Getting F-bounded polymorphism into shape

Abstract

We present a way to restrict recursive inheritance without sacrificing the benefits of F-bounded polymorphism. In particular, we distinguish two new concepts, materials and shapes, and demonstrate through a survey of 13.5 million lines of open-source genericJava code that these two concepts never actually overlap in practice. With this Material-Shape Separation, we prove that even naïve type-checking algorithms are sound and complete, some of which address problems that were unsolvable even under the existing proposals for restricting inheritance. We illustrate how the simplicity of our design reflects the design intuitions employed by programmers and potentially enables new features coming into demand for upcoming programming languages.

Java generics are turing complete

“…In most applications, partial correctness suffices, but not so in security critical applications (where users are malicious), nor in mission critical applications (where nontermination is costly). Since there cannot be a totally correct Java type checker, Theorem 1 strengthens the motivation behind research into alternative type systems, such as (Greenman et al 2014) and (Zhang et al 2015).…”

Sound gradual typing is nominally alive and well

“…In the presence of variant generics, subtyping is often undecidable, as Kennedy and Pierce [2007] and Grigore [2017] have shown. A promising approach is that of Greenman et al [2014], who identified a pattern in how generics are used in practice; a pattern that can be enforced by the language design to ensure that subtyping is decidable.…”

Java and scala's type systems are unsound: the existential crisis of null pointers

“…super when the wildcard has an implicit constraint, which Tate et al determined is extremely rare in practice [35]. Another solution is to only introduce implicit constraints between what Greenman et al call "materials" [13], which they have determined would be almost certainly be backwards compatible in practice. These constraints are not recursive, which would make it easier to determine if they introduce a new subtyping.…”