feat: add optional error data

api/error.go

@@ -1,6 +1,7 @@
 package api
 
 import (
+	"encoding/json"
 	"fmt"
 )
 
@@ -14,8 +15,31 @@ type Err struct {
 	Reason string `json:"reason"`
 	// associated HTTP status
 	Status int
+	// data that can be added to document the error
+	data []interface{} `json:"data"`
 }
 
 func (e Err) Error() string {
 	return fmt.Sprintf("[%d] %s", e.Code, e.Reason)
 }
+
+// With adds data to document the error and returns the updated error
+func (e Err) With(data ...interface{}) Err {
+	if e.data == nil {
+		e.data = make([]interface{}, 0)
+	}
+	e.data = append(e.data, data...)
+	return e
+}
+
+// MarshalJSON implements the 'json.Marshaler' interface and is used
+// to generate the JSON representation of the error
+func (e Err) MarshalJSON() ([]byte, error) {
+	fmt := make(map[string]interface{})
+	fmt["code"] = e.Code
+	fmt["reason"] = e.Reason
+	if len(e.data) > 0 {
+		fmt["data"] = e.data
+	}
+	return json.Marshal(fmt)
+}