Type Profiler: Ambitious Type Inference for Ruby 3 Yusuke Endoh (@mametter) RubyKaigi Takeout 2020 1

Goals of Ruby 3’s Static Analysis •Make Ruby programming easier • Bug detection before execution • Completion and document in IDE … with no type annotation in code! (Ruby 3 Type Challenge) 2 42 + "str" Is this a bug? 42.ti| Do you mean: 42.times {|i| | }

Ruby 3 will provide three “type” items 3 # app.rbs def inc: (Integer) -> Integer ① Type Signature Format (RBS) # app.rb def inc(n) n+1 end Ruby code ② Type Inference (ruby-type-profiler) ③ Type Check (Steep, Sorbet, RDL, …)

Ruby 3 Development Experience 4 gem lib.rb lib.rbs app app.rb app.rbs 42.ti| 42 + "str" Is this a bug? Do you mean: 42.times {|i| | } ② Type Inference • Generate an RBS prototype • Simply check the code ③ Type Checker • Verify .rb and .rbs • Serve as LSP server You can also write RBS manually if you want Type-guided linter Dynamic type checker More dedicated dev. environments Monitor and Harness Ruby code in run-time Code formatter by leveraging types Rubymine, Tabnine, and other development tools may use type information https://github.com/pocke/rubocop-typed https://github.com/ruby/rbs/blob/master/ lib/rbs/test/type_check.rb RBS may inspire other dreams Today we talk about ①RBS

Agenda ➔Type Profiler: Type Inference for Ruby 3 •Demo •How to use Type Profiler • Future plan 5

What is Type Profiler Abstract ("type-level") interpreter of Ruby Generates an RBS prototype by gathering what types a method accepts and returns 6 def foo(n) n.to_s end foo(42) Integer (not 42) String (not "42") def foo: (Integer) -> String

Why is the name "Type Profiler"? • Just for a historical reason • The initial version was runtime analysis (profiling) • Now it is a bit confusing with a normal profiler •Invite suggestions for the name • Should it start with "S"? (Steep, Sorbet, …) 7

Difference from traditional type system •Traditional type systems use intra-procedural (per-method) analysis • Can't handle unannotated method parameter well • … especially when there are many classes that respond to foo 8 def f(x) x.foo end What type is "x"? class Foo def foo; ...; end end class Bar def foo; ...; end end class Baz def foo; ...; end end ...

Difference from traditional type system •Solutions 1. Write type annotation → Avoid this 2. Infer type based on its usage → Too strict or too conservative • foo must be unique • or, structural type inference? 3. Use "inter-procedural" analysis • Pros: More powerful analysis • Cons: Slow and hard to control (Challenging) 9 def f(x: Foo) x.foo end def f(x) x.foo end f(Foo.new) "x" is a Foo! def f(x) x.foo end "x" is an object that responds to foo that accepts no argument and returns the same type of a return value of this method

There are many, many topics (but omit) Theoretical issues • Recursion and closures[RubyKaigi 2019] • Type-changing variable assignment • Container types and destructive operations[Osaka Ruby Kaigi 2019] • Flow-sensitive analysis[EuRuKo 2019] • Context-insensitive analysis[PPL 2019] • Aid of escape analysis • Cumbersome "untyped" type • Meta-programming features • etc, etc. Practical issues • Trade-off between precision and performance [Nagoya Ruby Kaigi 2019] • Tuple-like and sequential array • Method-local container type [Osaka Ruby Kaigi 2019] • Diagnosis features[Ruby 3 Summit] • Unreachable method analysis • Limitation of byte code • Super-rich Ruby features • Too complex Ruby features • etc, etc. 10

Agenda •Type Profiler: Type Inference for Ruby 3 ➔Demo • Simple case • Real-world program case • Library case •How to use • Future Plan 11

Demo 1: ao.rb •Simple case • A 3D renderer (~300 LoC) • Written by Hideki Miura • Original version was written by Syoyo Fujita https://code.google.com/archive/p/aobench/ •Analysis time < 1 sec. 12

Demo 1: ao.rb 13 class Vec attr_accessor x : Float attr_accessor y : Float attr_accessor z : Float def initialize : (Float, Float, Float) -> Float def vadd : (Vec) -> Vec def vsub : (Vec) -> Vec def vcross : (Vec) -> Vec def vdot : (Vec) -> Float def vlength : -> Float def vnormalize : -> Vec end NEW! attr_accessor Formerly, "def x=" and "def x"

Demo 1: ao.rb 14 class Sphere attr_reader center : Vec attr_reader radius : Float def initialize : (Vec, Float) -> Float def intersect : (Ray, Isect) -> Vec? end class Isect attr_accessor t : Float attr_accessor hit : bool attr_accessor pl : Vec attr_accessor n : Vec def initialize : -> Vec end NEW! optional type Formerly, "Vec | NilClass" NEW! bool type Formerly, "TrueClass | FalseClass"

Demo 2: Goodcheck • Real-world program case • A customizable linter for Ruby (~2000 LoC) • It has "hand-written" RBS • Analysis time < 30 sec. • Note: It requires many libraries which have no RBS • activesupport, concurrent-ruby, cgi, optparse, etc. • Type Profiler analyzed not only Goodcheck but also them • In future, we expect they have own RBS • Type Profiler can use RBS instead of the code itself 15

Manually reformatted to make comparison easy Demo 2: Goodcheck 16 class Goodcheck::Trigger attr_reader patterns : Array[(Goodcheck::Pattern::Literal | Goodcheck::Pattern::Regexp | Goodcheck::Pattern::Token)?] attr_reader globs : Array[Goodcheck::Glob?] attr_reader passes : Array[Array[untyped]] attr_reader fails : Array[Array[untyped]] attr_reader negated : bool ... end class Goodcheck::Trigger attr_reader patterns : Array[pattern] attr_reader globs: Array[Glob] attr_reader passes: Array[String] attr_reader fails: Array[String] attr_reader negated: bool ... end type Goodcheck::pattern = Pattern::Literal|Pattern::Regexp|Pattern::Token RBS inferred by Type Profiler Hand-written by Soutaro Wrong guess Type alias Not so bad? Extra optional Redundant namescope

Demo 2: Goodcheck 17 class Goodcheck::Pattern::Token attr_reader source : untyped attr_reader case_sensitive : true attr_reader variables : {} def initialize : (source: untyped, variables: {}, case_sensitive: true) -> true @regexp : Regexp def regexp : -> Regexp def test_variables : (untyped) -> bool def self.expand : (untyped, untyped, ?depth: Integer) -> Array[Regexp] def self.regexp_for_type : (name: untyped, type: :__, scanner: untyped) -> Regexp? def self.compile_tokens : (untyped, {}, case_sensitive: true) -> Regexp @@TYPES : {} end class Goodcheck::Pattern::Token attr_reader source: String attr_reader case_sensitive: bool attr_reader variables: Hash[Symbol, VarPattern] def initialize: (source: String, variables: Hash[Symbol, VarPattern], case_sensitive: bool) -> void def regexp: -> ::Regexp def self.expand: (String, String, ?depth: Integer) -> Array[::Regexp] def self.regexp_for_type: (name: Symbol, type: Symbol, scanner: StringScanner) -> ::Regexp def self.compile_tokens: (String source, Hash[Symbol, VarPattern] variables, case_sensitive: bool) -> void @@TYPES: Hash[Symbol, ^(String) -> ::Regexp] end RBS inferred by Type Profiler Hand-written by Soutaro Too specfic Failed to track the elements C lib constant (Lack of RBS) void is intended I think not so bad

Demo 3: diff-lcs •Real-world library case Famous algorithmic library •Hand-written entry point •Analysis time < 1 sec. 18 https://bestgems.org/ require_relative "lib/diff/lcs" class T; end Diff::LCS.diff([T.new]+[T.new], [T.new]+[T.new]) {}

Demo: diff-lcs 19 class Diff::LCS::Change include Comparable attr_reader action : String attr_reader position : Integer attr_reader element : (Array[T] | T)? def self.valid_action? : (String) -> untyped def initialize : (String, Integer, (Array[T] | T)?) -> nil def inspect : -> String ... def == : (untyped) -> bool def <=> : (untyped) -> Integer? def adding? : -> bool def deleting? : -> bool def unchanged? : -> bool def changed? : -> bool def finished_a? : -> bool def finished_b? : -> bool end predicate methods

Agenda •Type Profiler: Type Inference for Ruby 3 •Demo ➔ How to use Type Profiler • Planned experience • Specific usage •Future plan 20

Typical usage and experience (Plan) 1. Write an entry point program (if needed) 2. Apply Type Profiler 3. Partially write RBS for wrong-guessed methods 4. Re-apply Type Profiler 21 lib.rb app.rb Type Profiler lib.rbs (may include wrong guesses) partial RBS for difficult methods lib.rbs (final) ① ② ③ ④ "Partial RBS specification" has been implemented

How to use TP specifically Will be written until the RubyKaigi Takeout! (hopefully) https://github.com/mame/ruby-type-profiler 22 I have written the document

Agenda •Type Profiler: Type Inference for Ruby 3 •Demo •How to use Type Profiler ➔Future plan • Recent updates • Future plan • Conclusion 23

Recent updates • Improve cosmetics (attr_*, optional, bool, …) • Import Array and Hash methods from RBS • Type variable • Support Enumerable, Enumerator, and Struct • Support global variables • Improve flow-sensitive analysis • Improve analysis performance • Fix many, many bugs 24

Future plan • Until Ruby 3: Make it possible for plain Ruby code • Support partial RBS specification • Write document and release a gem • Continue to experiment, improve, etc, etc. • After the release: Support Sinatra app, Rails app… • Maybe need dedicated hard-coding for the frameworks • Concern is almost a language extension • ActiveRecord is super-meta feature • Please go easy on 🙏 25

Acknowledgment •Hideki Miura •Matz, Akr, Ko1 • Soutaro Matsumoto • Katsuhiro Ueno •Eijiro Sumii •Sorbet developers (Stripe, Shopify, etc.) •Jeff Foster 26

Conclusion •Type Profiler is crawling to you • Inviting suggestions for the tool name! • Ask me anything: @mametter (Twitter) 27