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// Copyright 2015 Canonical Ltd.
// Licensed under the LGPLv3, see LICENCE file for details.
package schema
import (
// The Coerce method of the Checker interface is called recursively when
// v is being validated. If err is nil, newv is used as the new value
// at the recursion point. If err is non-nil, v is taken as invalid and
// may be either ignored or error out depending on where in the schema
// checking process the error happened. Checkers like OneOf may continue
// with an alternative, for instance.
type Checker interface {
Coerce(v interface{}, path []string) (newv interface{}, err error)
// Any returns a Checker that succeeds with any input value and
// results in the value itself unprocessed.
func Any() Checker {
return anyC{}
type anyC struct{}
func (c anyC) Coerce(v interface{}, path []string) (interface{}, error) {
return v, nil
// OneOf returns a Checker that attempts to Coerce the value with each
// of the provided checkers. The value returned by the first checker
// that succeeds will be returned by the OneOf checker itself. If no
// checker succeeds, OneOf will return an error on coercion.
func OneOf(options ...Checker) Checker {
return oneOfC{options}
type oneOfC struct {
options []Checker
func (c oneOfC) Coerce(v interface{}, path []string) (interface{}, error) {
for _, o := range c.options {
newv, err := o.Coerce(v, path)
if err == nil {
return newv, nil
return nil, error_{"", v, path}
// pathAsPrefix returns a string consisting of the path elements
// suitable for using as the prefix of an error message. If path
// starts with a ".", the dot is omitted.
func pathAsPrefix(path []string) string {
if len(path) == 0 {
return ""
var s string
if path[0] == "." {
s = strings.Join(path[1:], "")
} else {
s = strings.Join(path, "")
if s == "" {
return ""
return s + ": "
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