Static type checking for Ruby is challenging because the language is dynamically typed. What exactly is the source of the difficulty? You might think of duck typing or defime_method? Totally! But not only them are the reason. In fact, only a few lines of typical and innocent looking Ruby code can make type checking much more complicated than you assume.

This talk is about the difficulties and type system extensions to make the type checking possible. Static type checking is not easy and requires unfamiliar type system features, but we can do that.