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Hack JSON Schema generator

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v4.102.2 2021-10-14 17:23 UTC


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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 use composer to install our dependencies. Running the following will install composer (if it doesn't exist already) and install the dependencies:

make composer-install

Running Tests

make test

Related Libraries

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


Hack JSON Schema is MIT-licensed.