PHP to Hack at Slack

1. Scott Sandler - June 2019

2. Scott Sandler Senior Staﬀ Engineer @ Slack 2015 - Present

9. ● Static type checker, hh_client ● Language Server Protocol ● VSCode, VIM, Emacs, and more ● Overview ● Static Type Checking ● Generics ○ Arrays ○ Hack Arrays ○ Shapes ● Standard Library ○ Pipeline ○ Typed Regex ● XHP

10. ● Overview ● Static Type Checking ● Generics ○ Arrays ○ Hack Arrays ○ Shapes ● Standard Library ○ Pipeline ○ Typed Regex ● XHP

12. “Generic programming is a style of computer programming in which algorithms are written in terms of types to-be-specified-later that are then instantiated when needed for specific types provided as parameters.” - Wikipedia Type Parameters like <T> allow classes, functions, and data structures to act on multiple types ● Overview ● Static Type Checking ● Generics ○ Arrays ○ Hack Arrays ○ Shapes ● Standard Library ○ Pipeline ○ Typed Regex ● XHP

22. ● Represent an array with known structure ● Named keys with known data types ○ Keys can be optional ● Structural subtyping ● Overview ● Static Type Checking ● Generics ○ Arrays ○ Hack Arrays ○ Shapes ● Standard Library ○ Pipeline ○ Typed Regex ● XHP

28. ● Inconsistent Argument Order ○ array_map($callback, $array); ○ array_filter($array, $callback); ● Misleading return types ○ strpos() returns int OR false ● Overview ● Static Type Checking ● Generics ○ Arrays ○ Hack Arrays ○ Shapes ● Standard Library ○ Pipeline ○ Typed Regex ● XHP

29. Read from inside out… or split into multiple lines and name each intermediate value? ● Overview ● Static Type Checking ● Generics ○ Arrays ○ Hack Arrays ○ Shapes ● Standard Library ○ Pipeline ○ Typed Regex ● XHP

30. ● Consistent Argument Order ● Consistent Return Types ○ null instead of false ● Namespaces: ○ Container Functions: Vec*, Dict*, Keyset*, C* ○ Basics: Str*, Regex*, Math* ○ More: IO*, PseudoRandom*, SecureRandom* ● |> Pipeline operator for chaining function calls ● Overview ● Static Type Checking ● Generics ○ Arrays ○ Hack Arrays ○ Shapes ● Standard Library ○ Pipeline ○ Typed Regex ● XHP

34. ● async/await cooperative multi-tasking ● is/as keywords for type assertion ● lambdas ● enums ● <<__Memoize>> caching ● hackfmt ● Function Autoloading ● VSCode debugging

35. Migrating Slack

37. ● <?php to <?hh ● Fix hh_client errors ● Blocking CI tests ○ hh_client ○ Linter ● Move code into functions ● Foundational Types ○ DB Shapes ○ Inferred returns ○ Untyped code ● Coverage Competition ● <?hh // strict ● Eliminate PHP deps ● HHVM 4.0 upgrade

38. Fix hh_client errors ● <?php to <?hh ● Fix hh_client errors ● Blocking CI tests ○ hh_client ○ Linter ● Move code into functions ● Foundational Types ○ DB Shapes ○ Inferred returns ○ Untyped code ● Coverage Competition ● <?hh // strict ● Eliminate PHP deps ● HHVM 4.0 upgrade

39. ● Hundreds of bugs ● Convince typechecker ● <?php to <?hh ● Fix hh_client errors ● Blocking CI tests ○ hh_client ○ Linter ● Move code into functions ● Foundational Types ○ DB Shapes ○ Inferred returns ○ Untyped code ● Coverage Competition ● <?hh // strict ● Eliminate PHP deps ● HHVM 4.0 upgrade

43. New Linter: HHAST ● Hack Abstract Syntax Tree ● More rigorous linters ● Migrations as well Old Linter: PHP_CodeSniﬀer ● Stream of tokens ● Each linter is a mini-PHP parser ● <?php to <?hh ● Fix hh_client errors ● Blocking CI tests ○ hh_client ○ Linter ● Move code into functions ● Foundational Types ○ DB Shapes ○ Inferred returns ○ Untyped code ● Coverage Competition ● <?hh // strict ● Eliminate PHP deps ● HHVM 4.0 upgrade

45. Foundational Types

46. After: AsyncMysqlClient ● Shapes for each table ● Async queries Before: mysqli ● Untyped arrays of strings ● Synchronous queries ● <?php to <?hh ● Fix hh_client errors ● Blocking CI tests ○ hh_client ○ Linter ● Move code into functions ● Foundational Types ○ DB Shapes ○ Inferred returns ○ Untyped code ● Coverage Competition ● <?hh // strict ● Eliminate PHP deps ● HHVM 4.0 upgrade

50. Untyped code ● <?php to <?hh ● Fix hh_client errors ● Blocking CI tests ○ hh_client ○ Linter ● Move code into functions ● Foundational Types ○ DB Shapes ○ Inferred returns ○ Untyped code ● Coverage Competition ● <?hh // strict ● Eliminate PHP deps ● HHVM 4.0 upgrade

51. Untyped code ● <?php to <?hh ● Fix hh_client errors ● Blocking CI tests ○ hh_client ○ Linter ● Move code into functions ● Foundational Types ○ DB Shapes ○ Inferred returns ○ Untyped code ● Coverage Competition ● <?hh // strict ● Eliminate PHP deps ● HHVM 4.0 upgrade

52. Type Coverage

53. Colorized Type Coverage ● <?php to <?hh ● Fix hh_client errors ● Blocking CI tests ○ hh_client ○ Linter ● Move code into functions ● Foundational Types ○ DB Shapes ○ Inferred returns ○ Untyped code ● Coverage Competition ● <?hh // strict ● Eliminate PHP deps ● HHVM 4.0 upgrade

54. Coverage Numbers ● <?php to <?hh ● Fix hh_client errors ● Blocking CI tests ○ hh_client ○ Linter ● Move code into functions ● Foundational Types ○ DB Shapes ○ Inferred returns ○ Untyped code ● Coverage Competition ● <?hh // strict ● Eliminate PHP deps ● HHVM 4.0 upgrade

55. Track it and pipe it to Slack ● <?php to <?hh ● Fix hh_client errors ● Blocking CI tests ○ hh_client ○ Linter ● Move code into functions ● Foundational Types ○ DB Shapes ○ Inferred returns ○ Untyped code ● Coverage Competition ● <?hh // strict ● Eliminate PHP deps ● HHVM 4.0 upgrade

56. <?hh // strict 1. All functions and arguments must be typed 2. No top-level code 3. Other dynamicism like $GLOBALS banned ● <?php to <?hh ● Fix hh_client errors ● Blocking CI tests ○ hh_client ○ Linter ● Move code into functions ● Foundational Types ○ DB Shapes ○ Inferred returns ○ Untyped code ● Coverage Competition ● <?hh // strict ● Eliminate PHP deps ● HHVM 4.0 upgrade

57. Eliminate PHP Dependencies 1. Smarty to XHP migration ○ Smarty Parser -> hack-codegen 2. Other dependencies: ○ Convert to Hack ○ Remove dependency ○ Run as PHP7 web service ● <?php to <?hh ● Fix hh_client errors ● Blocking CI tests ○ hh_client ○ Linter ● Move code into functions ● Foundational Types ○ DB Shapes ○ Inferred returns ○ Untyped code ● Coverage Competition ● <?hh // strict ● Eliminate PHP deps ● HHVM 4.0 upgrade

58. HHVM 4.0 Upgrade ● <?php to <?hh ● Fix hh_client errors ● Blocking CI tests ○ hh_client ○ Linter ● Move code into functions ● Foundational Types ○ DB Shapes ○ Inferred returns ○ Untyped code ● Coverage Competition ● <?hh // strict ● Eliminate PHP deps ● HHVM 4.0 upgrade

60. ● Thousands of bugs found and fixed ● Entire classes of problems eliminated ○ Call to undefined function ○ Missing include/require ○ Missing array keys ○ Missing null checks

61. ● Developer Surveys ● Increased confidence ● Increased productivity ● “When the type checker passes, my code usually works on the first try”

63. UI Integration Unit

64. UI Integration Unit Types

65. ● Types are the cheapest unit test ○ ...but only when you have a lot of them ● Types are the cheapest documentation ○ Especially complex types like shapes ● Typing is best done as part of feature work Lessons Learned

68. Yes, this is possible ● Generics RFC ● Arrow functions in 7.4 ● Property type hints ● JIT in PHP8 ● Static analysis: ○ phan: https://github.com/phan/phan ○ phpstan: https://github.com/phpstan/phpstan ○ psalm: https://github.com/vimeo/psalm

69. ● eval(), create_function(), extract(), compact() ● $GLOBAL variables ● $$variable variables ● call_user_func() ● &$references Avoid PHP’s most dangerous features

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