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(guile-tut.info)What are scripting and extension languages

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What are scripting and extension languages
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  A "scripting language" is a programming language which serves as glue
between other system programs.  In the UNIX world, the traditional
scripting language is the _Bourne shell_, which allows many UNIX
commands to be executed in sequence, or in a pipeline.  Traditional UNIX
commands are cleverly written to work well when put together in a
script.

  Other examples of UNIX scripting languages are AWK, Perl, Scsh (the
Scheme Shell: a Scheme interpreter enhanced to do good scripting),
Python, Tcl, Java ...

  UNIX programmers noticed, more than 25 years ago, that scripting
languages can do serious work, so the Bourne shell was written to have
variables, operators and control structures, just like a full-featured
programming language.

  What scripting languages have, that traditional programming languages
do not, is the ability to easily run an external program (or a pipeline
of external programs) and use the returned values and output from that
program in useful ways.

  An "extension language" is a programming language interpreter offered
by an application program, so that users can write macros or even
full-fledged programs to extend the original application.  Extension
languages have a C interface (it is usually C, but it could be any
other compiled language), and can be given access to the C data
structures.  Likewise, there are C routines to access the extension
language data structures.

  Extension languages abound in the software world, even though the name
_extension language_ is seldom used.  Examples are:

   * Emacs Lisp, the language used to program and customize GNU Emacs.

   * Tcl, John Ousterhout's general-purpose scripting and extension
     language.

   * The Lotus 1-2-3 macro language (any spreadsheet macro language,
     really).  I mention this one first because it is a classic, even
     though it is seldom used any more.

   * Other spreadsheet and database macro languages.

   * The Dominion empire-style game's _exec_ files.

   * Any syntax for a ".*rc" file you might have used.  Almost all
     programs end up parsing some kind of startup or configuration
     file.  The syntax for those can get pretty involved, thus
     justifying calling them "extension languages".  The _fvwm_ window
     manager, for example, parses a rather elaborate `.fvwmrc' file.

   * Brent Benson's libscheme.a, an embeddable Scheme interpreter.

   * Guile, the GNU extension language, which is the subject of this
     tutorial.


  One lesson we can learn from looking at classical large software
applications is that "writers of large programs" always end up throwing
in some kind of parser for configuration or scripting.

  Of the examples listed above, Emacs Lisp, Tcl, Libscheme and Guile
have an important property: they are not added as an afterthought for a
specific application.  They are general-purpose languages which a user
can learn (even in college courses) and then use to customize the
application program.

  This is a recent and (in my opinion) very exciting direction in
large-program software engineering: program designers can link in the
Guile or Tcl library from the very beginning, and tell their users "You
want to customize this program?  Just use Scheme (or Tcl, or whatever
language), which you already know!"