Danel Ahman

Dependent Types and Fibred Computational Effects

We study the interplay between dependent types and general computational effects. We define a language with both value types and terms, and computation types and terms, where types depend only on value terms. We use computational \(\varSigma \)-types to account for type-dependency in the sequential composition of computations. Our language design is justified by a natural class of categorical models. We account for both algebraic and non-algebraic effects. We also show how to extend the language with general recursion, using continuous families of cpos.

Normalization by Evaluation and Algebraic Effects

We examine the interplay between computational effects and higher types. We do this by presenting a normalization by evaluation algorithm for a language with function types as well as computational effects. We use algebraic theories to treat the computational effects in the normalization algorithm in a modular way. Our algorithm is presented in terms of an interpretation in a category of presheaves equipped with partial equivalence relations. The normalization algorithm and its correctness proofs are formalized in dependent type theory (Agda).

When is a container a comonad

Abbott, Altenkirch, Ghani and others have taught us that many parameterized datatypes (set functors) can be usefully analyzed via container representations in terms of a set of shapes and a set of positions in each shape. This paper builds on the observation that datatypes often carry additional structure that containers alone do not account for. We introduce directed containers to capture the common situation where every position in a datastructure determines another datastructure, informally, the sub-datastructure rooted by that position. Some natural examples are non-empty lists and node-labelled trees, and datastructures with a designated position (zippers). While containers denote set functors via a fully-faithful functor, directed containers interpret fully-faithfully into comonads. But more is true: every comonad whose underlying functor is a container is represented by a directed container. In fact, directed containers are the same as containers that are comonads. We also describe some constructions of directed containers. We have formalized our development in the dependently typed programming language Agda.

Update Monads: Cointerpreting Directed Containers

We introduce update monads as a generalization of state monads. Update monads are the compatible compositions of reader and writer monads given by a set and a monoid. Distributive laws between such monads are given by actions of the monoid on the set. We also discuss a dependently typed generalization of update monads. Unlike simple update monads, they cannot be factored into a reader and writer monad, but rather into similarly looking relative monads. Dependently typed update monads arise from cointerpreting directed containers, by which we mean an extension of an interpretation of the opposite of the category of containers into the category of set functors.

Handling fibred algebraic effects

We study algebraic computational effects and their handlers in the dependently typed setting. We describe computational effects using a generalisation of Plotkin and Pretnars effect theories, whose dependently typed operations allow us to capture precise notions of computation, e.g., state with location-dependent store types and dependently typed update monads. Our treatment of handlers is based on an observation that their conventional term-level definition leads to unsound program equivalences being derivable in languages that include a notion of homomorphism. We solve this problem by giving handlers a novel type-based treatment via a new computation type, the user-defined algebra type, which pairs a value type (the carrier) with a set of value terms (the operations), capturing Plotkin and Pretnars insight that effect handlers denote algebras. We then show that the conventional presentation of handlers can be routinely derived, and demonstrate that this type-based treatment of handlers provides a useful mechanism for reasoning about effectful computations. We also equip the resulting language with a sound denotational semantics based on families fibrations.

Directed Containers as Categories.

Directed containers make explicit the additional structure of those containers whose set functor interpretation carries a comonad structure. The data and laws of a directed container resemble those of a monoid, while the data and laws of a directed container morphism those of a monoid morphism in the reverse direction. With some reorganization, a directed container is the same as a small category, but a directed container morphism is opcleavage-like. We draw some conclusions for comonads from this observation, considering in particular basic constructions and concepts like the opposite category and a groupoid.

Recalling a witness: foundations and applications of monotonic state

We provide a way to ease the verification of programs whose state evolves monotonically. The main idea is that a property witnessed in a prior state can be soundly recalled in the current state, provided (1) state evolves according to a given preorder, and (2) the property is preserved by this preorder. In many scenarios, such monotonic reasoning yields concise modular proofs, saving the need for explicit program invariants. We distill our approach into the monotonic-state monad, a general yet compact interface for Hoare-style reasoning about monotonic state in a dependently typed language. We prove the soundness of the monotonic-state monad and use it as a unified foundation for reasoning about monotonic state in the F⋆ verification system. Based on this foundation, we build libraries for various mutable data structures like monotonic references and apply these libraries at scale to the verification of several distributed applications.

Constraint-Based Heuristic On-line Test Generation from Non-deterministic I/O EFSMs

We are investigating on-line model-based test generation from non-deterministic output-observable Input/Output Extended Finite State Machine (I/O EFSM) models of Systems Under Test (SUTs). We propose a novel constraint-based heuristic approach (Heuristic Reactive Planning Tester (xRPT)) for on-line conformance testing non-deterministic SUTs. An indicative feature of xRPT is the capability of making reasonable decisions for achieving the test goals in the on-line testing process by using the results of off-line bounded static reachability analysis based on the SUT model and test goal specification. We present xRPT in detail and make performance comparison with other existing search strategies and approaches on examples with varying complexity.