This paper describes a programming system based on the metaphor of communicating objects.

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An experimental system for declaring and inferring type in Smalltalk is described. (In the current Smalltalk language, the programmer supplies no type declarations.) The system provides the benefits of type declaration in regard to compile-time checking and documentation, while still retaining Smalltalk's flexibility. A type hierarchy, which is integrated with the existing Smalltalk class hierarchy, allows one type to inherit the traits of another type. A type may also have parameters, which are in turn other types.

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OpenSmalltalk-VM is a virtual machine (VM) for languages in the Smalltalk family (e.g. Squeak, Pharo) which is itself written in a subset of Smalltalk that can easily be translated to C. Development is done in Smalltalk, an activity we call "Simulation". The production VM is derived by translating the core VM code to C. As a result, two execution models coexist: simulation, where the Smalltalk code is executed on top of a Smalltalk VM, and production, where the same code is compiled to an executable through a C compiler. In this paper, we detail the VM simulation infrastructure and we report our experience developing and debugging the garbage collector and the just-in-time compiler (JIT) within it. Then, we discuss how we use the simulation infrastructure to perform analysis on the runtime, directing some design decisions we have made to tune VM performance. CCS Concepts • Software and its engineering → Runtime environments; Just-in-time compilers; Interpreters;