Dump runtime values to typed Hack source code.

v0.2.0 2024-02-15 06:53 UTC

This package is auto-updated.

Last update: 2024-05-15 07:53:37 UTC


Dump runtime values to typed Hack source code.

Why do you need this?

Run code during build-time, codegen the values you were interested in back to Hack code, write it to a Hack source file, and you have build-time compute.

The following snippet1 embodies the essence of this library perfectly.

function burn_a_value_to_constant<reify T>(
  string $constant_name,
  T $value,
  ExprDump\DumpOptions $dumper_options = shape(),
)[]: string {
  $type_name = TypeVisitor\visit<T, _, _>(new TypeVisitor\TypenameVisitor());
  $serialized_value = ExprDump\dump<T>($value, $dumper_options);
  return Str\format(
    'const %s %s = %s;',

All data that enters your program at runtime is typed as mixed, which is why you must cast it to a typed value in order to process it. If you do this safely, this incurs runtime costs. Doing it using an unsafe mechanism, such as HH\FIXME\UNSAFE_CAST or HH_FIXME[4110] is trading correctness for performance. But with burn_a_value_to_constant, the data doesn't enter the program at runtime. This data is already typed, so there is no need for casting.

Why the existing tools can not meet this need

In Hack, the same runtime value can represent two different types.

shape('name' => 'value') === dict['name' => 'value']; // true
shape('name' => 'value') === shape(SomeClass::CLASS_CONSTANT => 'value'); // true
vec[1, 2, 3] === tuple(1, 2, 3); // true
6 === MyEnum::ENUMERATOR; // true

HHVM is unable to distinguish between these types, but Hack is, and will emit errors if you initialize a value with the wrong type, even if the runtime value would be exactly the same as the correct initializer.

const (int, int) A_TUPLE = vec[1, 2]; // type error

ExprDump\dump<reify T>(T $value): string takes a type and a value. This gives it enough information to represent shapes as shapes, tuples as tuples, enums as enums, whilst not confusing them for dicts, vecs, and arraykeys.


// Like var_dump(), with Hack syntax, all type information is lost.
//  - shapes become dicts
//  - tuples become vecs
//  - enums become ints / strings

// There are multiple options that can be combined in any way.

// Encode Hack types and aliasses how you please.
// \App\user_id_from_int(4) types the value as an App\UserId.
ExprDump\dump<vec<shape('name' => string, 'friends' => vec<App\UserId>)>>(
    'custom_dumpers' => dict[
      App\UserId::class => (mixed $id)[] ==>
        '\App\user_id_from_int('.($id as int).')',

// Serialize enums as enumeration expression, such as Roles::ADMIN.
    'enum_definitions' => ExprDump\EnumDefinition::create(Roles::class),

// Keep class constant names in the dumped output.
// shape(\SomeClass::CONSTANT => 1) instead of shape('val' => 1)
ExprDump\dump<shape(SomeClass::CONSTANT => int)>(
    'shape_key_namer' => (
      ?string $_parent_shape_name,
      arraykey $key,
    )[] ==> {
      $prefix = '\\'.SomeClass::class.'::';
      return dict<arraykey, string>[
        SomeClass::CONSTANT => $prefix.'CONSTANT',
      ][$key] ?? null;

// Create a reusable dumper, call `->dump()`
$dumper = ExprDump\create_dumper<SomeType>(shape(
  // The options go here


Note about the stability of this api

This library depends on HTL\TypeVisitor to provide its functionality. TypeVisitor depends on unstable Hack apis, TypeStructure<T> and \HH\ReifiedGenerics\get_type_structure<T>(). For more details, see stability. This api has been unstable since 2016, so take this with a grain of salt.

In order to minimize the potential impact of a removal of these apis, you should not use this library in places where the performance of bootstrapping the dumper is critical. A far less performant variant of TypeVisitor could be written, even without these api affordances.


  1. This snippet burn_a_value_to_constant is excempt from the MIT license of this library. It is licensed to you under MIT-0 (MIT No Attribution). This excemption does not apply to the code called by this snippet. The MIT license still covers all other parts of this program.