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README.md

README.md

You're almost in Context. To get there, start the console:

  • On Mac OS, double-click the app containing this file.

  • On Windows, double-click windows/context.vbs

  • On Linux, run linux/context.sh

Hm, you're still reading this. Too bad. Well, perhaps a few comments about the structure of this thing are in order.

Context is a programming language, in the form of a virtual operating system. It runs on multiple host platforms (like Linux, Mac OS, and Windows), and looks like a normal app on each platform. However, it presents the programmer with a platform-independent model of system resources (like files, sockets, devices, and processes), through an object-oriented programming interface. A virtual machine supports the programming interface on each platform, using appropriate platform-specific system calls.

A summary of features one may expect to work in each release is in FEATURES.html.

This file is part of the Context app, which takes the form of an Apple app directory tree, augmented to be self-revealing on Linux and Windows as well. When you start the app, it opens a console, from which you may control a set of Smalltalk systems. The console presents a list of all installed systems, describing what each one does and its current state. You can use the console to resume and suspend each one. You can also use the console to find and install additional systems from the net, and publish them there.

objects, messages, and virtual machines

An object is a composition of state (a set of references to other objects) and behavior (sequences of instructions for manipulating those references). The instructions of an object's behavior are organized as methods, each of which corresponds to a message that the object understands. A Smalltalk system is a collection of objects sending messages to each other, thus invoking their behavior and changing their state.

The behavior is performed by an instruction processor, also known as a virtual machine. Each time an object sends a message to another object, the processor performs the instructions in the method corresonding to that message. To keep track of the receiving object of the message, which instruction it is performing, and intermediate results, the processor creates a context object corresponding to that particular invocation of the message.

In the course of performing a message, the processor will encounter instructions that describe sending another message. Each context keeps a reference to the context of the previous message the processor was performing. A chain of contexts is a process.

Other instructions direct the processor to perform a built-in operation, rather than send a message. These operations are primitives. There are primitives for basic functions that can be handled by the physical processor, like addition, and those which can be handled by the host operating system, like creating a network socket. The processor implements a process scheduling model. Some primitives request that the processor suspend the current process and run a different one instead. The processor uses the scheduling model to decide which process should run next.

Methods of instructions are generated by a compiler, from textual source written by humans or other objects. A compiler is an object like any other, part of the running system. In fact, source authors have an extensive set of tools (also known as an interactive development environment) for writing and compiling methods, all implemented as objects sending messages.

The source level is where humans typically reason about messages. The source of a method consists of one or more expressions. Here's an example of an expression:

3 squared

In that expression, the receiver object 3 is sent the message "squared". The selector of the message is the symbol "squared". This is an example of a unary message; there are no parameters. When there is at least one parameter, the selector may be a sequence of colon-terminated keywords. When there is only one paremeter, the selector may be a lone operator with no colon. Here's an expression with a message whose selector is a lone operator:

3 + 4

This is called a binary message. Here's an expression with a message whose selector is a sequence of keywords:

3 to: 11 by: 4

This is called a keyword message. The selector has two keywords, one for each parameter.

Every object is an instance of a class object. The class keeps a set of methods corresponding to the messages that its instances understand. Classes are chained into a hierarchy; each class has a superclass and zero or more subclasses. When the processor performs the sending of a message, it looks for a method corresponding to that message in the methods of the receiving object's class. If it doesn't find one, it continues searching in the superclasses of that class. Because the processor searches for appropriate methods in this way, we say that an instance of a class inherits behavior from the superclasses.

A method author can direct the compiler to create and install methods in any class at any time, while the system is running. This makes the system dynamic. The system is always running, and every modification that can be made to the system can be done with the system. This makes the system reflective; it models itself. Even the processor is implemented as objects sending messages.

All the objects in the system together form the memory of the processor. Like a laptop's processor, the method instruction processor can make a snapshot of its memory at a particular moment, suspend operation, and resume operation with the snapshot later. This is known as suspending and resuming a memory. The console lets you discover and manipulate memory snapshots.

sharing memories

The app directory is also a git repository, with a submodule repo nested within it for each locally-installed memory. Note that none of this structure is signed for any host platform. I'll set up a separate site for downloading properly-signed releases. When newly cloned, the master repo contains multiple installed memories: a console memory, and a development memory for each supported Smalltalk distribution.

Currently, Squeak is supported. The next target is Pharo. We intend to support all distributions, including:

Please create an issue if we missed one. Thanks!

For each memory you want to install locally, clone its repo directory to .../context.app/Contents/RUNME/memories/.

The console memory contains the objects necessary to support essential object behavior, to grow itself through live synchronization with other memories, and to communicate through web services. A development memory has modules installed to support a development environment (a graphical interface, compiler, inspectors, debugger, etc.).

A system includes a pair of memories: a subject and a history. Each history memory records the edit history of its subject, and synchronizes it with other subjects. It has no direct human interface support (it's headless), because it only needs to communicate with its subject memory. It communicates via remote messages: messages from objects in one memory to objects in another memory (run by a processor on any net-connected physical machine).

The console memory is also headless; there's a web server running there. The Context app resumes the console memory, and the console memory resumes its web server. The Context app then opens a host web browser that visits the console web server. The behavior of the console (for example, resuming a GUI memory) is then provided as a web service.

The console can interpret command links, special URLs which encode commands for controlling memories. When you click on a command link, the initial console answers a web page asking which live memory should perform the command. The chosen memory asks for confirmation before proceeding, through its own console. One thing a command can prescribe is module installation, using synchronization information in the link. Module and links for installing them can be cited together on web pages, indexed by search engines for later discovery.

The console memory can become the history memory for another system. By installing modules, any memory can grow to include a set of apps. You can also unload modules.

You're almost in Context. To get there, start the console:

  • On Mac OS, double-click the app containing this file.

  • On Windows, double-click windows/context.vbs

  • On Linux, run linux/context.sh

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