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ICFP 2016
Sun 18 - Sat 24 September 2016 Nara, Japan
Wed 21 Sep 2016 16:50 - 17:15 at Noh Theater - Session 12 Chair(s): Jeremy Gibbons

Effects and coeffects are two general, complementary aspects of program behaviour. They roughly correspond to computations which change the execution context (effects) versus computations which make demands on the context (coeffects). Effectful features include partiality, non-determinism, input-output, state, and exceptions. Coeffectful features include resource demands, variable access, notions of linearity, and data input requirements.
The effectful or coeffectful behaviour of a program can be captured and described via type-based analyses, with fine grained information provided by monoidal effect annotations and semiring coeffects. Various recent work has proposed models for such typed calculi in terms of graded (strong) monads for effects and graded (monoidal) comonads for coeffects.
Effects and coeffects have been studied separately so far, but in practice many computations are both effectful and coeffectful, e.g., possibly throwing exceptions but with resource requirements. To remedy this, we introduce a new general calculus with a combined effect-coeffect system. This can describe both the changes and requirements that a program has on its context, as well as interactions between these effectful and coeffectful features of computation. The effect-coeffect system has a denotational model in terms of effect-graded monads and coeffect-graded comonads where interaction is expressed via the novel concept of graded distributive laws. This graded semantics unifies the syntactic type theory with the denotational model. We show that our calculus can be instantiated to describe in a natural way various different kinds of interaction between a program and its evaluation context.

Wed 21 Sep
Times are displayed in time zone: Osaka, Sapporo, Tokyo change

16:50 - 17:40: Session 12Research Papers at Noh Theater
Chair(s): Jeremy GibbonsUniversity of Oxford, UK
16:50 - 17:15
Combining Effects and Coeffects via Grading
Research Papers
Marco GaboardiSUNY Buffalo, USA, Shin-ya KatsumataKyoto University, Japan, Dominic OrchardUniversity of Cambridge, UK, Flavien BreuvartInria, France, Tarmo UustaluTallinn University of Technology, Estonia
17:15 - 17:40
String Diagrams for Free Monads (Functional Pearl)
Research Papers
Maciej PirĂ³gKU Leuven, Belgium, Nicolas WuUniversity of Bristol, UK