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Hack JSON Schema generator

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Hack JSON Schema is a library for validating JSON inputs, using JSON schemas, in Hack.

Given a JSON schema file, you can generate a validator in Hack to validate incoming JSON against the schema. If the JSON conforms to the schema, the validator will returned typed output.

There are several benefits to generation:

  • We don't have to parse the JSON schema to validate the incoming object at runtime.
  • We can output typed shapes that are generated from the JSON schema, increasing the type safety of downstream code.



The most basic way to use this library is to generate a validator from a JSON schema file:

use type Slack\Hack\JsonSchema\Codegen\Codegen;

Codegen::forPath('/path/to/json-schema.json', shape(
  'validator' => shape(
    'file' => '/path/to/MyJsonSchemaValidator.php',
    'class' => 'MyJsonSchemaValidator',

/path/to/MyJsonSchemaValidator.php now exists with a class:

final class MyJsonSchemaValidator extends BaseValidator {
  ... class contents

Each validator has a validate method, which takes a decoded JSON object:

$json = json_decode($args['json_input'], true);
$validator = new MyJsonSchemaValidator($json);
if (!$validator->isValid()) {
  print_r("invalid_json", $validator->getErrors());

// JSON is valid, get typed object:
$validated = $validator->getValidatedInput();


If you have multiple JSON schemas that leverage the $ref attribute, you should prefer to use Codegen::forPaths over Codegen::forPath.

The workflow for Codegen::forPath is:

  • Given a JSON schema, "de-reference" the schema. De-referencing is the process of resolving all of the $ref paths with their actual schema. This creates a single de-referenced schema.
  • With the de-referenced schema, generate a validator.

This works well if you only have one primary schema, but if you have multiple schemas, each with common refs, you'll start to generate a lot of duplicate code.

In these cases, you can use Codegen::forPaths.

use type Slack\Hack\JsonSchema\Codegen\Codegen;

$schemas = vec['/path/to/json-schema-1.json', '/path/to/json-schema-2.json', '/path/to/json-schema-3.json'];
Codegen::forPaths($schemas, shape(
  'validator' => shape(
    'refs' => shape(
       'unique' => shape(
          'source_root' => '/path/to',
          'output_root' => '/path/gen'

By defining the source_root and output_root we can generate unique validators per $ref we come across. We can then re-use those validators when generating other validators.


Installing Dependencies

We handle all dependencies through Docker. It's as simple as:

make install

Running Tests

make test

Related Libraries

This library was inspired by the ideas in these related libraries:


Hack JSON Schema is MIT-licensed.