errors: captures error message fields in struct

closes Issue #44

Author: santhosh-tekuri

errors.go

@@ -3,6 +3,8 @@ package jsonschema
 import (
 	"fmt"
 	"strings"
+
+	"github.com/santhosh-tekuri/jsonschema/v5/msg"
 )
 
 // InvalidJSONTypeError is the error type returned by ValidateInterface.
@@ -69,7 +71,7 @@ type ValidationError struct {
 	KeywordLocation         string             // validation path of validating keyword or schema
 	AbsoluteKeywordLocation string             // absolute location of validating keyword or schema
 	InstanceLocation        string             // location of the json value within the instance being validated
-	Message                 string             // describes error
+	Message                 fmt.Stringer       // captures the message and data used in constructing it
 	Causes                  []*ValidationError // nested validation errors
 }
 
@@ -81,10 +83,11 @@ func (ve *ValidationError) add(causes ...error) error {
 }
 
 func (ve *ValidationError) causes(err error) error {
-	if err := err.(*ValidationError); err.Message == "" {
-		ve.Causes = err.Causes
+	var e = err.(*ValidationError)
+	if _, ok := e.Message.(msg.Empty); ok {
+		ve.Causes = e.Causes
 	} else {
-		ve.add(err)
+		ve.add(e)
 	}
 	return ve
 }

example_extension_test.go

@@ -31,6 +31,15 @@ func (powerOfCompiler) Compile(ctx jsonschema.CompilerContext, m map[string]inte
 	return nil, nil
 }
 
+type powerOfSchemaMsg struct {
+	got  interface{}
+	want int64
+}
+
+func (m powerOfSchemaMsg) String() string {
+	return fmt.Sprintf("%v not powerOf %v", m.got, m.want)
+}
+
 type powerOfSchema int64
 
 func (s powerOfSchema) Validate(ctx jsonschema.ValidationContext, v interface{}) error {
@@ -42,7 +51,7 @@ func (s powerOfSchema) Validate(ctx jsonschema.ValidationContext, v interface{})
 			n = n / pow
 		}
 		if n != 1 {
-			return ctx.Error("powerOf", "%v not powerOf %v", v, pow)
+			return ctx.Error("powerOf", powerOfSchemaMsg{got: v, want: pow})
 		}
 		return nil
 	default:

extension.go

@@ -1,5 +1,7 @@
 package jsonschema
 
+import "fmt"
+
 // ExtCompiler compiles custom keyword(s) into ExtSchema.
 type ExtCompiler interface {
 	// Compile compiles the custom keywords in schema m and returns its compiled representation.
@@ -75,7 +77,7 @@ type ValidationContext struct {
 	result          validationResult
 	validate        func(sch *Schema, schPath string, v interface{}, vpath string) error
 	validateInplace func(sch *Schema, schPath string) error
-	validationError func(keywordPath string, format string, a ...interface{}) *ValidationError
+	validationError func(keywordPath string, msg fmt.Stringer) *ValidationError
 }
 
 // EvaluatedProp marks given property of object as evaluated.
@@ -104,8 +106,8 @@ func (ctx ValidationContext) Validate(s *Schema, spath string, v interface{}, vp
 // Error used to construct validation error by extensions.
 //
 // keywordPath is relative-json-pointer to keyword.
-func (ctx ValidationContext) Error(keywordPath string, format string, a ...interface{}) *ValidationError {
-	return ctx.validationError(keywordPath, format, a...)
+func (ctx ValidationContext) Error(keywordPath string, msg fmt.Stringer) *ValidationError {
+	return ctx.validationError(keywordPath, msg)
 }
 
 // Group is used by extensions to group multiple errors as causes to parent error.