# In celebration of Philip Wadler's 60th birthday

## Accepted papers

Faris Abou-Saleh, James Cheney, Jeremy Gibbons, James McKinna and Perdita Stevens
Bidirectional transformations (bx) have primarily been modeled as pure functions, and do not account for the possibility of the side-effects that are available in most programming languages. Recently several formulations of bx that use monads to account for effects have been proposed, both among practitioners and in academic research. The combination of bx with effects turns out to be surprisingly subtle, leading to problems with some of these proposals and increasing the complexity of others. This paper reviews the proposals for monadic lenses to date, and offers some improved definitions, paying particular attention to the obstacles to naively adding monadic effects to existing definitions of pure bx such as lenses and symmetric lenses, and the subtleties of equivalence of symmetric bidirectional transformations in the presence of effects.
Robert Atkey, Sam Lindley and J. Garrett Morris
Conflation confers concurrency
Session types provide a static guarantee that concurrent programs respect communication protocols. Recent work has explored a correspondence between proof rules and cut reduction in linear logic and typing and evaluation of process calculi. This paper considers two approaches to extend logically-founded process calculi. First, we consider extensions of the process calculus to more closely resemble pi-calculus. Second, inspired by denotational models of process calculi, we consider conflating dual types. Most interestingly, we observe that these approaches coincide: conflating tensor and par allows processes to share multiple channels; conflating plus and with provides nondeterminism; and finally, conflating of-course and why-not yields access points, a rendezvous mechanism for initiating session-typed communication. Access points are particularly expressive: for example, they are sufficient to encode concurrent state and general recursion.
Nick Benton, Andrew Kennedy, Martin Hofmann and Vivek Nigam
Counting successes: effects and transformations for non-deterministic programs
We give a simple effect system for non-deterministic programs, tracking static approximations to the number of results that may be produced by each computation. A relational semantics for the effect system establishes the soundness of both the analysis and its use in effect-based program transformations.
Andrew Black, Kim Bruce and James Noble
The essence of inheritance
Programming languages serve a dual purpose: to communicate programs to computers, and to communicate programs to human beings. Indeed, it is this dual purpose that makes programming language design a constrained and challenging problem. The thesis of this paper is that inheritance is an essential aspect of that second purpose: it is a tool to improve communication. Our argument is that most humans understand new concepts most readily by first looking at a number of concrete examples, and later abstracting over those examples. Inheritance mirrors this process: it provides a formal mechanism for moving from the concrete to the abstract.
John T. O'Donnell and Cordelia Hall
Pointlessness is better than listlessness
Extensible sparse functional arrays (ESFA) is a persistent data structure with an implementation that performs every operation in O(1) time and space. There is no requirement for single threading of arrays, and no performance penalty for sharing. The implementation is an example of hardware/software co-design and also of active data structures. This makes the work interdisciplinary, and it builds on many ideas from functional programming. These include the definition of new array operations, observations about mutability and purity, monads, program transformations to remove unnecessary data structures, equational reasoning, and fixing space leaks in the garbage collector. This paper summarises a recently published exposition of the system, focusing on drawing connections with some of these foundational ideas. It also presents some new results on a simulation of ESFA on a GPU. A surprising result is that although the simulation has high overheads and limited parallelism, it can outperform a state of the art red black tree implementation when there is a large amount of sharing between arrays.
Hugh Leather and Janne Irgens
The lambda calculus: practice and principle
The Lambda Calculus has perplexed students of computer science for millennia, rendering many incapable of understanding even the most basic precepts of functional programming. This paper gently introduces the core concepts to the lay reader, assuming only a minimum of background knowledge in category theory, quantum chromodynamics, and paleomagnetism.

In addition, this paper goes on to its main results, showing how the Lambda Calculus can be used to easily prove the termination of Leibniz' Hailstone numbers for all $n > 0$, to show that matrix multiplication is possible in linear time, and to guarantee Scottish independence.
Simon Gay
Subtyping supports safe session substitution
Session types describe the structure of bi-directional point-to-point communication channels by specifying the sequence and format of messages on each channel. A session type defines a communication protocol. Type systems that include session types are able to statically verify that communication-based code generates, and responds to, messages according to a specified protocol. It is natural to consider subtyping for session types, but the literature contains conflicting definitions. It is now folklore that one definition is based on safe substitutability of channels, while the other is based on safe substitutability of processes. We explain this point in more detail, and show how to unify the two views.
Neil Ghani, Fredrik Nordvall Forsberg and Federico Orsanigo
Proof relevant parametricity
Parametricity is one of the foundational principles which underpin our understanding of modern programming languages. Roughly speaking, parametricity expresses the hidden invariants of programs satisfy by formalising the intuition that programs map related inputs to related outputs. Traditionally parametricity is formulated with proof-irrelevant relations but programming in Type Theory requires an extension to proof-relevant relations. But then one might ask … can our proofs be parametric? This paper shows how this can be done and, excitingly, our answer requires a trip into the world of higher dimensional parametricity.
Jeremy Gibbons
List comprehensions are a widely used programming construct, in languages such as Haskell and Python and in technologies such as Microsoft's Language Integrated Query. They generalize from lists to arbitrary monads, yielding a lightweight idiom of imperative programming in a pure functional language. When the monad has the additional structure of a so-called ringad, corresponding to "empty" and "union" operations, then it can be seen as some kind of collection type, and the comprehension notation can also be extended to incorporate aggregations. Ringad comprehensions represent a convenient notation for expressing database queries. The ringad structure alone does not provide a good explanation or an efficient implementation of relational joins; but by allowing heterogeneous comprehensions, involving both bag and indexed table ringads, we show how to accommodate these too.
Ralf Hinze and Dan Marsden
Dragging proofs out of pictures
String diagrams provide category theory with a new and very distinctive visual flavour. We demonstrate that they are an effective tool for equational reasoning using a variety of examples evolving around the composition of monads. A deductive approach is followed, discovering necessary concepts as we go along. In particular, we show that the Yang--Baxter equation arises naturally when composing three monads. We are also concerned with the pragmatics of string diagrams. Diagrams are carefully arranged to provide geometric intution for the proof steps being followed. We introduce thick wires to distinguish composite functors and suggest a two-dimensional analogue of parenthesis to partion diagrams.
John Hughes
Experiences with QuickCheck: testing the hard stuff and staying sane
QuickCheck, well known in the Haskell community, generates random tests from formal specifications, and simplifies failing tests to minimal counterexamples. Almost ten years ago, I founded Quviq AB to build a commercial tool based on the same ideas, and use it to help improve the quality of customers' testing. The need to test different kinds of customer software has forced a number of extensions, many based on state machine models. In this paper I explain how these models work using a simple example in C, and describe how they were used in a large project for the car industry, and to track down a race condition that was plaguing one of Sweden's fastest growing companies. These are war stories that both entertain and inform!
Graham Hutton and Patrick Bahr
Cutting out continuations.
In the field of program transformation, one often transforms programs into continuation-passing style to make their flow of control explicit, and then immediately removes the resulting continuations using defunctionalisation to make the programs first-order. In this article, we show how these two transformations can be fused together into a single transformation step that cuts out the need to first introduce and then eliminate continuations. Our approach is calculational, uses standard equational reasoning techniques, and is widely applicable.
Conor McBride
I got plenty o’ nuttin’
Work to date on combining linear types and dependent types has deliberately and successfully avoided doing so. Entirely fit for their own purposes, such systems wisely insist that types depend only on the replicable sublanguage, thus sidestepping the issue of counting uses of limited-use data either within types or in ways which are only really needed to shut the typechecker up. As a result, the linear implication (‘lollipop’) stubbornly remains a non-dependent S -o T. This paper defines and establishes the basic metatheory of a type theory supporting a ‘dependent lollipop’ (x:S) -o T[x], where what the input used to be is in some way commemorated by the type of the output. Usage is tracked with resource annotations belonging to an arbitrary rig, or ‘riNg without Negation’. The key insight is to use the rig’s zero to mark information in contexts which is present for purposes of contemplation rather than consumption, like a meal we remember fondly but cannot eat twice. We can have plenty of nothing with no additional runtime resource, and nothing is plenty for the construction of dependent types.
Martin Odersky, Nada Amin, Tiark Rompf, Sandro Stucki and Samuel Gruetter
The essence of dependent object types
Focusing on path dependent types, the paper develops foundations for Scala from first principles. Starting from a simple calculus D$_{<:}$ of dependent functions, it adds records, intersections and recursion to arrive at DOT, a calculus for dependent object types. The paper shows an encoding of System F with subtyping in D$_{<:}$ and demonstrates the expressiveness of DOT by modeling a range of Scala constructs in it.
Jennifer Paykin and Steve Zdancewic
Linear lambda-mu is CP (more or less)
In this paper we compare Wadler's CP calculus for classical linear processes to a linear version of Parigot's lambda-mu calculus for classical logic. We conclude that linear lambda-mu is more or less'' CP, in that it equationally corresponds to a polarized version of CP. The comparison is made by extending a technique from Mellies and Tabareau's tensor logic that correlates negation with polarization. The polarized CP, which is written CP+- and pronounced CP more or less,'' is an interesting bridge in the landscape of Curry-Howard interpretations of logic.
Simon Peyton Jones, Stephanie Weirich, Richard A. Eisenberg and Dimitrios Vytiniotis
A reflection on types
The ability to perform type tests at runtime blurs the line between statically-typed and dynamically-typed languages. Recent developments in Haskell's type system allow even programs that use reflection to themselves be statically typed, using a type-indexed runtime representation of types called TypeRep.
Tiark Rompf
The essence of multi-stage evaluation in LMS
Embedded domain-specific languages (DSLs) are the subject of wide-spread interest, and a variety of implementation techniques exist. Some of them have been invented, and some of them discovered. Many are based on a form of \emph{generative} or \emph{multi-stage} programming, where the host language program builds up DSL terms during its evaluation. In this paper, we examine the execution model of LMS (Lightweight Modular Staging), a framework for embedded DSLs in Scala, and link it to evaluation in a two-stage lambda calculus. This clarifies the semantics of certain ad-hoc implementation choices, and provides guidance for implementing similar multi-stage evaluation facilities in other languages.
Andreas Rossberg
1ML with special effects
We take another look at 1ML, a language in the ML tradition, but with core and modules merged into one unified language, rendering all modules first-class values. 1ML already comes with a simple form of effect system that distinguishes pure from impure computations. Now we enrich it with effect polymorphism: by introducing effect declarations and, more interestingly, abstract effect specifications, effects can be parameterised over, and treated as abstract or concrete in the type system, very much like types themselves. Because type generativity in 1ML is controlled by (im)purity effects, this yields a somewhat exotic novel notion of _generativity polymorphism_ -- that is, a given functor can be asked to behave as either generative'' or applicative''. And this time, we even get to define an interesting (poly)monad for that!
Manuel Serrano
The computer scientist nightmare
This text relates the story of particularly shocking bug that occurred during the development of a Web application. The bug is first described. The battle to understand and fix it is then presented. This experimental report concludes with some questionings about the way we conceive programming languages and programming environments.
Avraham Shinnar and Jerome Simeon
A branding strategy for business types
In the course of building a compiler from business rules to a database run-time, we encounter the need for a type system that includes a class hierarchy and subtyping in the presence of complex record operations. Since our starting point is based on structural typing and targets a data-centric language, we develop an approach inspired by Wadler's work on XML types. Our proposed type system has strong similarities with brand objects, combining nominal and structural typing, and is designed to support a rich set of operations on records commonly found in query languages. We show soundness of the type system and illustrate its use on two of the languages involved in our compiler for business rules: a calculus for pattern matching with aggregation (CAMP) that captures rules semantics, and the nested relational algebra (NRA) used for optimization and code generation. We show type soundness for both languages. The approach and correctness proofs are fully mechanized using the Coq proof-assistant.
The recursive union of some gradual types
We study union types and recursive types in the setting of a gradually typed lambda calculus. Our goal is to obtain a foundational account for languages that enable recursively defined data structures to be passed between static and dynamically typed regions of a program. We discuss how traditional sum types are not appropriate for this purpose and instead study a form of true union in the tradition of soft typing [Cartwright and Fagan, 1991] and occurrence typing [Tobin-Hochstadt and Felleisen, 2008]. Regarding recursive types, our formulation is based on the axiomatization of subtyping by Brand and Henglein (1998).

This paper defines three artifacts. First, in the context of the simply typed lambda calculus, we define the semantics of our unions and integrate them with equi-recursive types. Second, we add a dynamic type to obtain a gradually typed lambda calculus. The semantics of this calculus is defined by translation to the third artifact, a blame calculus [Wadler and Findler, 2009] extended with unions and equi-recursive types.
Bernardo Toninho and Nobuko Yoshida
Certifying data in multiparty session types
Multiparty session types (MPST) are a typing discipline for ensuring the coordination and orchestration of multi-agent communication in concurrent and distributed programs. However, by mostly focusing on the communication aspects of concurrency, MPST are often unable to capture important data invariants in programs. In this work we propose to increase the expressiveness of MPST by considering a notion of value dependencies in order to certify invariants of exchanged data in concurrent and distributed settings.
Peter Thiemann
A delta for hybrid type checking
A hybrid type checker defers parts of the type checking to run time. The static part of the checker performs a best effort at verifying subtyping constraints. Constraints that cannot be proved by the static checker, are reified as run-time casts in an intermediate blame calculus.

The goal of this work is to simplify these casts by exploiting context information. To this end, we develop a coercion calculus that corresponds to the blame calculus via a pair of translations and we define the formal framework to simplify these coercions. We give a concrete instance of the calculus and demonstrate that simplification can be regarded as a synthesis problem.
David Turner
Recursion equations as a programming language
This paper was written in 1981 and published in “Functional Programming and its Applications”, Cambridge University Press, January 1982, editors Darlington, Henderson and Turner. The volume comprised the lecture notes from a summer school on Functional Programming held at Newcastle University in July 1981, attended by 80 people. The paper includes an overview of Kent Recursive Calculator, a simple functional programming system based on higher order recursion equations and a series of programming examples. It is probably the earliest paper using list comprehensions applied to lazy lists, and has the first published account of the “list of successes” method of eliminating backtracking, here applied to the eight queens problem. The method didn’t yet have a name. It was Phil Wadler who saw its importance and coined the phrase “list of successes” in his 1985 paper. It was also Phil who invented the term “list comprehensions” for what are here called “ZF expressions”.