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2nd Hessian Workshop on Programming Languages

Workshop program

10:30 Arrival

11:00 Welcome

11:10 Session I (Session Chair: Paolo Giarrusso)


Rita Loogen

Skeleton Composition in Eden

Eden is a parallel functional programming language which extends Haskell with constructs for the definition and instantiation of parallel processes. Processes evaluate function applications remotely in parallel. The programmer has control over process granularity, data distribution, communication topology, and evaluation site, but need not manage synchronisation and data exchange between processes. The latter are performed by the parallel runtime system through implicit communication channels, transparent to the programmer. Common and sophisticated parallel communication patterns and topologies, so-called algorithmic skeletons, are provided as higher-order functions in a user-extensible skeleton library written in Eden. In this talk we will take a look at the composability of said skeletons.


Oliver Bracevac

The Cloud Calculus

Over the last few years, cloud computing has emerged as the reference model for on-demand, fault-tolerant and scalable computations. Despite the popularity of this model, little research has been devoted to the investigation of theoretical foundations. We propose to fill this gap with a calculus that captures the essential aspects of cloud computing at the proper level of abstraction. Our minimal cloud calculus is suitable for reasoning about cloud applications, can be easily extended to capture more semantic details and is accompanied by a simple yet elegant metatheory.

12:00 Coffee Break

12:15 Session II (Session Chair: Sebastian Erdweg)


Yufei Cai

Nominal Functors

we present Nominal Functors, a concise, expressive DSL for traversable functors in Scala. Users write the type constructor of the functor, the system generates the corresponding fmap method. Every n-constructor datatype has at least 2^n traversable functors associated with it; the DSL grants the user easy access to all of them. This allows us to find a unifying connection between generic programming techniques such as scrap-your-boilerplate, almost-compositional-functions and biplate: Each technique essentially creates a different traversable functor from a given datatype.


Pascal Wittmann

A Language for the Specification and Efficient Implementation of Type Systems

Type systems are important tools to detect semantic inconsistencies, to establish abstractions and to guide the programmer in the development process. However, there is currently a lack of established tools supporting the development of type systems. Tools like lexer and parser generators. We introduce a declarative specification language for type systems, that allows to specify type systems in a natural deductive style. We generate two products from a specification: A first-order formula representation to facilitate the use of automated theorem provers and an efficient type checker. Both results aim to make the development cycle for type systems faster and to narrow the gap between theory and practice.

13:05 Lunch

14:05 Incremental and reactive computation (Session Chair: Klaus Ostermann)


Joscha Drechsler

Distributed REScala: An Update Propagation Algorithm for Distributed Reactive Programming

Reactive programming improves the design of reactive applications by relocating the logic for managing dependencies between dependent values away from the application logic to the language implementation. Many distributed applications are reactive. Yet, existing change propagation algorithms are not suitable in a distributed setting.
We propose Distributed REScala, a reactive language with a change propagation algorithm that works without centralized knowledge about the topology of the dependency structure among reactive values and avoids unnecessary propagation of changes, while retaining safety guarantees (glitch freedom). Distributed REScala enables distributed reactive programming, bringing the benefits of reactive programming to distributed applications. We demonstrate the enabled design improvements by a case study. We also empirically evaluate the performance of our algorithm in comparison to other algorithms in a simulated distributed setting.


Mirko Köhler

i3QL: Language-Integrated Live Data Views

An incremental computation updates its result based on a change to its input, which is often an order of magnitude faster than a recomputation from scratch. In particular, incrementalization can make expensive computations feasible for settings that require short feedback cycles, such as interactive systems, IDEs, or (soft) real-time systems.
We present i3QL, a general-purpose programming language for specifying incremental computations. i3QL provides a declarative SQL-like syntax and is based on incremental versions of operators from relational algebra, enriched with support for general recursion. We integrated i3QL into Scala as a library, which enables programmers to use regular Scala code for non-incremental subcomputations of an i3QL query and to easily integrate incremental computations into larger software projects. To improve performance, i3QL optimizes user-defined queries by applying algebraic laws and partial evaluation. We describe the design and implementation of i3QL and its optimizations, demonstrate its applicability, and evaluate its performance.


Lightning talk

15:20 Coffee Break

15:50 Session IV (Session Chair: Tillmann Rendel)


Edlira Kuci

Abstract Dependent Classes

Software reuse provides means to combine existing components to implement new functionalities. Dependent Classes are a powerful solution that combines a lot of desirable features, including dynamic composition, feature selection and static type checking.
However, several research questions are still open. An important issue is that many interesting applications of Dependent Classes involve the use of abstract classes. However the calculus of Dependent Classes does not support them. Type checking of abstract declarations for dependent classes is challenging, because it requires completeness and uniqueness checks for classes dispatched by dependent types. We envisage that the existing calculus of Dependent Classes can support abstract classes, show the changes that are necessary and provide a roadmap for further research in the field.


Jonathan Brachthäuser

Typesafe Extensible Functional Objects

Statically typed, class based languages usually do not allow adding new interface implementations after an object has been created. A famous solution to this problem is the decorator pattern. However, multiple decorators that add new methods cannot be composed and also late binding is not supported by default. To solve this problem, we encode objects as extensible terminal coalgebras: this allows both typesafe static composition as well as dynamic extension of already constructed objects. Additional extensions like support for private state, super-references and selective open recursion can easily be implemented on top of the core encoding.


Sebastian Erdweg

Capture-Avoiding and Hygienic Program Transformations

Program transformations in terms of abstract syntax trees compromise referential integrity by introducing variable capture. Variable capture occurs when in the generated program a variable declaration accidentally shadows the intended target of a variable reference. Existing transformation systems either do not guarantee the avoidance of variable capture or impair the implementation of transformations.
We present an algorithm called name-fix that automatically eliminates variable capture from a generated program by systematically renaming variables. name-fix is guided by a graph representation of the binding structure of a program, and requires name-resolution algorithms for the source language and the target language of a transformation. name-fix is generic and works for arbitrary transformations in any transformation system that supports origin tracking for names. We verify the correctness of name-fix and identify an interesting class of transformations for which name-fix provides hygiene. We demonstrate the applicability of name-fix for implementing capture-avoiding substitution, inlining, lambda lifting, and compilers for two domain-specific languages.

17.05 Closing

18.00 Dinner