Dependent pattern matching is an intuitive way to write programs and proofs in dependently typed languages. It is reminiscent of both pattern matching in functional languages and case analysis in on-paper mathematics. However, in general, it is incompatible with new type theories such as homotopy type theory (HoTT). As a consequence, proofs in such theories are typically harder to write and to understand. The source of this incompatibility is the reliance of dependent pattern matching on the so-called K axiom – also known as the uniqueness of identity proofs – which is inadmissible in HoTT. In this paper, we propose a new criterion for dependent pattern matching without K, and prove it correct by a translation to eliminators in the style of Goguen et al. (2006 Algebra, Meaning, and Computation). Our criterion is both less restrictive than existing proposals, and solves a previously undetected problem in the old criterion offered by Agda. It has been implemented in Agda and is the first to be supported by a formal proof. Thus, it brings the benefits of dependent pattern matching to contexts where we cannot assume K, such as HoTT.

Effect systems have the potential to help software developers, but their practical adoption has been very limited. We conjecture that this limited adoption is due in part to the difficulty of transitioning from a system where effects are implicit and unrestricted to a system with a static effect discipline, which must settle for conservative checking in order to be decidable. To address this hindrance, we develop a theory of gradual effect checking, which makes it possible to incrementally annotate and statically check effects, while still rejecting statically inconsistent programs. We extend the generic type-and-effect framework of Marino and Millstein with a notion of unknown effects, which turns out to be significantly more subtle than unknown types in traditional gradual typing. We appeal to abstract interpretation to develop and validate the concepts of gradual effect checking. We also demonstrate how an effect system formulated in the framework of Marino and Millstein can be automatically extended to support gradual checking. We use gradual effect checking to develop a fully gradual type-and-effect framework, which permits interaction between static and dynamic checking for both effects and types.

We present a new approach to automated reasoning about higher-order programs by endowing symbolic execution with a notion of higher-order, symbolic values. To validate our approach, we use it to develop and evaluate a system for verifying and refuting behavioral software contracts of components in a functional language, which we call soft contract verification. In doing so, we discover a mutually beneficial relation between behavioral contracts and higher-order symbolic execution. Contracts aid symbolic execution by providing a rich language of specifications serving as a basis of symbolic higher-order values; the theory of blame enables modular verification and leads to the theorem that verified components can't be blamed; and the run-time monitoring of contracts enables soft verification whereby verified and unverified components can safely interact. Conversely, symbolic execution aids contracts by providing compile-time verification and automated test case generation from counter-examples to verification. This relation between symbolic exuection and contracts engenders a virtuous cycle encouraging the gradual use of contracts.

Our approach is able to analyze first-class contracts, recursive data structures, unknown functions, and control-flow-sensitive refinements of values, which are all idiomatic in dynamic languages. It makes effective use of off-the-shelf solvers to decide problems without heavy encodings. Counterexample search is sound and relatively complete with respect to a first-order solver for base type values and counter-examples are reported as concrete values, including functions. Therefore, it can form the basis of automated verification and bug-finding tools for higher-order programs. The approach is competitive with a range of existing tools—including type systems, flow analyzers, and model checkers—on their own benchmarks. We have built a prototype to analyze programs written in Racket and report on its effectiveness in verifying and refuting contracts.

Homotopy type theory is an extension of Martin-Löf type theory, based on a correspondence with homotopy theory and higher category theory. In homotopy type theory, the propositional equality type is proof-relevant, and corresponds to paths in a space. This allows for a new class of datatypes, called higher inductive types, which are specified by constructors not only for points but also for paths. In this paper, we consider a programming application of higher inductive types. Version control systems such as Darcs are based on the notion of patches—syntactic representations of edits to a repository. We show how patch theory can be developed in homotopy type theory. Our formulation separates formal theories of patches from their interpretation as edits to repositories. A patch theory is presented as a higher inductive type. Models of a patch theory are given by maps out of that type, which, being functors, automatically preserve the structure of patches. Several standard tools of homotopy theory come into play, demonstrating the use of these methods in a practical programming context.

Current systems for safely manipulating values containing names only support simple binding structures for those names. As a result, few tools exist to safely manipulate code in those languages for which name problems are the most challenging. We address this problem with Romeo, a language that respects α-equivalence on its values, and which has access to a rich specification language for binding, inspired by attribute grammars. Our work has the complex-binding support of David Herman's λm, but is a full-fledged binding-safe language like Pure FreshML.

Generative type abstractions – present in Haskell, OCaml, and other languages – are useful concepts to help prevent programmer errors. They serve to create new types that are distinct at compile time but share a run-time representation with some base type. We present a new mechanism that allows for zero-cost conversions between generative type abstractions and their representations, even when such types are deeply nested. We prove type safety in the presence of these conversions and have implemented our work in GHC.

The 19th ACM SIGPLAN International Conference on Functional Programming (ICFP) took place on September 1–3, 2014 in Gothenburg, Sweden. After the conference, the programme committee, chaired by Manuel Chakravarty, selected several outstanding papers and invited their authors to submit to this special issue of JFP. We acted as editors for these submissions. This issue includes the six accepted papers, each of which provides substantial new material beyond the original conference version. The selected papers demonstrate both the quality and the breadth of the conference, with a strong emphasis on types and their applications, and ranging from compilation methods through contract verification to homotopy type theory.