate interface {
	error
	Errors() []error
}

// NewAggregate converts a slice of errors into an Aggregate interface, which
// is itself an implementation of the error interface.  If the slice is empty,
// this returns nil.
func NewAggregate(errlist []error) Aggregate {
	if len(errlist) == 0 {
		return nil
	}
	return aggregate(errlist)
}

// This helper implements the error and Errors interfaces.  Keeping it private
// prevents people from making an aggregate of 0 errors, which is not
// an error, but does satisfy the error interface.
type aggregate []error

// Error is part of the error interface.
func (agg aggregate) Error() string {
	if len(agg) == 0 {
		// This should never happen, really.
		return ""
	}
	if len(agg) == 1 {
		return agg[0].Error()
	}
	result := fmt.Sprintf("[%s", agg[0].Error())
	for i := 1; i < len(agg); i++ {
		result += fmt.Sprintf(", %s", agg[i].Error())
	}
	result += "]"
	return result
}

// Errors is part of the Aggregate interface.
func (agg aggregate) Errors() []error {
	return []error(agg)
}

// Matcher is used to match errors.  Returns true if the error matches.
type Matcher func(error) bool

// FilterOut removes all errors that match any of the matchers from the input
// error.  If the input is a singular error, only that error is tested.  If the
// input implements the Aggregate interface, the list of errors will be
// processed recursively.
//
// This can be used, for example, to remove known-OK errors (such as io.EOF or
// os.PathNotFound) from a list of errors.
func FilterOut(err error, fns ...Matcher) error {
	if err == nil {
		return nil
	}
	if agg, ok := err.(Aggregate); ok {
		return NewAggregate(filterErrors(agg.Errors(), fns...))
	}
	if !matchesError(err, fns...) {
		return err
	}
	return nil
}

// matchesError returns true if any Matcher returns true
func matchesError(err error, fns ...Matcher) bool {
	for _, fn := range fns {
		if fn(err) {
			return true
		}
	}
	return false
}

// filterErrors returns any errors (or nested errors, if the list contains
// nested Errors) for which all fns return false. If no errors
// remain a nil list is returned. The resulting silec will have all
// nested slices flattened as a side effect.
func filterErrors(list []error, fns ...Matcher) []error {
	result := []error{}
	for _, err := range list {
		r := FilterOut(err, fns...)
		if r != nil {
			result = append(result, r)
		}
	}
	return result
}

// Flatten takes an Aggregate, which may hold other Aggregates in arbitrary
// nesting, and flattens them all into a single Aggregate, recursively.
func Flatten(agg Aggregate) Aggregate {
	result := []error{}
	if agg == nil {
		return nil
	}
	for _, err := range agg.Errors() {
		if a, ok := err.(Aggregate); ok {
			r := Flatten(a)
			if r != nil {
				result = append(result, r.Errors()...)
			}
		} else {
			if err != nil {
				result = append(result, err)
			}
		}
	}
	return NewAggregate(result)
}