Use error structs instead of duplicated strings

errors.go

@@ -0,0 +1,57 @@
+package mapstructure
+
+import (
+	"fmt"
+	"reflect"
+)
+
+// DecodeError is a generic error type that holds information about
+// a decoding error together with the name of the field that caused the error.
+type DecodeError struct {
+	name string
+	err  error
+}
+
+func newDecodeError(name string, err error) *DecodeError {
+	return &DecodeError{
+		name: name,
+		err:  err,
+	}
+}
+
+func (e *DecodeError) Name() string {
+	return e.name
+}
+
+func (e *DecodeError) Unwrap() error {
+	return e.err
+}
+
+func (e *DecodeError) Error() string {
+	return fmt.Sprintf("'%s' %s", e.name, e.err)
+}
+
+// ParseError is an error type that indicates a value could not be parsed
+// into the expected type.
+type ParseError struct {
+	Expected reflect.Value
+	Value    interface{}
+	Err      error
+}
+
+func (e *ParseError) Error() string {
+	return fmt.Sprintf("cannot parse '%s' as '%s': %s",
+		e.Value, e.Expected.Type(), e.Err)
+}
+
+// UnconvertibleTypeError is an error type that indicates a value could not be
+// converted to the expected type.
+type UnconvertibleTypeError struct {
+	Expected reflect.Value
+	Value    interface{}
+}
+
+func (e *UnconvertibleTypeError) Error() string {
+	return fmt.Sprintf("expected type '%s', got unconvertible type '%s', value: '%v'",
+		e.Expected.Type(), reflect.TypeOf(e.Value), e.Value)
+}