ICFP Workshops 2020
25th ACM SIGPLAN International Conference on Functional Programming (ICFP 2020)
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5th ACM SIGPLAN International Workshop on Type-Driven Development (TyDe 2020), August 23, 2020, Virtual Event, USA

TyDe 2020 – Proceedings

Contents - Abstracts - Authors

5th ACM SIGPLAN International Workshop on Type-Driven Development (TyDe 2020)

Frontmatter

Title Page


Welcome from the Chairs
Welcome to the 5th ACM SIGPLAN International Workshop on Type-Driven Development (TyDe'20), co-located with the International Conference on Functional Programming (ICFP 2020). The workshop aims to show how static type information may be used effectively in the development of computer programs.
TyDe is a merging of two previous workshops: the Workshop on Dependently Typed Programming and the Workshop on Generic Programming. These two research areas have a rich history and bridge both theory and practice. Novel techniques explored by both communities have gradually spread to more mainstream languages. This workshop aims to bring together leading researchers and practitioners in generic programming and dependently typed programming from around the world, and features contributions capturing the state of the art in these important areas.
The call for submissions sought both full papers (up to 12 pages, published in the ACM Digital Library) and extended abstracts (up to 2 pages, not formally published but posted on the workshop webpage). Each submission was evaluated by at least three members of the program committee: full papers for the novelty and significance of their results, and extended abstracts for relevance and interest to the TyDe community. We received 4 full papers and 7 extended abstracts, of which 2 and 7 respectively were accepted. The submission period overlapped substantially with the beginning of the COVID-19 pandemic, which likely contributed to a lower normal number of submissions compared to previous years. Nevertheless, the submissions that we received presented fascinating new ideas and results and we are pleased to be able to put together an excellent online program.

Papers

Strongly Bounded Termination with Applications to Security and Hardware Synthesis
Thomas Reynolds, William L. Harrison, Rohit Chadha, and Gerard Allwein
(University of Missouri, USA; Oak Ridge National Laboratory, USA; US Naval Research Laboratory, USA)
Termination checking is a classic static analysis, and, within this focus, there are type-based approaches that formalize termination analysis as type systems (i.e., so that all well-typed programs terminate). But there are situations where a stronger termination property (which we call strongly-bounded termination) must be determined and, accordingly, we explore this property via a variant of the simply-typed λ-calculus called the bounded-time λ-calculus (BTC). This paper presents the BTC and its semantics and metatheory through a Coq formalization. Important examples (e.g., hardware synthesis from functional languages and detection of covert timing channels) motivating strongly-bounded termination and BTC are described as well.

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Practical Dependent Type Checking using Twin Types
Víctor López Juan and Nils Anders Danielsson
(Chalmers University of Technology, Sweden; University of Gothenburg, Sweden)
People writing proofs or programs in dependently typed languages can omit some function arguments in order to decrease the code size and improve readability. Type checking such a program involves filling in each of these implicit arguments in a type-correct way. This is typically done using some form of unification.
One approach to unification, taken by Agda, involves sometimes starting to unify terms before their types are known to be equal: in some cases one can make progress on unifying the terms, and then use information gleaned in this way to unify the types. This flexibility allows Agda to solve implicit arguments that are not found by several other systems. However, Agda's implementation is buggy: sometimes the solutions chosen are ill-typed, which can cause the type checker to crash.
With Gundry and McBride's twin variable technique one can also start to unify terms before their types are known to be equal, and furthermore this technique is accompanied by correctness proofs. However, so far this technique has not been tested in practice as part of a full type checker.
We have reformulated Gundry and McBride's technique without twin variables, using only twin types, with the aim of making the technique easier to implement in existing type checkers (in particular Agda). We have also introduced a type-agnostic syntactic equality rule that seems to be useful in practice. The reformulated technique has been tested in a type checker for a tiny variant of Agda. This type checker handles at least one example that Coq, Idris, Lean and Matita cannot handle, and does so in time and space comparable to that used by Agda. This suggests that the reformulated technique is usable in practice.

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