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unmarshal-error.go
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/
unmarshal-error.go
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package goxtag
import (
"fmt"
"reflect"
)
// All "Reason" fields within CannotUnmarshalError will be constants and part of
// this list
const (
nonPointer = "non-pointer value"
nodeNotFound = "node not found in document"
nilDestination = "destination is nil"
arrayLengthMismatch = "array length does not match document elements found"
customUnmarshalError = "a custom Unmarshaler implementation threw an error"
typeConversionError = "a type conversion error occurred"
mapIsNotSupportedError = "map type is not currently supported"
multipleNodesDetected = "multiple nodes detected for selector"
)
// CannotUnmarshalError represents an error returned by the goqxtag Unmarshaler
// and helps consumers in programmatically diagnosing the cause of their error.
type CannotUnmarshalError struct {
Err error
Val string
FldOrIdx interface{}
V reflect.Value
Reason string
XPath string
}
// This type is a mid-level abstraction to help understand the error printing logic
type errChain struct {
chain []*CannotUnmarshalError
val string
tail error
}
// tPath returns the type path in the same string format one might use to access
// the nested value in go code. This should hopefully help make debugging easier.
func (e errChain) tPath() string {
nest := ""
for _, err := range e.chain {
if err.FldOrIdx != nil {
switch nesting := err.FldOrIdx.(type) {
case string:
switch err.V.Type().Kind() {
case reflect.Map:
nest += fmt.Sprintf("[%q]", nesting)
case reflect.Struct:
nest += fmt.Sprintf(".%s", nesting)
}
case int:
nest += fmt.Sprintf("[%d]", nesting)
case *int:
nest += fmt.Sprintf("[%d]", *nesting)
default:
nest += fmt.Sprintf("[%v]", nesting)
}
}
}
return nest
}
func (e errChain) last() *CannotUnmarshalError {
return e.chain[len(e.chain)-1]
}
// Error gives a human-readable error message for debugging purposes.
func (e errChain) Error() string {
last := e.last()
// Avoid panic if we cannot get a type name for the Value
t := "unknown: invalid value"
if last.V.IsValid() {
t = last.V.Type().String()
}
msg := "could not unmarshal "
if e.val != "" {
msg += fmt.Sprintf("value %q ", e.val)
}
v := e.chain[0].V
if v.CanAddr() {
msg += fmt.Sprintf(
"into '%s%s' (type %s): %s",
v.Type(),
e.tPath(),
t,
last.Reason,
)
}
if last.XPath != "" {
msg += fmt.Sprintf(" tag: '%s'", last.XPath)
}
// If a generic error was reported elsewhere, report its message last
if e.tail != nil {
msg = msg + ": " + e.tail.Error()
}
return msg
}
// Traverse e.Err, printing hopefully helpful type info until there are no more
// chained errors.
func (e *CannotUnmarshalError) unwind() *errChain {
str := &errChain{chain: []*CannotUnmarshalError{}}
for {
str.chain = append(str.chain, e)
if e.Val != "" {
str.val = e.Val
}
// Terminal error was of type *CannotUnmarshalError and had no children
if e.Err == nil {
return str
}
if e2, ok := e.Err.(*CannotUnmarshalError); ok {
e = e2
continue
}
// Child error was not a *CannotUnmarshalError; print its message
str.tail = e.Err
return str
}
}
func (e *CannotUnmarshalError) Error() string {
return e.unwind().Error()
}