Allowing Type Checking in Method Signatures

We are working to provide optional _static_ type checking for Ruby, but also open for optional _runtime_ type checking too. This idea is to implement a runtime type checking library which cooperates with RBS type definitions.

Brush up RBS and related tools for practical Rails apps

RBS is still a developing technology and its immediate applicability is somewhat unknown. In particular, it is an important task to verify and improve the practicality of RBS in Rails apps, where Ruby is typically used