Combining Static and Dynamic Typing in Ruby

Abstract

Many popular scripting languages such as Ruby, Python, and Perl are dynamically typed. Dynamic typing provides many advantages such as terse, flexible code and the ability to use highly dynamic language constructs, such as an eval method that evaluates a string as program text. However these dynamic features have traditionally obstructed static analyses leaving the programmer without the benefits of static typing, including early error detection and the documentation provided by type annotations. In this dissertation, we present Diamondback Ruby (DRuby), a tool that blends static and dynamic typing for Ruby. DRuby provides a type language that is rich enough to precisely type Ruby code, without unneeded complexity. DRuby uses static type inference to automatically discover type errors in Ruby programs and provides a type annotation language that serves as verified documentation of a method's behavior. When necessary, these annotations can be checked dynamically using runtime contracts. This allows statically and dynamically checked code to safely coexist, and any runtime errors are properly blamed on dynamic code. To handle dynamic features such as eval, DRuby includes a novel dynamic analysis and transformation that gathers per-application profiles of dynamic feature usage via a program's test suite. Based on these profiles, DRuby transforms the program before applying its type inference algorithm, enforcing type safety for dynamic constructs. By leveraging a program's test suite, our technique gives the programmer an easy to understand trade-off: the more dynamic features covered by their tests, the more static checking is achieved. We evaluated DRuby on a benchmark suite of sample Ruby programs. We found that our profile-guided analysis and type inference algorithms worked well, discovering several previously unknown type errors. Furthermore, our results give us insight into what kind of Ruby code programmers ``want'' to write but is not easily amenable to traditional static typing. This dissertation shows that it is possible to effectively integrate static typing into Ruby without losing the feel of a dynamic language.