From 94e9f5cbd41b4bde5cdf30700e223b94757932ff Mon Sep 17 00:00:00 2001
From: Aleksey Levenstein <levenleven@github.com>
Date: Tue, 12 Dec 2023 09:48:34 +0200
Subject: [PATCH] docs: document custom keyword validation

---
 README.md | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/README.md b/README.md
index fca5266..58b3a65 100644
--- a/README.md
+++ b/README.md
@@ -489,6 +489,24 @@ Options:
   -v, --version                    Show version
 ```
 
+## Custom keyword validation
+
+Custom keyword validation can be achieved by extending spec vocab. E.g. to error on fields marked with `deprecated` keyword one can:
+
+```ruby
+class Deprecated < JSONSchemer::Keyword
+  def error(formatted_instance_location:, **)
+    "#{formatted_instance_location} is deprecated"
+  end
+
+  def validate(instance, instance_location, keyword_location, _context)
+    result(instance, instance_location, keyword_location, false)
+  end
+end
+
+JSONSchemer::Draft202012::Vocab::VALIDATION['deprecated'] = Deprecated
+```
+
 ## Development
 
 After checking out the repo, run `bin/setup` to install dependencies. Then, run `rake test` to run the tests. You can also run `bin/console` for an interactive prompt that will allow you to experiment.