This is release 3.0 of Elk, the Extension Language Kit.


What is Elk?
------------

Elk is an implementation of the Scheme programming language.
In contrast to existing, stand-alone Scheme systems Elk has been
designed specifically as an embeddable, reusable extension language
subsystem for applications written in C or C++.

Developers using Elk can deliver applications with different components
written in different languages, such as an efficient core written in
C or C++ and an extensible user interface layer implemented in Scheme.
To help building hybrid application architectures, Elk supports a
tightly-knit interworking of the C/C++ parts of applications with
Scheme code.

Elk is also useful as a stand-alone Scheme implementation, in particular
as a platform for rapid prototyping of X11-based Scheme programs.

The Elk project was started in 1987 to support ISOTEXT, a multimedia
document editor that has been developed at the Technical University of
Berlin.  The first freely available version, Elk 1.0, was published in
USENET in September 1989.  Since then, Elk has been successfully used as
the extension language framework for numerous applications (commercial
products as well as free software projects).


Getting Elk
-----------

You can obtain the Elk 3.0 distribution as well as additional information
about Elk in the World Wide Web at
   
   http://www.informatik.uni-bremen.de/~net/elk

The distribution is also available for anonymous FTP from a number of
servers including these:
   
   ftp://ftp.x.org/contrib/devel_tools/elk-3.0.tar.gz
   ftp://ftp.uni-bremen.de/pub/programming/languages/scheme/elk/elk-3.0.tar.gz


A non-trivial example application using Elk as its extension language
is available as source and pre-compiled binaries (`unroff' is a troff
translator with back-ends for HTML and the -ms and -man macros):

  http://www.informatik.uni-bremen.de/~net/unroff


What is new in Elk 3.0?
-----------------------

The major improvements in Elk 3.0 are a new Scheme object representation
and a new, completely rewritten C/C++ Programmer's Manual for Elk.

The new object representation has been suggested by Craig McPheeters,
who also contributed an initial set of patches.  Its advantages are:

   o  the heap size is no longer limited (except by the amount of
      virtual memory that can be addressed);
   o  `fixnums' now occupy an entire C int;
   o  the number of Scheme types is no longer limited to 128
   o  the new format improves compile-time checking and eases debugging;
   o  Elk now ports easily to 64-bit platforms such as the DEC/Alpha.

The new C/C++ Programmer's Manual is a complete specification of the
C/C++ interface to Elk; it is intended for authors of extensible,
Elk-based applications and for extension writers.  Topics range from
the general architecture of extensible applications and the use of
dynamic loading to advanced techniques such as weak data structures and
cooperation with the garbage collector.

Also new in Elk 3.0 is a POSIX-style regular expression extension.
Elk has been ported to a number of new platforms (among them Linux,
BSD/OS, AIX 4.1, and HP-UX 10.0).  A full list of changes is in the
distribution (see the files CHANGES and MIGRATE).


Elk features
------------

  o  Full incremental, dynamic loading

     This facility enables Scheme code to load compiled Scheme extensions
     into the running interpreter (or into the application) on demand.
     Complex Elk-based applications can be decomposed into dynamically
     loadable components to avoid large, monolithic executables.
     Furthermore, user-supplied extension need not be written entirely in
     Scheme; they may include an efficient, low-level layer written in C
     or C++.

     Dynamic loading in Elk is supported on many platforms and is not
     restricted to a dlopen() interface.  Elk provides automatic
     initialization of dynamically loaded extensions and takes care of
     C++ static constructors/destructors embedded in object files.

  o  Freezing of fully customized applications into executable files

     Elk provides a new Scheme primitive `dump' which freezes the dynamic
     runtime image of the Scheme interpreter into an executable file
     (including an enclosing application if present).  This facility
     resembles unexec() in Emacs, but the new executable resumes execution
     by returning from the call to `dump' by which that executable was
     created (not unlike fork() in UNIX).  Dynamic loading and `dump'
     increase the usability of Elk as the backbone of complex applications.

  o  Powerful C/C++ interface for language interoperability

     Elk provides for a tight integration of the C/C++ core of applications
     (or extensions) with the extension language.  Applications can define
     their own Scheme primitives (three calling disciplines are supported),
     define application-specific first-class Scheme types with customized
     print and read functions, convert objects between Scheme types and
     C/C++ types in various ways, implement weak data structures, raise
     Scheme errors, define Scheme variables and symbols, evaluate
     S-expressions encoded as C strings, and utilize the garbage collector.

  o  Full Scheme bindings for X11 and Motif

     Several dynamically loadable extensions provide full Scheme access to
     the X11/OpenWindows Xlib, to the application programmer interface of
     the Xt intrinsics, and to the Athena and OSF/Motif widget sets.
     Using these extensions, the graphical user-interfaces of Elk-based
     applications can be built entirely in the extension language.

  o  UNIX interface

     Elk provides Scheme access to most UNIX system calls and common C
     library functions.  The UNIX extension supports a wide range of
     different UNIX platforms without restricting its functionality to the
     lowest common denominator or to the POSIX 1003.1 functions.

  o  Stop-and-copy and generational, incremental garbage collection

     Elk employs two garbage collection strategies selectable at compile
     time: a traditional stop-and-copy garbage collector and a generational
     garbage collector which is more efficient and thus reduces the time the
     application is disrupted by a garbage collection.  On platforms with
     advanced memory management, `incremental' mode can be enabled for the
     generational garbage collector to further reduce wait times.

  o  Non-standard Scheme features

     In addition to the standard Scheme core, Elk supports first-class
     environments, error handling, provide/require and autoloading,
     fluid bindings and dynamic-wind, simple `eval-twice'-style macros,
     property lists, string ports and bidirectional ports, shell-style
     `tilde expansion' in filenames, an interactive top-level written
     in Scheme, a Scheme debugger and a pretty printer, arbitrary length
     bitstrings, and Scheme records.

  o  Comprehensive documentation

     The distribution includes 230+ pages of fully indexed documentation.
     All manuals exist as troff input files which can be translated to HTML
     (with `unroff') for online browsing in addition to producing typeset-
     quality printed versions.

  o  Distributed in legally unencumbered form

     The copyright/license agreement permits free redistribution and use
     of Elk in commercial products.


Why is Elk using Scheme?
------------------------

As extensions can get as large and complex as freestanding programs,
extension language programmers (usually the users of the application)
deserve the same language features that other programmers are
accustomed to.  By using a general-purpose programming language rather
than a specialized scripting language, non-trivial extensions can
benefit from the structuring functionality inherent in real programming
languages (such as Lisp).

Members of the Lisp language family are particularly suitable as an
extension language:  Lisp has a simple syntax but powerful semantics,
it allows for small implementations, and its interactive nature
supports rapid prototyping and encourages users to explore and test
solutions to problems in an incremental way.

Consequently, Lisp has become increasingly popular for this purpose, to
the point where the abundance of different dialects has grown into a
problem.  Of the standardized dialects of Lisp, only Scheme is suitably
modest, yet sufficiently general, to serve as a reusable extension
language for a wide range of applications.  Scheme is orthogonal and
well-defined, and it is small enough to not dominate the application it
serves and to be fully understood with acceptable effort.


Oliver Laumann  <net@informatik.uni-bremen.de>