able to list products

internal/domain/error.go

@@ -1,7 +1,12 @@
 package domain
 
+import (
+	"fmt"
+)
+
 const (
-	UnexpectedStatusError SimpleError = "unexpected status"
+	ErrNotImplemented   SimpleError = "not implemented"
+	ErrUnexpectedStatus SimpleError = "unexpected status"
 )
 
 type SimpleError string
@@ -9,3 +14,23 @@ type SimpleError string
 func (err SimpleError) Error() string {
 	return string(err)
 }
+
+type ValidationError struct {
+	Field  string
+	Reason string
+}
+
+func (err *ValidationError) Error() string {
+	return fmt.Sprintf("field %s is not valid for reason %s", err.Field, err.Reason)
+}
+
+func NewValidationError(field, reason string) *ValidationError {
+	if field == "" {
+		panic("field not provided")
+	}
+
+	return &ValidationError{
+		Field:  field,
+		Reason: reason,
+	}
+}