Section: New Results

Constructs for Dynamic Languages

Participants : Stéphane Ducasse, Marcus Denker, Veronica Uquillas-Gomez, Gwenael Casaccio, Camillo Bruni, Jean-Baptiste Arnaud, Damien Pollet.

To support our research on secure dynamic languages, we focused on improving language infrastructure.

Efficient Proxies in Smalltalk. A proxy object is a surrogate or placeholder that controls access to another target object. Proxy objects are a widely used solution for different scenarios such as remote method invocation, future objects, behavioral reflection, object databases, inter-languages communications and bindings, access control, lazy or parallel evaluation, security, among others. Most proxy implementations support proxies for regular objects but they are unable to create proxies for classes or methods. Proxies can be complex to install, have a significant overhead, be limited to certain type of classes, etc. Moreover, most proxy implementations are not stratified at all and there is no separation between proxies and handlers. We present Ghost, a uniform, light-weight and stratified general purpose proxy model and its Smalltalk implementation. Ghost supports proxies for classes or methods. When a proxy takes the place of a class it intercepts both, messages received by the class and lookup of methods for messages received by instances. Similarly, if a proxy takes the place of a method, then the method execution is intercepted too. [22]

Bootstrapping a Smalltalk. Smalltalk is a reflective system. It means that it is defined in itself in a causally connected way. Traditionally, Smalltalk systems evolved by modifying and cloning what is called an image (a chunk of memory containing all the objects at a given point in time). During the evolution of the system, objects representing it are modified. However, such an image modification and cloning poses several problems: (1) There is no operational machine-executable algorithm that allows one to build a system from scratch. A system object may be modified but it may be difficult to reproduce its exact state before the changes. Therefore it is difficult to get a reproducible process. (2) As a consequence, certain classes may not have been initialized since years. (3) Finally, since the system acts as a living system, it is not simple to evolve the kernel for introducing new abstractions without performing some kind of brain surgery on oneself. There is a need to have a step by step process to build Smalltalk kernels from scratch. After an analysis of past and current practices to mutate or generate kernels, we describe a kernel bootstrap process step-by-step. First the illusion of the existence of a kernel is created via stubs objects. Second the classes and meta-classes hierarchy are generated. Code is compiled and finally information needed by the virtual machine and execution are generated and installed. [15]

Flexible Object Layouts: Enabling Lightweight Language Extensions by Intercepting Slot Access. Programming idioms, design patterns and application libraries often introduce cumbersome and repetitive boilerplate code to a software system. Language extensions and external DSLs (domain specific languages) are sometimes introduced to reduce the need for boilerplate code, but they also complicate the system by introducing the need for language dialects and inter-language mediation.To address this, we propose to extend the structural reflective model of the language with object layouts, layout scopes and slots. Based on the new reflective language model we can 1) provide behavioral hooks to object layouts that are triggered when the fields of an object are accessed and 2) simplify the implementation of state-related language extensions such as stateful traits. By doing this we show how many idiomatic use cases that normally require boilerplate code can be more effectively supported.We present an implementation in Smalltalk, and illustrate its usage through a series of extended examples. [25]