Presentation about Hack programming language used by Facebook

2. Agenda
 What is the Hack Language ?
 Hack Modes
 Type Annotations
 Nullable
 Generics
 Collections
 Type Aliasing
 Constructor Argument Promotion
 PHP to Hack Conversion DEMO

3. Not on Agenda, but interesting
 Async - asynchronous programming support via
async/await
 XHP – an extension of the PHP and Hack syntax
where XML elements/blocks become valid
expressions
 Lamba expressions – a more advanced version of PHP
closures

4. What is the Hack Language ?
 Language for HHVM that interoperates with PHP
 Developed and used by Facebook
 Open-Sourced in March 2014
 Basically, PHP with a ton of extra features
 Hack = Fast development cycle of PHP + discipline of
strong typing
 Combines strong typing with weak typing => gradual
typing (you can strong type only the parts you want)
 Write cleaner/safer code while maintaining good
compatibility with existing PHP codebases

5. What is the Hack Language ? (II)
 Hack code uses “<?hh” instead of “<?php”. Not
mandatory but, if you do it, you won’t be able to jump
between Hack and HTML anymore
 Hack and PHP can coexist in the same project. Good if
you want an incremental switch to Hack.
 PHP code can call Hack code and vice-versa
 Closing tags (“?>”) are not supported
 Naming your files “*.hh” can be a good practice

6. Hack Modes (I)
 Intended for maximum flexibility converting PHP to
Hack
 Helps when running “hackificator”
 Each file can have only one Hack mode
 Default is “partial”
 Declared at the top: “<?hh // strict”

7. Hack Modes (II)
 Strict:
 <?hh // strict
 Type checker will catch every error
 All code in this mode must be correctly annotated
 Code in strict mode cannot call non-Hack code.

8. Hack Modes (III)
 Partial:
 <?hh // partial
 <?hh
 Default mode
 Allows calling non-Hack code
 Allows ommitting some function parameters

9. Hack Modes (IV)
 Decl:
 <?hh // decl
 used when annotating old APIs
 Allows Hack code written in strict mode to call into legacy
code. Just like in partial mode, but without having to fix
reported problems

10. Type Annotations (I)
 Readability by helping other developers understand the
purpose and intention of the code. Many used
comments to do this, but Hack formalizes it instead
 Correctness by forbidding unsafe coding practices
 Make use of automatic and solid refactoring tools of a
strong-typed language.

11. Type Annotations (II)
 class MyExampleClass {
 public int $number;
 public CustomClass $myObject;
 private string $theName;
 protected array $mystuff;

 function doStuff(string $name, bool $withHate): string {
 if ($withHate && $name === 'Satan') {
 return '666 HAHA';
 }
 return '';
 }
 }

12. Type Annotations (III)
 Possible annotations:
 Primitive types: int, string, float, bool, array
 User-defined classes: MyClass, Vector<mytype>
 Generics: MyClass<T>
 Mixed: mixed
 Void: void
 Typed arrays: array<Foo>, array<string, array<string, MyClass>>
 Tuples: e.g. tuple(string, bool)
 XHP elements
 Closures: (function(type_1, type_2): return_type)
 Resources: resource
 this: this

13. Nullable
 Safer way to deal with nulls
 In type annotation, add “?” to the type to make it
nullable. e.g. ?string
 Normal PHP code does allow some annotation:
 public function doStuff(MyClass $obj)
 But passing null to this results in a fatal error
 So in Hack you can do this:
 public function doStuff(?MyClass $obj)
 It’s best not to abuse this feature, but to use it when you
really need it

14. Generics (I)
 Allows classes and methods to be parameterized
 Generics code can be statically checked for correctness
 Offers a great alternative against using a top-level object + a
bunch of instanceof calls + type casts
 In most languages, classes must be used as types. But Hack
allows primitives too
 They are immutable: once a type has been associated, it can
not be changed
 In Hack, objects can be used as types too
 Nullable types are also supported
 Interfaces and Traits support Generics too
 See docs for more details

15. Generics (II)
 class MyClass<K> {
 private K $param;
 public function __construct(K $param) {
 $this->param = $param;
 }
 public function getParameter(): K {
 return $this->param;
 }
 }

16. Collections (I)
 Classes specialized for data storage and retrieval
 PHP arrays are too general, they offer a “one-size-fits-all”
approach
 In both Hack and PHP, arrays are not objects. But
Collections are.
 Seeing the keyword "array" in a piece of code doesn't
make it clear how that array will be used
 But each Collection class is best suited for specific
situations.
 So code is clearer and, if used correctly, can be a
performance improvement

17. Collections (II)
 They are traversable with “foreach” loops
 Support bracket notations: $mycollection[$index]
 Since they are objects, they are not copied when passed
around as parameters
 They integrate and work very well with Generics
 Immutable variants of the Collection classes also exist.
This is to ensure that they are not changed when passed
around
 Classes are: Vector, Map, Set, Pair
 Immutable collections: ImmVector, ImmMap, ImmSet

18. Collections (III)
 Vector example:
 $vector = Vector {6, 5, 4, 9};
 $vector->add(97);
 $vector->add(31);
 $vector->get(1);
 $x = $vector[2];
 $vector->removeKey(2);
 $vector[] = 999;
 foreach ($vector as $key => $value) {...}

19. Collections (IV)
 Map example:
 $map = Map {“key1" => 1, “key2" => 2, “key3" => 3};
 $b = $map->get(“key2");
 $map->remove(“key2");
 $map->contains(“key1");
 foreach ($map as $key => $value) {...}

20. Type Aliasing
 Just like PHP has aliasing support in namespaces, so
does Hack provide the same functionality for any type
 Two modes of declaration:
 type MyType = string;
 newtype MyType = string;
 The 2nd is called “opaque aliasing” and is only visible in
the same file
 Composite types can also be declared:
 newtype Coordinate = (float, float);
 These are implemented as tuples, so that is how they must
be created in order to be used

21. Constructor Argument Promotion (I)
 Before:
 class User {
 private string $name;
 private string $email;
 private int $age;
 public function __construct(string $name, string $email, int $age)
{
 $this->name = $name;
 $this->email = $email;
 $this->age = $age;
 }
 }

22. Constructor Argument Promotion (II)
 After:
 class User {
 public function __construct(
 private string $name,
 private string $email,
 private int $age
 ) {}
 }

23. PHP to Hack
Conversion
DEMO

24. Q & A