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1. A transient semantics for Typed Racket

   https://doi.org/10.22152/programming-journal.org/2022/6/9  

   Greenman, Lazarek (January 2022, Programming)

   Mixed-typed languages enable programmers to link typed and untyped components in various ways. Some offer rich type systems to facilitate the smooth migration of untyped code to the typed world; others merely provide a convenient form of type Dynamic together with a conventional structural type system. Orthogonal to this dimension, Natural systems ensure the integrity of types with a sophisticated contract system, while Transient systems insert simple first-order checks at strategic places within typed code. Furthermore, each method of ensuring type integrity comes with its own blame-assignment strategy. Typed Racket has a rich migratory type system and enforces the types with a Natural semantics. Reticulated Python has a simple structural type system extended with Dynamic and enforces types with a Transient semantics. While Typed Racket satisfies the most stringent gradual-type soundness properties at a significant performance cost, Reticulated Python seems to limit the performance penalty to a tolerable degree and is nevertheless type sound. This comparison raises the question of whether Transient checking is applicable to and beneficial for a rich migratory type system. This paper reports on the surprising difficulties of adapting the Transient semantics of Reticulated Python to the rich migratory type system of Typed Racket. The resulting implementation, Shallow Typed Racket, is faster than the standard Deep Typed Racket but only when the Transient blame assignment strategy is disabled. For language designers, this report provides valuable hints on how to equip an existing compiler to support a Transient semantics. For theoreticians, the negative experience with Transient blame calls for a thorough investigation of this strategy. 

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2. Corpse reviver: sound and efficient gradual typing via contract verification

   https://doi.org/10.1145/3434334  

   Moy, Cameron; Nguyễn, Phúc_C; Tobin-Hochstadt, Sam; Van_Horn, David (January 2021, Proceedings of the ACM on Programming Languages)

   Gradually typed programming languages permit the incremental addition of static types to untyped programs. To remain sound, languages insert run-time checks at the boundaries between typed and untyped code. Unfortunately, performance studies have shown that the overhead of these checks can be disastrously high, calling into question the viability of sound gradual typing. In this paper, we show that by building on existing work on soft contract verification, we can reduce or eliminate this overhead. Our key insight is that while untyped code cannot be trusted by a gradual type system, there is no need to consider only the worst case when optimizing a gradually typed program. Instead, we statically analyze the untyped portions of a gradually typed program to prove that almost all of the dynamic checks implied by gradual type boundaries cannot fail, and can be eliminated at compile time. Our analysis is modular, and can be applied to any portion of a program. We evaluate this approach on a dozen existing gradually typed programs previously shown to have prohibitive performance overhead—with a median overhead of 2.5× and up to 80.6× in the worst case—and eliminate all overhead in most cases, suffering only 1.5× overhead in the worst case. 

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3. A Universal Session Type for Untyped Asynchronous Communication

   Balzer, Stephanie; Pfenning, Frank; Toninho, Bernardo (August 2018, Leibniz international proceedings in informatics)

   In the simply-typed lambda-calculus we can recover the full range of expressiveness of the untyped lambda-calculus solely by adding a single recursive type U = U -> U. In contrast, in the session-typed pi-calculus, recursion alone is insufficient to recover the untyped pi-calculus, primarily due to linearity: each channel just has two unique endpoints. In this paper, we show that shared channels with a corresponding sharing semantics (based on the language SILL_S developed in prior work) are enough to embed the untyped asynchronous pi-calculus via a universal shared session type U_S. We show that our encoding of the asynchronous pi-calculus satisfies operational correspondence and preserves observable actions (i.e., processes are weakly bisimilar to their encoding). Moreover, we clarify the expressiveness of SILL_S by developing an operationally correct encoding of SILL_S in the asynchronous pi-calculus. 

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4. A Formal Model of Checked C

   https://doi.org/10.1109/CSF54842.2022.9919657  

   Li, Liyi; Liu, Yiyun; Postol, Deena; Lampropoulos, Leonidas; Van Horn, David; Hicks, Michael (August 2022, 2022 IEEE 35th Computer Security Foundations Symposium (CSF))

   We present a formal model of Checked C, a dialect of C that aims to enforce spatial memory safety. Our model pays particular attention to the semantics of dynamically sized, potentially null-terminated arrays. We formalize this model in Coq, and prove that any spatial memory safety errors can be blamed on portions of the program labeled unchecked; this is a Checked C feature that supports incremental porting and backward compatibility. While our model's operational semantics uses annotated (“fat”) pointers to enforce spatial safety, we show that such annotations can be safely erased. Using PLT Redex we formalize an executable version of our model and a compilation procedure to an untyped C-like language, as well as use randomized testing to validate that generated code faithfully simulates the original. Finally, we develop a custom random generator for well-typed and almost-well-typed terms in our Redex model, and use it to search for inconsistencies between our model and the Clang Checked C implementation. We find these steps to be a useful way to co-develop a language (Checked C is still in development) and a core model of it. 

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5. How Profilers Can Help Navigate Type Migration

   https://doi.org/10.1145/3622817  

   Greenman, Ben; Felleisen, Matthias; Dimoulas, Christos (October 2023, Proceedings of the ACM on Programming Languages)

   Sound migratory typing envisions a safe and smooth refactoring of untyped code bases to typed ones. However, the cost of enforcing safety with run-time checks is often prohibitively high, thus performance regressions are a likely occurrence. Additional types can often recover performance, but choosing the right components to type is difficult because of the exponential size of the migratory typing lattice. In principal though, migration could be guided by off-the-shelf profiling tools. To examine this hypothesis, this paper follows the rational programmer method and reports on the results of an experiment on tens of thousands of performance-debugging scenarios via seventeen strategies for turning profiler output into an actionable next step. The most effective strategy is the use of deep types to eliminate the most costly boundaries between typed and untyped components; this strategy succeeds in more than 50% of scenarios if two performance degradations are tolerable along the way. 

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