267 lines
12 KiB
Plaintext
267 lines
12 KiB
Plaintext
$Id$
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This is Elk 3.99, a pre-release of Elk 4.0, the Extension Language Kit.
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What is Elk?
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------------
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Elk is an implementation of the Scheme programming language.
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In contrast to existing, stand-alone Scheme systems Elk has been
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designed specifically as an embeddable, reusable extension language
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subsystem for applications written in C or C++.
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Developers using Elk can deliver applications with different components
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written in different languages, such as an efficient core written in
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C or C++ and an extensible user interface layer implemented in Scheme.
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To help building hybrid application architectures, Elk supports a
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tightly-knit interworking of the C/C++ parts of applications with
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Scheme code.
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Elk is also useful as a stand-alone Scheme implementation, in particular
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as a platform for rapid prototyping of X11-based Scheme programs.
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The Elk project was started in 1987 to support ISOTEXT, a multimedia
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document editor that has been developed at the Technical University of
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Berlin. The first freely available version, Elk 1.0, was published in
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USENET in September 1989. Since then, Elk has been successfully used as
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the extension language framework for numerous applications (commercial
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products as well as free software projects).
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Getting Elk
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-----------
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You can obtain the Elk 3.99 distribution as well as additional information
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about Elk in the World Wide Web at
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http://sam.zoy.org/projects/sam/
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The distribution is also available for anonymous FTP from a number of
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servers including these:
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ftp://ftp.x.org/contrib/devel_tools/elk-3.0.tar.gz
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ftp://ftp.uni-bremen.de/pub/programming/languages/scheme/elk/elk-3.0.tar.gz
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A non-trivial example application using Elk as its extension language
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is available as source and pre-compiled binaries (`unroff' is a troff
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translator with back-ends for HTML and the -ms and -man macros):
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http://www.informatik.uni-bremen.de/~net/unroff
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Elk features
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------------
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o Full incremental, dynamic loading
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This facility enables Scheme code to load compiled Scheme extensions
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into the running interpreter (or into the application) on demand.
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Complex Elk-based applications can be decomposed into dynamically
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loadable components to avoid large, monolithic executables.
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Furthermore, user-supplied extension need not be written entirely in
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Scheme; they may include an efficient, low-level layer written in C
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or C++.
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Dynamic loading in Elk is supported on many platforms and is not
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restricted to a dlopen() interface. Elk provides automatic
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initialization of dynamically loaded extensions and takes care of
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C++ static constructors/destructors embedded in object files.
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o Freezing of fully customized applications into executable files
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Elk provides a new Scheme primitive `dump' which freezes the dynamic
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runtime image of the Scheme interpreter into an executable file
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(including an enclosing application if present). This facility
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resembles unexec() in Emacs, but the new executable resumes execution
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by returning from the call to `dump' by which that executable was
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created (not unlike fork() in UNIX). Dynamic loading and `dump'
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increase the usability of Elk as the backbone of complex applications.
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o Powerful C/C++ interface for language interoperability
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Elk provides for a tight integration of the C/C++ core of applications
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(or extensions) with the extension language. Applications can define
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their own Scheme primitives (three calling disciplines are supported),
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define application-specific first-class Scheme types with customized
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print and read functions, convert objects between Scheme types and
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C/C++ types in various ways, implement weak data structures, raise
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Scheme errors, define Scheme variables and symbols, evaluate
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S-expressions encoded as C strings, and utilize the garbage collector.
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o Full Scheme bindings for X11 and Motif
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Several dynamically loadable extensions provide full Scheme access to
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the X11/OpenWindows Xlib, to the application programmer interface of
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the Xt intrinsics, and to the Athena and OSF/Motif widget sets.
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Using these extensions, the graphical user-interfaces of Elk-based
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applications can be built entirely in the extension language.
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o UNIX interface
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Elk provides Scheme access to most UNIX system calls and common C
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library functions. The UNIX extension supports a wide range of
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different UNIX platforms without restricting its functionality to the
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lowest common denominator or to the POSIX 1003.1 functions.
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o Stop-and-copy and generational, incremental garbage collection
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Elk employs two garbage collection strategies selectable at compile
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time: a traditional stop-and-copy garbage collector and a generational
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garbage collector which is more efficient and thus reduces the time the
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application is disrupted by a garbage collection. On platforms with
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advanced memory management, `incremental' mode can be enabled for the
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generational garbage collector to further reduce wait times.
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o Non-standard Scheme features
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In addition to the standard Scheme core, Elk supports first-class
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environments, error handling, provide/require and autoloading,
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fluid bindings and dynamic-wind, simple `eval-twice'-style macros,
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property lists, string ports and bidirectional ports, shell-style
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`tilde expansion' in filenames, an interactive top-level written
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in Scheme, a Scheme debugger and a pretty printer, arbitrary length
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bitstrings, and Scheme records.
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o Comprehensive documentation
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The distribution includes 230+ pages of fully indexed documentation.
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All manuals exist as troff input files which can be translated to HTML
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(with `unroff') for online browsing in addition to producing typeset-
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quality printed versions.
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o Distributed in legally unencumbered form
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The copyright/license agreement permits free redistribution and use
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of Elk in commercial products.
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Why is Elk using Scheme?
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------------------------
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As extensions can get as large and complex as freestanding programs,
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extension language programmers (usually the users of the application)
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deserve the same language features that other programmers are
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accustomed to. By using a general-purpose programming language rather
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than a specialized scripting language, non-trivial extensions can
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benefit from the structuring functionality inherent in real programming
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languages (such as Lisp).
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Members of the Lisp language family are particularly suitable as an
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extension language: Lisp has a simple syntax but powerful semantics,
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it allows for small implementations, and its interactive nature
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supports rapid prototyping and encourages users to explore and test
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solutions to problems in an incremental way.
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Consequently, Lisp has become increasingly popular for this purpose, to
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the point where the abundance of different dialects has grown into a
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problem. Of the standardized dialects of Lisp, only Scheme is suitably
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modest, yet sufficiently general, to serve as a reusable extension
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language for a wide range of applications. Scheme is orthogonal and
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well-defined, and it is small enough to not dominate the application it
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serves and to be fully understood with acceptable effort.
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The Elk distribution
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--------------------
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Here is a brief roadmap for the subdirectories and files included in
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the distribution.
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elk-3.0 --+-- README Explains the purpose and release status of Elk
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+-- CHANGES Lists the changes between this and earlier releases
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| of Elk
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+-- MIGRATE Explains how C/C++ code (applications or extensions)
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| written for older versions of Elk may have to be
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| modified to make it work with this version
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+-- INSTALL Instructions for configuring, compiling, and
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| installing Elk; a brief description of the files that
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| get installed in the process; and a description of
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| the structure of the Makefiles and the purpose of
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| Makefile.local and `build' in each source directory
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+-- MACHINES Additional, platform-specific advice for installing
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| and using Elk, such as compiler bugs, unsupported
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| features, problems with older OS versions and other
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| pitfalls
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+-- BUGS Information about known problems with this release
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+-- TODO Ideas, improvements and projects for future releases
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+-- COPYING The copyright status of the distribution
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+-- AUTHORS A list of people who have contributed significantly
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| to Elk; acknowledgments and credits
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+-- include/ The include files to be #included by applications
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| that use Elk as their extension language, and by
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| extensions to Elk. Including scheme.h from this
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| directory causes all the other .h files to be
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| included in the right order. The include files may
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| or may not use ANSI/ISO-C prototypes, depending on
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| the config file you have chosen.
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+-- src/ The C source files of the Scheme interpreter
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+-- scm/ Scheme files that are loaded during runtime. These
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| are copied to a destination directory specified in
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| config/site when Elk is installed.
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+-- lib --, This directory tree holds the C source for various
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| | Elk extensions that can be loaded into the Scheme
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| | interpreter or linked with an application
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| +-- xlib/ The C source files of the X11 Xlib extension
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| +-- xt/ The C source files of the Xt (X11 Toolkit
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| | Intrinsics) extension
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| +-- xaw/ The Scheme interfaces to the X11 Athena widgets.
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| | There is one .d file for each widget class.
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| | Each of these is compiled into a C source file
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| | when running `make' and then compiled into a
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| | dynamically loadable object.
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| +-- xm/ The .d files for the Motif widgets
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| +-- unix/ The C source files of the UNIX extension
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| `-- misc/ The C source files of the record extension, the
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| bitstring extension, the regular expression
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| extension, and various other dynamically
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| loadable Elk extensions
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+-- doc/ The directory tree holding the documentation for
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| Elk as troff input files and pre-generated
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| PostScript files. See doc/README for a roadmap
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| of the `doc' tree.
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+-- examples --, A collection of demonstration programs for Elk
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| | and the various extensions (mostly in Scheme)
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| +-- scheme Basic Scheme demos (collected from USENET
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| | and other sources)
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| +-- xlib Programs demonstrating the Xlib, Athena,
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| +-- xaw and Motif extensions
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| +-- xm
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| +-- unix Example programs for the UNIX extension
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| +-- regexp A demonstration of the regexp extension
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| `-- c++ A few simple C++ programs demonstrating
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| use of Elk with C++ applications (see
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| README in this directory)
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`-- util/ Various utilities, some of which may aid in preparing
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a config file for an as yet unsupported platform.
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