194 lines
8.5 KiB
Plaintext
194 lines
8.5 KiB
Plaintext
This is release 3.0 of Elk, 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 3.0 distribution as well as additional information
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about Elk in the World Wide Web at
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http://www.informatik.uni-bremen.de/~net/elk
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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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What is new in Elk 3.0?
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-----------------------
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The major improvements in Elk 3.0 are a new Scheme object representation
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and a new, completely rewritten C/C++ Programmer's Manual for Elk.
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The new object representation has been suggested by Craig McPheeters,
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who also contributed an initial set of patches. Its advantages are:
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o the heap size is no longer limited (except by the amount of
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virtual memory that can be addressed);
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o `fixnums' now occupy an entire C int;
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o the number of Scheme types is no longer limited to 128
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o the new format improves compile-time checking and eases debugging;
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o Elk now ports easily to 64-bit platforms such as the DEC/Alpha.
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The new C/C++ Programmer's Manual is a complete specification of the
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C/C++ interface to Elk; it is intended for authors of extensible,
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Elk-based applications and for extension writers. Topics range from
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the general architecture of extensible applications and the use of
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dynamic loading to advanced techniques such as weak data structures and
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cooperation with the garbage collector.
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Also new in Elk 3.0 is a POSIX-style regular expression extension.
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Elk has been ported to a number of new platforms (among them Linux,
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BSD/OS, AIX 4.1, and HP-UX 10.0). A full list of changes is in the
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distribution (see the files CHANGES and MIGRATE).
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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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Oliver Laumann <net@informatik.uni-bremen.de>
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