Principal Type Schemes for Gradual Programs With updates and corrections since publication (Latest: May 16, 2017) Ronald Garcia ∗ Matteo Cimini y Software Practices Lab Indiana University Department of Computer Science
[email protected] University of British Columbia
[email protected] Abstract to Siek and Taha (2006) has been used to integrate dynamic checks Gradual typing is a discipline for integrating dynamic checking into into a variety of type structures, including object-oriented (Siek a static type system. Since its introduction in functional languages, and Taha 2007), substructural (Wolff et al. 2011), and ownership it has been adapted to a variety of type systems, including object- types (Sergey and Clarke 2012). The key technical pillars of grad- oriented, security, and substructural. This work studies its applica- ual typing are the unknown type, ?, the consistency relation among tion to implicitly typed languages based on type inference. Siek gradual types, and support for the entire spectrum between purely and Vachharajani designed a gradual type inference system and dynamic and purely static checking. A gradual type checker rejects algorithm that infers gradual types but still rejects ill-typed static type inconsistencies in programs, accepts statically safe code, and programs. However, the type system requires local reasoning about instruments code that could plausibly be safe with runtime checks. type substitutions, an imperative inference algorithm, and a subtle This paper applies the gradual typing approach to implicitly correctness statement. typed languages, like Standard ML, OCaml, and Haskell, where This paper introduces a new approach to gradual type inference, a type inference (a.k.a. type reconstruction) procedure is integral to type checking.