BABEL 2001 final programme with abstracts

Firenze, Italy, 8th September 2001

9:00-10:30. Session 1

Invited Talk: Towards a Principled Multi-Language Infrastructure
Zhong Shao (Yale University)

Sun's Java architecture introduced a safe virtual machine (VM) in which an ensemble of software components developed independently could smoothly interoperate. The goal of Microsoft's Common Language Runtime (CLR) is to generalize this approach and allow components in many source languages to interoperate safely. CLR supports flexible interoperation by compiling various source languages into a common intermediate language and by using a unified type system. However, the type system in CLR (and Java VM) enforces only conventional type safety in an object-oriented system. Therefore, higher-level specifications (e.g., resource bounds, generalized access control, formal software protocols) cannot be enforced. Because conventional type systems are too inflexible for real applications, developers often bypass the type system, producing code that steps outside the managed part of the VM; such components cannot be verified.

At Yale we have been developing typed common intermediate languages (named FLINT) that can support safely not only the standard object-oriented model, but also higher-order generic (polymorphic) programming and Java-style reflection (introspection). Unlike CLR, our type system is independent of any particular programming model, yet it is capable of expressing all valid propositions and proofs in higher-order predicate logic (so it can be used to capture and verify advanced program properties). The rich type system of FLINT makes it possible to typecheck both compiler intermediate code and low level machine code; this allows typechecking to take place at any phase of compilation, even after optimizations and register allocation. It also leads to a smaller and more extensible VM because low-level native routines that would otherwise be in VM can now be verified and moved into a certified library. This talk describes our vision of the FLINT system, outline our approach to its design, and survey the technologies that can be brought to support its implementation.

A framework for interoperability
Kathleen Fisher (AT&T Labs, Research), Riccardo Pucella (Cornell University) and John Reppy (Lucent Technologies, Bell Labs)

Practical implementations of high-level languages must provide access to libraries and system services that have APIs specified in a low-level language (usually C). An important characteristic of such mechanisms is the foreign-interface policy that defines how to bridge the semantic gap between the high-level language and C. For example, IDL-based tools generate code to marshall data into and out of the high-level representation according to user annotations. The design space of foreign-interface policies is large and there are pros and cons to each approach. Rather than commit to a particular policy, we choose to focus on the problem of supporting a gamut of interoperability policies. In this paper, we describe a framework for language interoperability that is is expressive enough to support very efficient implementations of a wide range of different foreign-interface policies. We describe two tools that implement substantially different policies on top of our framework and present benchmarks that demonstrate their efficiency.

11:00-12:30. Session 2

Alice in the Land of Oz - An Interoperability-based Implementation of a Functional Language on Top of a Relational Language
Leif Kornstaedt (Universitšt des Saarlandes)

This paper reports practical experience in implementing Alice, an extension of Standard ML, on top of an existing implementation of Oz. This approach yields a high-quality implementation with little effort. The combination is an advanced programming system for both Oz and Alice, which offers more than either language on its own.

No-Longer-Foreign: Teaching an ML compiler to speak C "natively"
Matthias Blume (Lucent Technologies, Bell Labs)

We present a new foreign-function interface for SML/NJ. It is based on the idea of data- level interoperability--the ability of ML programs to inspect as well as manipulate C data structures directly. The core component of this work is an encoding of the complete C type system in ML types. The encoding makes extensive use of a ``folklore'' typing trick, taking advantage of ML's polymorphism, its type constructors, its abstraction mechanisms, and even functors. A small low-level component which deals with ``new'' C types (struct or union declarations) as well as program linkage is hidden from the programmer's eye by a simple program-generator tool that translates C declarations to corresponding ML glue code. As a competent ML programmer you would not even have to know C to be able to use this new facility!

ILX: Extending the .NET Common IL for Functional Language Interoperability
Don Syme (Microsoft Research, Cambridge)

This paper describes several extensions to the .NET Common Intermediary Language (CIL), each of which is designed to enable easier implementation of typed high-level programming languages on the .NET platform, and to promote closer integration and interoperability between these languages. In particular we aim for easier interoperability between components whose interfaces are expressed using function types, discriminated unions and parametric polymorphism, regardless of the languages in which these components are implemented. We show that it is possible to add these constructs to an existing, ``real world'' intermediary language and that this allows corresponding subsets of constructs to be compiled uniformly, which in turn will allow programmers to use these constructs seamlessly between different languages. In this paper we discuss the motivations for our extensions, which are together called Extended IL (ILX), and describe them via examples. In this setting, many of the traditional responsibilities of the backend of a compiler must be moved to ILX and the execution environment, in particular those related to representation choices and low-level optimizations. We have modified a Haskell compiler to generate this language, and have implemented an assembler that translates the extensions to regular or polymorphic CIL code.

14:00-15:30. Session 3

Compiling Mercury to the .NET Common Language Runtime
Tyson Dowd, Fergus Henderson (University of Melbourne) and Peter Ross (Mission Critical, Belgium)

The .NET Common Language Runtime (CLR) offers a new opportunity to experiment with multi-language interoperation, and provides a relatively rare chance to explore deep interoperation of a wide range of programming language paradigms. This article describes how the logic/functional programming language Mercury is compiled to the CLR. We describe the problems with generating code for the CLR and discuss the initial findings regarding the interoperation potential of this platform, and then suggest improvements to both systems that may give smoother interoperation.

Object-Oriented Style Overloading for Haskell
Mark Shields and Simon Peyton Jones (Microsoft Research, Cambridge)

Haskell has a sophisticated mechanism for overloading identifiers with multiple definitions at distinct types. Object-oriented programming has a similar notion of overloading for methods names. Unfortunately, it is not possible to encode object-oriented overloading directly using Haskell overloading. This deficiency becomes particularly tiresome when Haskell programs wish to call methods imported from an object-oriented library. We explore various encodings of object-oriented classes into Haskell, demonstrate precisely where Haskell's existing type class system is unsatisfactory, and propose two refinements. We proceed in three stages. Firstly, we discuss various ways of accommodating sub-typing; we conclude that a simple encoding using Haskell classes is better for our purpose than a more substantial language extension. Second, we introduce a new notion of closed class, and show how this enables improvement of constraints beyond what is possible in Haskell. Closed classes make it easy to encode the truely ad hoc overloading of object-oriented methods without the need for name mangling or gratuitous type annotations. Thirdly, we allow overlapping instances, and define what it means for one instance to be better than another. This mechanism will turn out to subsume the rather complex overloading resolution rules used by object-oriented languages to select the most-specific method at a call site. In the Appendix, we present type checking and inference rules, as well as details of constraint entailment and simplification. However, this workshop paper is somewhat exploratory: the design may shift once we gain experience with an implementation, and we have not devoted any time to showing any formal properties of our system.

Annotations for Portable Intermediate Languages
Fermin Reig (University of Glasgow)

This paper identifies high-level program properties that can be discovered by static analysis in a compiler front-end, and that are useful for classical low-level optimizations. We suggest suitable annotations to convey these properties to the code generator.

16:00-17:30. Session 4

Active Oberon for .NET: An Exercise in Object Model Mapping
Jurg Gutknecht (ETH Zurich)

Active Oberon is a substantial evolution of the programming language Oberon. It has been specially designed for component-oriented software development and in particular for language interoperability. In the context of a joint project with Microsoft Research, the new language has been implemented on top of the new .net platform. Conceptual highlights of Active Oberon are (a) a notion of active object types, (b) an abstraction concept called definition, (c) a module construct, and (d) a block statement. The first three concepts are in fact powerful combinations of concepts: Active objects integrate reactive methods with active behavior, definitions combine units of implementation with units of inheritance, and modules are both name space and singleton object. Block statements unify the grouping of statements that share common processing attributes. In essence, the notion of subclassing or type extension used by most object-oriented languages has been replaced in Active Oberon by the more general concept of definition in combination with a generic type OBJECT and two relations called IMPLEMENTS and REFINES respectively. This article is a report on a work in progress. We divide our presentation into three parts: (a) Short recall of the history of programming languages developed at ETH, (b) extensive conceptual overview of Active Oberon's object model called Active Object System (AOS), (c) discussion of the mapping of the AOS into the Virtual Object System (VOS) exposed by .NET.

Language-Agnostic Approaches to Mobile Code
Peter Housel, Christian Stork, Vivek Haldar, Niall Dalton and Michael Franz (University of California, Irvine)

The Java Virtual Machine is biased towards transporting Java programs. As a consequence, the less similar to Java the language one programs in, the less efficient the transport via JVM becomes. Microsoft's .NET transport format fares better in this respect because it has a more flexible type system and instruction set, but it is not extensible, and for example, has no provision for supporting explicit programmer-specified parallelism. Microsoft's format addresses performance issues by including a variant of SSA along with the bytecode, but these are not verified and hence require authentication-based trust. We have been exploring the design space for mobile-code technologies. This is a project with a time horizon of 5-10 years aiming to arrive at meta-level insights, but we are simultaneously also working on proof-of-concept implementations. One focus is on turning semantic properties of programming language into well-formedness properties of the mobile-code format itself, thereby removing the expensive verification process required with byte-code formats. A beneficial side effect of such encodings is compactness - illegal programs are not expressible and hence do not dilute the entropy. Our first results are encouraging: in spite of being fully generic, parametrized by an abstract grammar, applying our technique to Java yields an encoding that is consistently more compact than the best known dedicated compression scheme for Java.

Tail call elimination on the Java Virtual Machine
Michel Schinz and Martin Odersky (Ecole Polytechnique Fťdťrale de Lausanne)

A problem that often has to be solved by compilers for functional languages targeting the Java Virtual Machine is the elimination of tail calls. This paper explains how we solved it in our Funnel compiler and presents some experimental results about the impact our technique has on both performance and size of the compiled programs.