slang.tm

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#% -*- mode: tm; mode: fold -*-

#% text-macro definitions #%{{{
#i linuxdoc.tm

#d slang \bf{S-Lang}
#d slrn \bf{slrn}
#d jed \bf{jed}
#d kw#1 \tt{$1}
#d exmp#1 \tt{$1}
#d var#1 \tt{$1}
#d ldots ...
#d times *
#d math#1 $1
#d sc#1 \tt{$1}
#d verb#1 \tt{$1}
#d sldxe \bf{sldxe}
#d url#1 <htmlurl url="$1" name="$1">
#d slang-library-reference \bf{The \slang Library Reference}
#d chapter#1 <chapt>$1<p>
#d preface <preface>
#d tag#1 <tag>$1</tag>
#d appendix <appendix>

#d NULL <tt>NULL</tt>
#d kbd#1 <tt>$1</tt>

#d documentstyle book

#%}}}

\linuxdoc

\begin{\documentstyle}

\title A Guide to the S-Lang Language
\author John E. Davis, \tt{[e-mail]}
\date \__today__

\toc

#i preface.tm

\chapter{Introduction} #%{{{

   \slang is a powerful interpreted language that may be embedded into
   an application to make the application extensible.  This enables
   the application to be used in ways not envisioned by the programmer,
   thus providing the application with much more flexibility and
   power.  Examples of applications that take advantage of the
   interpreter in this way include the \jed editor and the \slrn
   newsreader.

\sect{Language Features}

   The language features both global and local variables, branching
   and looping constructs, user-defined functions, structures,
   datatypes, and arrays.  In addition, there is limited support for
   pointer types.  The concise array syntax rivals that of commercial
   array-based numerical computing environments.

\sect{Data Types and Operators} #%{{{

   The language provides built-in support for string, integer (signed
   and unsigned long and short), double precision floating point, and
   double precision complex numbers.  In addition, it supports user
   defined structure types, multi-dimensional array types, and
   associative arrays.  To facilitate the construction of
   sophisticated data structures such as linked lists and trees, a
   `reference' type was added to the language.  The reference type
   provides much of the same flexibility as pointers in other
   languages.  Finally, applications embedding the interpreter may
   also provide special application specific types, such as the
   \var{Mark_Type} that the \jed editor provides.

   The language provides standard arithmetic operations such as
   addition, subtraction, multiplication, and division.  It also
   provides support for modulo arithmetic as well as operations at
   the bit level, e.g., exclusive-or.  Any binary or unary operator
   may be extended to work with any data type.  For example, the
   addition operator (\var{+}) has been extended to work between
   string types to permit string concatenation.

   The binary and unary operators work transparently with array types.
   For example, if \var{a} and \var{b} are arrays, then \exmp{a + b}
   produces an array whose elements are the result of element by
   element addition of \var{a} and \var{b}.  This permits one to do
   vector operations without explicitly looping over the array
   indices.

#%}}}

\sect{Statements and Functions} #%{{{

   The \slang language supports several types of looping constructs and
   conditional statements.  The looping constructs include \kw{while},
   \kw{do...while}, \kw{for}, \kw{forever}, \kw{loop}, \kw{foreach},
   and \kw{_for}. The conditional statements include \kw{if},
   \kw{if-then-else}, and \kw{!if}.

   User defined functions may be defined to return zero, one, or more
   values.  Functions that return zero values are similar to
   `procedures' in languages such as PASCAL.  The local variables of a
   function are always created on a stack allowing one to create
   recursive functions.  Parameters to a function are always passed by
   value and never by reference. However, the language supports a
   \em{reference} data type that allows one to simulate pass by
   reference.

   Unlike many interpreted languages, \slang allows functions to be
   dynamically loaded (function autoloading).  It also provides
   constructs specifically designed for error handling and recovery as
   well as debugging aids (e.g., function tracebacks).

   Functions and variables may be declared as private belonging to a
   namespace associated with the compilation unit that defines the
   function or variable.  The ideas behind the namespace implementation
   stems from the C language and should be quite familiar to any one
   familiar with C.

#%}}}

\sect{Error Handling} #%{{{

   The \slang language defines a construct called an \em{error-block}
   that may be used for error handling and recovery.  When a non-fatal
   run-time error is encountered, any error blocks that have been
   defined are executed as the run-time stack unwinds.  An error block
   can optionally clear the error and the program will continue
   running after the statement that triggered the error.  This
   mechanism is somewhat similar to try-catch in C++.

#%}}}

\sect{Run-Time Library} #%{{{

   Functions that compose the \slang run-time library are called
   \em{intrinsics}.  Examples of \slang intrinsic functions available
   to every \slang application include string manipulation functions
   such as \var{strcat}, \var{strchop}, and \var{strcmp}.  The \slang
   library also provides mathematical functions such as \var{sin},
   \var{cos}, and \var{tan}; however, not all applications enable the
   use of these intrinsics.  For example, to conserve memory, the 16
   bit version of the \jed editor does not provide support for any
   mathematics other than simple integer arithmetic, whereas other
   versions of the editor do support these functions.

   Most applications embedding the languages will also provide a set of
   application specific intrinsic functions.  For example, the \jed
   editor adds over 100 application specific intrinsic functions to
   the language.  Consult your application specific documentation to
   see what additional intrinsics are supported.

#%}}}

\sect{Input/Output}

   The language supports C-like stdio input/output functions such as
   \var{fopen}, \var{fgets}, \var{fputs}, and \var{fclose}.  In
   addition it provides two functions, \var{message} and \var{error},
   for writing to the standard output device and standard error.
   Specific applications may provide other I/O mechanisms, e.g.,
   the \jed editor supports I/O to files via the editor's
   buffers.

\sect{Obtaining \slang} #%{{{

  Comprehensive information about the library may be obtained via the
  World Wide Web from \tt{http://www.s-lang.org}.

  \slang as well as some programs that embed it are freely available
  via anonymous ftp in the United States from
\begin{itemize}
  \item \url{ftp://space.mit.edu/pub/davis}.
\end{itemize}
  It is also available outside the United States from the following
  mirror sites:
\begin{itemize}
    \item \url{ftp://ftp.uni-stuttgart.de/pub/unix/misc/slang/}
    \item \url{ftp://ftp.fu-berlin.de/pub/unix/news/slrn/}
    \item \url{ftp://ftp.ntua.gr/pub/lang/slang/}
\end{itemize}

  The Usenet newsgroup \var{alt.lang.s-lang} was created for \slang
  programmers to exchange information and share macros for the various
  programs the embed the language.  The newsgroup \var{comp.editors}
  can be a useful resource for \slang macros for the \jed editor.
  Similarly, \slrn users will find \var{news.software.readers} to be a
  valuable source of information.
  
  Finally, two mailing lists dealing with the \slang library have been
  created: 
\begin{itemize}
     \item \tt{[e-mail]}
     \item \tt{[e-mail]}
\end{itemize}
  The first list is for announcements of new releases of the library, while the
  second list is intended for those who use the library for their own code
  development.  To subscribe to the announcement list, send an email to
  \tt{[e-mail]} and include
  the word \tt{subscribe} in the body of the message.  To subscribe to
  the developers list, use the address
  \tt{[e-mail]}.

#%}}}

#%}}}

\chapter{Overview of the Language} #%{{{

   This purpose of this section is to give the reader a feel for the
   \slang language, its syntax, and its capabilities.  The information
   and examples presented in this section should be sufficient to
   provide the reader with the necessary background to understand the
   rest of the document.

\sect{Variables and Functions} #%{{{

   \slang is different from many other interpreted languages in the
   sense that all variables and functions must be declared before they
   can be used.

   Variables are declared using the \kw{variable} keyword, e.g.,
#v+
     variable x, y, z;
#v-
   declares three variables, \var{x}, \var{y}, and \var{z}.  Note the
   semicolon at the end of the statement.  \em{All \slang statements must
   end in a semi-colon.}

   Unlike compiled languages such as C, it is not necessary to specify
   the data type of a \slang variable.  The data type of a \slang
   variable is determined upon assignment.  For example, after
   execution of the statements
#v+
     x = 3;
     y = sin (5.6);
     z = "I think, therefore I am.";
#v-
   \var{x} will be an integer, \var{y} will be a
   double, and \var{z} will be a string.  In fact, it is even possible
   to re-assign \var{x} to a string:
#v+
     x = "x was an integer, but now is a string";
#v-
   Finally, one can combine variable declarations and assignments in
   the same statement:
#v+
     variable x = 3, y = sin(5.6), z = "I think, therefore I am.";
#v-

   Most functions are declared using the \kw{define} keyword.  A
   simple example is
#v+
      define compute_average (x, y)
      {
         variable s = x + y;
         return s / 2.0;
      }
#v-
   which defines a function that simply computes the average of two
   numbers and returns the result.  This example shows that a function
   consists of three parts: the function name, a parameter list, and
   the function body.

   The parameter list consists of a comma separated list of variable
   names.  It is not necessary to declare variables within a parameter
   list; they are implicitly declared.  However, all other \em{local}
   variables used in the function must be declared.  If the function
   takes no parameters, then the parameter list must still be present,
   but empty:
#v+
      define go_left_5 ()
      {
         go_left (5);
      }
#v-
   The last example is a function that takes no arguments and returns
   no value.  Some languages such as PASCAL distinguish such objects
   from functions that return values by calling these objects
   \em{procedures}.  However, \slang, like C, does not make such a
   distinction.
   
   The language permits \em{recursive} functions, i.e., functions that
   call themselves.  The way to do this in \slang is to first declare
   the function using the form:
\begin{tscreen}
     define \em{function-name} ();
\end{tscreen}
   It is not necessary to declare a parameter list when declaring a
   function in this way.

   The most famous example of a recursive function is the factorial
   function.  Here is how to implement it using \slang:
#v+
     define factorial ();   % declare it for recursion
     
     define factorial (n)
     {
        if (n < 2) return 1;
        return n * factorial (n - 1);
     }
#v-
   This example also shows how to mix comments with code.  \slang uses
   the `\var{%}' character to start a comment and all characters from
   the comment character to the end of the line are ignored.

#%}}}

\sect{Strings} #%{{{

   Perhaps the most appealing feature of any interpreted language is
   that it frees the user from the responsibility of memory management.
   This is particularly evident when contrasting how
   \slang handles string variables with a lower level language such as
   C.  Consider a function that concatenates three strings.  An
   example in \slang is:
#v+
     define concat_3_strings (a, b, c)
     {
        return strcat (a, strcat (b, c));
     }
#v-
   This function uses the built-in
   \var{strcat} function for concatenating two strings.  In C, the
   simplest such function would look like:
#v+
     char *concat_3_strings (char *a, char *b, char *c)
     {
        unsigned int len;
        char *result;
        len = strlen (a) + strlen (b) + strlen (c);
        if (NULL == (result = (char *) malloc (len + 1)))
          exit (1);
        strcpy (result, a);
        strcat (result, b);
        strcat (result, c);
        return result;
     }
#v-
   Even this C example is misleading since none of the issues of memory
   management of the strings has been dealt with.  The \slang language
   hides all these issues from the user.

   Binary operators have been defined to work with the string data
   type.  In particular the \var{+} operator may be used to perform
   string concatenation.  That is, one can use the
   \var{+} operator as an alternative to \var{strcat}:
#v+
      define concat_3_strings (a, b, c)
      {
         return a + b + c;
      }