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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.
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Sound, heuristic type annotation inference for Ruby
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.
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- Award ID(s):
- 1918233
- PAR ID:
- 10223731
- Date Published:
- Journal Name:
- Dynamic Languages Symposium
- Page Range / eLocation ID:
- 112 to 125
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Chung-Kil Hur (Ed.)Dynamic object-oriented languages, such as Python, Ruby, and Javascript are widely used nowadays. A distinguishing feature of dynamic object-oriented languages is that objects, the fundamental runtime data representation, are highly dynamic, meaning that a single constructor may create objects with different types and objects can evolve freely after their construction. While such dynamism facilitates fast prototyping, it brings many challenges to program understanding. Many type systems have been developed to aid programming understanding, and they adopt various types and techniques to represent and track dynamic objects. However, although many types and techniques have been proposed, it is unclear which one suits real dynamic object usages best. Motivated by this situation, we perform an empirical study on 50 mature Python programs with a focus on object dynamism and object type models. We found that (1) object dynamism is highly prevalent in Python programs, (2) class-based types are not precise to handle dynamic behaviors, as they introduce type errors for 52% of the evaluated polymorphic attributes, (3) typestate-based types, although mostly used in static languages, matches the behaviors of dynamic objects faithfully, and (4) some well-designed but still lightweight techniques for objectbased types, such as argument type separation and recency abstraction can precisely characterize dynamic object behaviors. Those techniques are suitable for building precise but still concise object-based types.more » « less
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Abstract Gradual typing allows programs to enjoy the benefits of both static typing and dynamic typing. While it is often desirable to migrate a program from more dynamically typed to more statically typed or vice versa, gradual typing itself does not provide a way to facilitate this migration. This places the burden on programmers who have to manually add or remove type annotations. Besides the general challenge of adding type annotations to dynamically typed code, there are subtle interactions between these annotations in gradually typed code that exacerbate the situation. For example, to migrate a program to be as static as possible, in general, all possible combinations of adding or removing type annotations from parameters must be tried out and compared. In this paper, we address this problem by developing migrational typing , which efficiently types all possible ways of replacing dynamic types with fully static types for a gradually typed program. The typing result supports automatically migrating a program to be as static as possible or introducing the least number of dynamic types necessary to remove a type error. The approach can be extended to support user-defined criteria about which annotations to modify. We have implemented migrational typing and evaluated it on large programs. The results show that migrational typing scales linearly with the size of the program and takes only 2–4 times longer than plain gradual typing.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1145/3426422.3426985
