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Many researchers have explored retrofitting static type systems to dynamic languages. This raises the question of how to add type annotations to code that was previously untyped. One obvious solution is type inference. However, in complex type systems, in particular those with structural types, type inference typically produces most general types that are large, hard to understand, and unnatural for programmers. To solve this problem, we introduce InferDL, a novel Ruby type inference system that infers sound and useful type annotations by incorporating heuristics that guess types. For example, we might heuristically guess that a parameter whose name ends in "count" is an integer. InferDL works by first running standard type inference and then applying heuristics to any positions for which standard type inference produces overly-general types. Heuristic guesses are added as constraints to the type inference problem to ensure they are consistent with the rest of the program and other heuristic guesses; inconsistent guesses are discarded. We formalized InferDL in a core type and constraint language. We implemented InferDL on top of RDL, an existing Ruby type checker. To evaluate InferDL, we applied it to four Ruby on Rails apps that had been previously type checked with RDL, and hence had type annotations. We found that, when using heuristics, InferDL inferred 22% more types that were as or more precise than the previous annotations, compared to standard type inference without heuristics. We also found one new type error. We further evaluated InferDL by applying it to six additional apps, finding five additional type errors. Thus, we believe InferDL represents a promising approach for inferring type annotations in dynamic languages. 

Many researchers have explored ways to bring static typing to dynamic languages. However, to date, such systems are not precise enough when types depend on values, which often arises when using certain Ruby libraries. For example, the type safety of a database query in Ruby on Rails depends on the table and column names used in the query. To address this issue, we introduce CompRDL, a type system for Ruby that allows library method type signatures to include type-level computations (or comp types for short). Combined with singleton types for table and column names, comp types let us give database query methods type signatures that compute a table’s schema to yield very precise type information. Comp types for hash, array, and string libraries can also increase precision and thereby reduce the need for type casts. We formalize CompRDL and prove its type system sound. Rather than type check the bodies of library methods with comp types—those methods may include native code or be complex—CompRDL inserts run-time checks to ensure library methods abide by their computed types. We evaluated CompRDL by writing annotations with type-level computations for several Ruby core libraries and database query APIs. We then used those annotations to type check two popular Ruby libraries and four Ruby on Rails web apps. We found the annotations were relatively compact and could successfully type check 132 methods across our subject programs. Moreover, the use of type-level computations allowed us to check more expressive properties, with fewer manually inserted casts, than was possible without type-level computations. In the process, we found two type errors and a documentation error that were confirmed by the developers. Thus, we believe CompRDL is an important step forward in bringing precise static type checking to dynamic languages.