S-Lang 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.
The S-Lang distribution contains a standalone application called slsh that may be used for writing S-Lang scripts and full-blown S-Lang based applications. For example, the author has used slsh to create a mediacenter for his home entertainment system that integrates internet radio and tv, podcasts, digital pictures and video, CDs, and so forth. The use of slsh in such non-interactive modes is discussed in the chapter on slsh.
slsh also may be used interactively and has full access to all components of the S-Lang interpreter. With features such as customizable command-line editing, history recall and completion, slsh is a convenient environment for learning and using the language. In fact, as you are reading this manual, it is recommended that you use slsh in its interactive mode as an aid to understanding the language.
While a standard S-Lang installation includes slsh, some some binary distributions package slsh separately from the S-Lang library, and as such must be installed separately. For example, on Debian Linux it can be installed via
apt-get install slsh
When called without arguments, slsh will start in interactive mode
by issuing a (customizable)
slsh> prompt and waits for input.
While most of the time one would enter S-Lang statements at the
prompt, slsh also accepts some other commands, most notably
slsh> help Most commands must end in a semi-colon. If a command begins with '!', then the command is passed to the shell. Examples: !ls, !pwd, !cd foo, ... Special commands: help <help-topic> apropos <something> start_log( <optional-log-file> ); start logging input to a file (default is slsh.log) stop_log(); stop logging input save_input (<optional-file>); save all previous input to a file (default: slsh.log) quit;
Although the language normally requires variables to be declared before use, it is not necessary to do so when using slsh interactively. For example, in this document you will see examples such as
At the slsh command line, the use of the
variable x = [1:10]; variable y = sin (x^2);
variablekeyword in such statements is optional:
slsh> x = [1:10]; y = sin(x^2);
As the above example suggests, one use of slsh is as a sophisticated calculator. For example,
This is especially true when combined with modules, e.g.,
slsh> sin (1.24) + 3*cos (1.3*PI); -0.817572
In this example, the
slsh> require ("fits"); slsh> require ("histogram"); slsh> tbl = fit_read_table ("evt1a.fits"); slsh> engrid = [min(tbl.energy):max(energy):#1024]; slsh> spectrum = hist1d (tbl.energy[where(tbl.status==0)], engrid);
fitsmodule was used to read data from a binary file called
evt1a.fits, and the
histogrammodule was used to bin the data in the energy column into a histogram to create a spectrum. The expression involving
wherefilters the data by accepting only those energy values whose status is set to 0. The
histogrammodules are not distributed with S-Lang but may be obtained separately-- see http://www.jedsoft.org/slang/modules/ for links to them. For more information about modules, see the Modules chapter in this document.
For more information about using slsh, see the chapter on slsh.
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.
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, lists, and
associative arrays. To facilitate the construction of
sophisticated data structures such as linked lists and trees, the
language also includes a ``reference'' type. 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
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, including user-defined types.
For example, the addition operator (
+) 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
b are arrays, then
a + b
produces an array whose elements are the result of element by
element addition of
b. This permits one to do
vector operations without explicitly looping over the array
The S-Lang language supports several types of looping constructs and
conditional statements. The looping constructs include
_for. The conditional statements include
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 reference data type that allows one to simulate pass by reference.
Unlike many interpreted languages, S-Lang 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 stem from the C language and should be quite familiar to any one familiar with C.
The S-Lang language has a try/throw/catch/finally exception model
whose semantics are similar to that of other languages. Users may
also extend the exception class hierarchy with user-defined
ERROR_BLOCK based exception model of S-Lang
1.x is still supported but deprecated.
Functions that compose the S-Lang run-time library are called
intrinsics. Examples of S-Lang intrinsic functions available
to every S-Lang application include string manipulation functions
strcmp. The S-Lang
library also provides mathematical functions such as
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.
Operating systems that support dynamic linking allow a slang interpreter to dynamically link additional libraries of intrinsic functions and variables into the interpreter. Such loadable objects are called modules. A separate chapter of this manual is devoted to this important feature.
The language supports C-like stdio input/output functions such as
addition it provides two functions,
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
Comprehensive information about the library may be obtained via the World Wide Web from http://www.jedsoft.org/slang/. In particular see http://www.jedsoft.org/slang/download.html for downloading the latest version of the library.
Users with generic questions about the interpreter are encouraged to
post questions to the Usenet newsgroup
specific questions relating to the use of S-Lang within some
application may be better answered in an application-specific forum.
For example, users with questions about using S-Lang as embedded in
the jed editor are more likely to be answered in the
comp.editors newsgroup or on the jed mailing list. Similarly
users with questions concerning slrn will find
news.software.readers to be a valuable source of information.
Developers who have embedded the interpreter are encouraged to join the S-Lang mailing list. To subscribe to the list or just browse the archives, visit http://www.jedsoft.org/slang/mailinglists.html.