Ruby Programming with Type Checking Soutaro Matsumoto (@soutaro)

Steep • https://github.com/soutaro/steep • Gradual typing for Ruby • Type annotation as a comment • Type definitions by programmers • Local type inference • Structural subtyping (between object types)

Released 0.3.0, yesterday

Released 0.3.0, yesterday Automatic implements Type-case on case expression Branch specific type annotation Constant type declaration Instance variable type declaration Typing polymorphic methods Revised type inference algorithm nil type Casting to any type masgn support Scaffolding type definitions

Comparison with Sorbet Steep Sorbet Type Inference Local Type Inference Type System Generics, union type, any type, context sensitive typing Subtyping Structural Nominal Signature Special Syntax in Another File Ruby code Type Annotation Comment Ruby code Performance Slow Fast

Demo • Introductory example • Simple Ruby program with two classes

Type Checking Steps 1. Write signatures 2. Write Ruby program with annotation 3. Run type checker 4. Goto 1 (2)

Demo • Type checking existing Ruby program • 4 years old production Rails application

Strategy • Even though Rails uses a lot of metaprogramming, there is a concrete type representation eventually • Programmers can write down that application's state as type definitions

• 1 controller / 2 models • Minimal set of classes/methods Demo Result

• 1 controller / 2 models • Minimal set of classes/methods Demo Result

• 1 controller / 2 models • Minimal set of classes/methods Demo Result For Models

• 1 controller / 2 models • Minimal set of classes/methods Demo Result For Rails

• 1 controller / 2 models • Minimal set of classes/methods Demo Result For Other Libraries

Only 7 lines of annotations

Type Checking Difficulties • Missing library types • Type system expressiveness (params) • Abused metaprogramming (ActiveRecord)

Missing Library Types • 40% of the signatures I wrote was type of libraries • If you ship your gems with type definitions, we can use that without writing their signatures

Type System Expressiveness • Tuple types and record types can improve • Workaround: Use ActionArgs ⚡
 def show: (id: Integer, org_id: Integer) -> void def params: -> Hash def params: -> { id: String, org_id: String } Not Implemented Yet

Abused Metaprogramming • Steep itself does not understand Rails association methods (has_many) or even builtin ones (attr_reader) • Generate signatures by some tools with Rails knowledge • A plugin to generate signature from AR models? • Smarter scaffold command?

Recap • Try Ruby programming with type checking using Steep
 $ gem install steep • Give me a feedbacks!