Implement the custom format validator

README.md

@@ -142,6 +142,21 @@ true
 
 The validator supports all the formats specified by draft 4 (`date-time`, `email`, `hostname`, `ipv4`, `ipv6`), with the exception of the `uri` format which has confusing/broken requirements in the official test suite (see https://github.com/json-schema/JSON-Schema-Test-Suite/issues/77).
 
+### Custom formats
+
+The [JSON schema spec][format-spec] states that the `format` property "allows values to be constrained beyond what the other tools in JSON Schema can do". To support this, you can configure a callback validator function which gets called when a `format` property is encountered that is not one of the builtin formats:
+
+```elixir
+config :ex_json_schema,
+  :custom_format_validator,
+  {MyModule, :my_validator}
+```
+
+The configured function is called with the arguments `(format, data)` and is expected to return either `true` or `false`, depending whether the data is valid for the given format. For compatibility with JSON schema, it is expected to return `true` when the format is unknown by your callback function.
+
+[format-spec]: https://json-schema.org/understanding-json-schema/reference/string.html#format
+
+
 ## License
 
 Released under the [MIT license](LICENSE).

lib/ex_json_schema/validator/format.ex

@@ -29,7 +29,16 @@ defmodule ExJsonSchema.Validator.Format do
     end
   end
 
-  defp do_validate(_, _) do
-    []
+  defp do_validate(format, data) do
+    case Application.fetch_env(:ex_json_schema, :custom_format_validator) do
+      :error ->
+        []
+
+      {:ok, {mod, fun}} ->
+        case apply(mod, fun, [format, data]) do
+          true -> []
+          false -> [%Error{error: %Error.Format{expected: format}, path: ""}]
+        end
+    end
   end
 end

test/ex_json_schema/validator_test.exs

@@ -522,6 +522,48 @@ defmodule ExJsonSchema.ValidatorTest do
     )
   end
 
+  test "unknown formats are ignored" do
+    assert :ok == validate(%{"format" => "custom-format"}, "asdfsadf")
+  end
+
+  defmodule MyFormatValidator do
+    def validate_format("always_error", _data) do
+      false
+    end
+
+    def validate_format("zipcode", data) do
+      Regex.match?(~r/^\d+$/, data)
+    end
+  end
+
+  test "configuring a custom formatter" do
+    Application.put_env(
+      :ex_json_schema,
+      :custom_format_validator,
+      {MyFormatValidator, :validate_format}
+    )
+
+    on_exit(fn ->
+      Application.delete_env(:ex_json_schema, :custom_format_validator)
+    end)
+
+    assert_validation_errors(
+      %{"format" => "always_error"},
+      "",
+      [{"Expected to be a valid always_error.", "#"}],
+      [%Error{error: %Error.Format{expected: "always_error"}, path: "#"}]
+    )
+
+    assert :ok == validate(%{"format" => "zipcode"}, "12345")
+
+    assert_validation_errors(
+      %{"format" => "zipcode"},
+      "asdf",
+      [{"Expected to be a valid zipcode.", "#"}],
+      [%Error{error: %Error.Format{expected: "zipcode"}, path: "#"}]
+    )
+  end
+
   test "passing the formatter as an option" do
     assert :ok = validate(%{"type" => "string"}, "foo", error_formatter: Error.StringFormatter)