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URL: http://www-internal.alphanet.ch/~schinz/scsh-faq.html
Frequently Asked Questions
**************************
This is the scsh Frequently Asked Questions list of 13 March 2003.
This article is provided as is without any express or implied
warranties. While every effort has been taken to ensure the accuracy of
the information contained in this article, the maintainer assumes no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.
Meta-questions
**************
This section contains questions and answers about this FAQ, its
authors, etc.
What is the aim of this FAQ?
============================
The aim of this FAQ is to provide some help and documentation to
people interested in scsh, a Unix shell that uses Scheme as its
scripting language. It is mainly aimed towards those who do not know
much about scsh or Scheme. This explains why some questions that might
seem trivial to the seasoned Scheme programmer are included anyway.
Who wrote this FAQ?
===================
Most of the FAQ was written and is still maintained by Michel Schinz
(<schinz@alphanet.ch>) and Eric Marsden (<emarsden@laas.fr>). To send
mail about the FAQ in general, please do not use these personal
addresses, but the alias mentioned below (*note Contact::).
What was changed recently in this FAQ?
======================================
Here is a list of recent changes. The name of the person who
suggested the change (either explicitly by sending a mail, or
implicitly by posting in the newsgroup) is mentioned in parentheses.
1. 2000/02/29 v2.16 language comparisons, example error handler,
mention guile-scsh.
2. 2000/08/28 v2.17 binary RPMs for linux distributions, s48.org.
3. 2000/09/05 v2.18 Olin Shivers' e-mail/web page location updated,
authors section added, history shortened, regexp section updated.
4. 2000/12/22 v2.19 win32 binaries
5. 2001/01/15 v2.20 HTML versions of the scsh manual/paper mentioned
(Dorai Sitaram).
6. 2001/05/28 v2.21 building on MacOS X, Sourceforge site.
7. 2001/06/08 v2.22 scsh version 0.5.3 released.
8. 2001/06/14 v2.23 section about the guile port updated (Paul
Emsley).
9. 2001/07/02 v2.24 Matthias Radestock's Scheme FAQ mentioned, URL for
R5RS updated.
10. 2001/09/06 v2.25 fix Wily URL (Bengt Kleberg)
11. 2001/10/19 v2.26 added Martin Gasbichler among the scsh authors,
referenced his bugs page.
12. 2002/01/10 v2.27 updates for the 0.6 release, http://www.scsh.net,
Mike Sperber added among the authors.
13. 2002/02/04 v2.28 improved perl code, thanks to John W. Krahn.
14. 2002/02/26 v2.29 minor update for scsh 0.6.1.
15. 2002/03/20 v2.30 Python example corrected (Beni Cherniavksy)
16. 2002/05/20 v2.31 minor update for scsh 0.6.2.
17. 2002/05/24 v2.32 URL for Windows binary updated (Reini Urban)
18. 2002/06/21 v2.33 Mentioned Cash (partial scsh port to OCaml).
19. 2002/08/16 v2.34 Fix exception-handling example, include OpenBSD in
the list of supported platforms, update the FreeBSD ports
information.
20. 2002/12/17 v2.35 Rewrote the part about MzScheme (Paul Steckler)
21. 2003/01/13 v2.36 minor update for scsh 0.6.3.
22. 2003/02/12 rewrote part comparing scsh with other scripting
languages (Yoann Padioleau), added Ruby script, updated scsh
license, removed reference to CIG, dropped version numbers for
this FAQ.
23. 2003/02/14 removed broken and outdated link to notes on the scsh
Web sever, added reference to the "new" Scheme 48 documentation
(Michael Sperber), changed home pages of Rees and Kelsey.
24. 2003/02/18 corrected scsh example script to include dot files
(Sriram Thaiyar).
25. 2003/03/13 added reference to Sunterlib (Anthony Carrico).
Where do I get the latest version of this FAQ?
==============================================
The latest version of the FAQ can be found at the "scsh FAQ
home-page":
`http://www-internal.alphanet.ch/~schinz/scsh-faq.html'
This home-page contains three versions of this FAQ: an ASCII
version, an HTML version and an Info version. If you have access to
the World Wide Web, I strongly recommend that you get the HTML version,
since all the hyperlinks can be followed just by clicking on them.
Apart from that, this document is posted on the 13th of each month to
the newsgroups `comp.lang.scheme.scsh', `comp.lang.scheme',
`comp.unix.shell', `comp.answers' and `news.answers'.
Where do I send comments about this FAQ?
========================================
Comments about this FAQ should be sent to the following address:
<scsh-faq@alphanet.ch>.
Please help us in producing a useful document by sending suggestions
and material for this FAQ. If you find stylistic, grammatical or syntax
errors, please also report them.
General
*******
This section contains general questions about scsh: what it is,
where to find it, etc.
What is scsh?
=============
Scsh is a Scheme shell. That is, it is a Unix shell which uses Scheme
as its scripting language. It was designed and written by Olin Shivers,
Brian Carlstrom, Martin Gasbichler and Mike Sperber, and is built on
top of Scheme 48, an implementation of Scheme written by Jonathan Rees
and Richard Kelsey.
Scsh currently includes the following features:
- A complete Posix interface.
- A very complete support for networking, with high and low level
interfaces. An additional network package, including an HTTP
server, SMTP support, etc. is also available separately.
- Powerful string manipulation functions: pattern matching, file-name
manipulations, etc.
- AWK-like macros.
- An s-expression-based notation for regular expressions (SREs).
- Threads.
However, it is currently aimed primarily at scripting use, rather
than interactive use (*note Interactive scsh::).
How do you pronounce scsh?
==========================
According to Olin, scsh is pronounced "skishhhh" (it rhymes with
"fish").
What is the current version of scsh?
====================================
The current version (as of 13 March 2003) is 0.6.3.
What are the licensing terms for scsh?
======================================
Scsh is distributed under a BSD-like open source licence. Here are
the exact terms, which can be found in the file `COPYING' of the
distribution:
Copyright (c) 1993-2002 Richard Kelsey and Jonathan Rees Copyright
(c) 1994-2002 by Olin Shivers and Brian D. Carlstrom. Copyright
(c) 1999-2002 by Martin Gasbichler. Copyright (c) 2001-2002 by
Michael Sperber.
All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met: 1. Redistributions of source code must retain the above
copyright notice, this list of conditions and the following
disclaimer. 2. Redistributions in binary form must reproduce the
above copyright notice, this list of conditions and the
following disclaimer in the documentation and/or other
materials provided with the distribution. 3. The name of the
authors may not be used to endorse or promote products derived
from this software without specific prior written permission.
THIS SOFTWARE IS PROVIDED BY THE AUTHORS "AS IS" AND ANY EXPRESS OR
IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY
DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
Previous to version 0.5.2 scsh was distributed under a more
restrictive licence.
What is Scheme?
===============
Scheme is a small and elegant programming language of the Lisp
family, originally designed by Guy Lewis Steele Jr. and Gerald Jay
Sussman. It includes powerful features like first-class procedures and
continuations, and is statically scoped (like Pascal). For more
information, refer to the Scheme FAQ (*note Getting the docs::).
What is Scheme 48?
==================
Scheme 48 is a small and portable Scheme implementation written by
Jonathan Rees and Richard Kelsey. It is based on a virtual machine
architecture (i.e. it does not compile to native code).
Scheme 48 implements all the features described in R5RS (*note
Getting the docs::) and some extensions like exceptions and a module
system.
How does scsh compare to other common scripting languages?
==========================================================
Many scripting languages are in use today, and comparing scsh with
all of them would be impossible. Here, we therefore restrict ourselves
to the following three popular scripting languages: Perl, Python and
Ruby. Below, they will be collectively referred to as PP&R.
To simplify the comparison, it will be split in three parts: first we
will look at the programming languages themselves, that is the
abstractions they offer to the programmer; then, we will look at the
implementation(s) of these languages; finally, we will look at the
(standard) libraries available for these different languages.
Underlying programming languages
--------------------------------
From a programming language perspective, scsh is different than PP&R
in that it is based on a general-purpose programming language, namely
Scheme. PP&R, on the other hand, were all designed to be scripting
languages originally. Today many people use them for relatively large
applications which do not qualify as scripts, but scripts were the
primary target of these languages. This sometimes shows in their
design, for example when it comes to variable declarations which are by
default optional in PP&R but not in Scheme.
Scheme is also special in that it is a functional programming
language, whereas Python and Ruby are object-oriented and Perl is
procedural (even though most modern Perl code is written using
objects). This does not mean that it is not possible to do functional
programming with PP&R or object-oriented programming with Scheme. It
just means that in Ruby and Python the object-oriented programming
style is encouraged both by the language itself (e.g. everything is an
object) and the standard libraries. The same holds for Scheme and the
functional programming style.
Another major difference between Scheme and PP&R is the syntax. PP&R
all have infix syntax with different notations for different concepts.
For example, the `if' conditional expression is syntactically different
from, say, a function call. Scheme, on the other hand, has a very
regular prefix syntax based on so-called s-expressions. In Scheme, an
`if' looks no different than a function call.
This very regular syntax make it possible to extend Scheme using a
sophisticated macro system. Such macros are used in several crucial
places in scsh, for example to support regular expressions with a
syntax based on s-expressions (see below), or to provide a mechanism to
iterate over the contents of a file in a way reminiscent of the AWK
language.
It should finally be noted that Scheme, Ruby and at least one
implementation of Python (Stackless Python) give the programmer a way
to capture the current continuation. This makes it possible to
implement, directly in the language, very powerful control features
including coroutines. More about continuations can be found, for
example, in the Scheme FAQ (*note Getting the docs::).
Language implementations
------------------------
Perl, Python, Ruby and scsh are relatively close to each other when
it comes to the features offered by their current implementation(s).
Ruby, Python and scsh have an interactive mode which enables one to
enter expression and see the result of their evaluation. Perl does not
include such a mode by default, even though its debugger can be used as
a basic interactive evaluator. Ruby, Python and scsh also provide a
debugger.
Libraries
---------
Because of their popularity, there is an impressive amount of
third-party libraries available for PP&R, which cover many application
domains. Currently, the same cannot be said about scsh, although a few
third-party libraries are available (*note Scsh code archive::).
That said, scsh has excellent support for writing scripts in a Posix
environment, which is its main domain of application. A great design
effort has been put into making Posix look nice from scsh. This is
accomplished not only by providing nicer names for Posix functions, but
also by altering their behaviour. This contrasts with PP&R which often
use both the name and the behaviour of the standard Posix functions.
One example situation where scsh diverges both from Posix and PP&R is
child process management. With Posix and PP&R, when child a process
exits, its parent process has to wait on it explicitly. If it fails to
do so, the dead child process (actually some information about it, like
its exit status) remains in the kernel's process table, as a zombie
process. This can be a serious problem, since the kernel's process
table can eventually become full with zombie processes.
Scsh can manage dead children processes in an automatic fashion
through a technique called process reaping. The basic idea is that scsh
takes care of waiting for child processes and stores their exit status
in the Scheme data-structure associated to them. The scsh programmer is
then free to wait or simply ignore its children, without having to
worry about zombie processes.
Another strength of scsh is its handling of regular expressions.
Since regular expressions are important in many scripting applications,
pretty much all scripting languages provide support for them, and PP&R
or scsh are no exceptions. It should be noted that the regular
expressions offered by scsh have some limitations when compared to
PP&R, like the lack of non-greedy versions of some operators. On the
other hand, scsh's regular expressions have some very interesting
properties not found in PP&R:
* a notation based on s-expressions instead of plain strings, which
makes it unnecessary to quote special characters in strings, and
makes it possible to lay out regular expressions nicely, comment
them, etc.
* a representation of regular expressions as trees instead of
strings, which eases their manipulation by programs, for example
to compose large regular expressions out of small ones, or to
write functions building regular expressions.
Example script
--------------
The following code snippets aim to provide an idea of how scsh
compares with other common scripting languages. They all print a list
of all the executables available in the current PATH to the standard
output (improvements to these examples are welcome).
- `sh'
#!/bin/sh
IFS=':'
for d in $PATH; do
for f in $d/*; do
[ -x $f -a ! -d $f ] && echo $f
done
done
- `perl'
What is the sound of Perl? Is it not the sound of a wall that
people have stopped banging their head against?
- _Larry Wall_
#!/usr/local/bin/perl
for my $dir (split /:/, $ENV{PATH}) {
opendir DIR, $dir or die "can't opendir $dir: $!";
-x "$dir/$_" && !-d _ && print "$_\n" for readdir DIR;
closedir DIR;
}
- `python'
#!/usr/local/bin/python
import os, string, stat
for d in string.split(os.environ['PATH'], ':'):
for f in os.listdir(d):
mode = os.lstat(d + '/' + f)[stat.ST_MODE]
if not stat.S_ISDIR(mode):
print f
- `ruby'
#!/usr/bin/ruby
ENV["PATH"].split(/:/).each {|path|
Dir.foreach(path) {|file|
puts(file) if File.stat(File.join(path, file)).executable?
}
}
- `scsh'
#!/usr/local/bin/scsh -s
!#
(define (executables dir)
(with-cwd dir
(filter file-executable? (directory-files dir #t))))
(define (writeln x) (display x) (newline))
(for-each writeln
(append-map executables ((infix-splitter ":") (getenv "PATH"))))
Where can I get scsh?
=====================
The latest version of scsh can be downloaded from the scsh home page,
located at `http://www.scsh.net/'.
There is also a SourceForge projet page for scsh at
`http://scsh.sourceforge.net/'.
Binaries for the Debian distribution of GNU/Linux are available from
`http://www.debian.org/Packages/stable/interpreters/scsh.html'. A
binary RPM for Red Hat Linux for x86 is available in the shells group of
the libc6 contrib archive (say `rpmfind scsh').
Binaries for win32 using Cygwin32 are available at
`http://xarch.tu-graz.ac.at/autocad/lisp/cl/scsh-0.5.2-cygwin-bin.tgz'
(thanks to Reini Urban). These require a recent version of cygwin1.dll.
And please notice that this is an old (0.5.2) version of scsh, not the
latest.
Where can I find documentation about scsh?
==========================================
The main documentation about scsh is the scsh manual. It is
included in the distribution, in `<prefix>/lib/scsh/doc/scsh-manual'
where `<prefix>' is the location where you installed scsh.
You may also want to take a look at the technical report describing
the design of scsh. It is also included in the distribution, in
`<prefix>/lib/scsh/doc/scsh-paper'.
The documentation about Scheme 48 is also worth reading, since it
describes features like the module system, the interface with C, the
command processor and some interesting libraries. It is in
`<prefix>/lib/scsh/doc/s48-manual'.
Also, since scsh is written on top of a Scheme system, you have
access to the great power of Scheme. However, no Scheme documentation
is available with scsh, so you may wish to obtain the standard Scheme
references as well. Here are some useful pointers:
- The official specification for Scheme is "The Revised^5 Report on
the Algorithmic Language Scheme", often abbreviated R5RS. This is
the document you should use to look up details about Scheme. It is
available in various formats at:
`http://www.schemers.org/Documents/Standards/'
- An excellent and up-to-date Scheme FAQ was written by Matthias
Radestock and is available at:
`http://www.schemers.org/Documents/FAQ/'.
An older FAQ was maintained by Mark Kantrowitz and Barry Margolin
and, for those interested, is still available at:
`http://www.faqs.org/faqs/scheme-faq/'.
- Schemers.org (`http://www.schemers.org/') is a collection of Scheme
resources maintained by the Programming Languages Team at Rice
University.
- The Scheme home-page is located at:
`http://www.swiss.ai.mit.edu/projects/scheme/'
- The Scheme 48 home-page is located at `http://s48.org/'.
- There are many good books about Scheme, for example: "Structure and
Interpretation of Computer Programs" (2nd ed.) by Harold Abelson
and Gerald Jay Sussman, MIT Press, 1996 or "Scheme and the Art of
Programming" by George Springer and Daniel P. Friedman, MIT Press,
1989. The full text of the first one is available online at the
following address: `http://mitpress.mit.edu/sicp/'.
For more references, see the Scheme FAQ.
The home-pages of the various people involved in the design of
Scheme, Scheme 48 or scsh may also be of interest to you. Here are
some links:
- Gerald Jay Sussman:
`http://www-swiss.ai.mit.edu/~gjs/gjs.html'
- Jonathan A. Rees:
`http://mumble.net/jar/'
- Richard Kelsey:
`http://s48.org/~kelsey/'
- Olin Shivers:
`http://www.cc.gatech.edu/fac/Olin.Shivers/'
- Brian D. Carlstrom:
`http://www-swiss.ai.mit.edu/~bdc/'
- Martin Gasbichler:
`http://www-pu.informatik.uni-tuebingen.de/users/gasbichl/'
- Mike Sperber:
`http://www-pu.informatik.uni-tuebingen.de/users/sperber/'
Is there some kind of reference card for scsh?
==============================================
Not exactly. There is a small list of all of scsh's functions in the
file `doc/cheat.txt', but it has not been updated for quite some time.
However, it would be great to have a nice TeXified reference card,
which would include R5RS functions as well (something like the nice
Perl reference card).
Which newsgroups and mailing-lists are related to scsh?
=======================================================
Currently, there is a mailing-list and a newsgroup which are
completely equivalent.
The newsgroup is `comp.lang.scheme.scsh'.
To (un)subscribe to the mailing-list, send a message to
<scsh-request@zurich.ai.mit.edu>. To submit a message to the
mailing-list, send it to <scsh@zurich.ai.mit.edu>.
Also, `comp.lang.scheme', which talks about Scheme in general, may
be of interest to you. If Scheme is your first functional language, you
might also want to read `comp.lang.functional'.
Does scsh run on my system?
===========================
Currently, scsh runs without modification on the following systems:
DEC Ultrix, Harris CXUX, HP-UX, IBM AIX, Linux, FreeBSD (*note Scsh on
FreeBSD::), OpenBSD, NetBSD, NeXTSTEP, SGI IRIX, Solaris, SunOS, MacOS
X and Win32. It should also run without too many changes on other 32
bits UNIX platforms (for 64 bit platforms like Digital Unix, *note
Porting scsh::)
Is scsh easy to port?
=====================
On 32 bits UNIX machines, yes, usually. If your system isn't already
supported, take a look at the file `doc/install.txt' which contains
porting instructions.
Porting scsh to 64 bit UNIX machines (or, more generally, non-32 bit
machines) is currently harder. The main reason is that this requires
modifications to the Scheme 48 virtual machine (VM). This VM is written
in PreScheme, a dialect of Scheme, and the PreScheme compiler isn't
distributed with scsh. It is, however, included in standard Scheme 48
distributions, which are available on Richard Kelsey's FTP site:
`ftp://ftp.nj.nec.com/pub/kelsey/'
(you'll also find a paper about PreScheme there).
In any case, never try to hack the C code generated by the PreScheme
compiler (file `scheme48vm.c'); this is ugly and you'll have to restart
from scratch for the next release of Scheme 48.
Apart from the problems with the Scheme 48 VM, there are also some
problems with scsh: the current version contains C code that assumes
32-bitness. This occurs mainly in the foreign-function interfaces (that
is, interface between Scheme and C), where integers are converted
between their Scheme and C representation.
Since v0.5.2 there is also a port to Win32 which uses the cygwin32
toolkit (thanks to Brian Carlstrom).
Can I run scsh under some other Scheme implementation?
======================================================
Currently, scsh is tightly bound to Scheme 48 because it uses two
non-standard features of Scheme 48: its module system and its foreign
function interface (FFI). This does not mean that porting it to another
Scheme implementation is impossible, but it is certainly hard.
Mike Sperber and Richard Kelsey are drafting a SRFI for a standard
FFI based on the current Scheme 48 FFI. This would help in making scsh
portable, since its C part could be easily reused with other
implementations using this FFI.
Matthew Flatt will be implementing this standard FFI for PLT Scheme.
Therefore, PLT Scheme may become a viable alternative to Scheme 48 for
scsh users. PLT Scheme is an umbrella name for a family of
implementations of Scheme, which includes DrScheme and MzScheme. More
information about it can be found at:
`http://www.plt-scheme.org/'
There is also a near-complete port of scsh to the Guile interpreter
by Gary Houston, which you can access by cvsweb at
`http://subversions.gnu.org/cgi-bin/cvsweb/guile/guile-scsh/'. It needs
the `guilerxspencer' and `rxspencer' packages, available at
`http://arglist.com/guile/'.
While scsh was designed primarily for Scheme, most of it can be
ported with some adaptations to other languages. We are currently aware
of one such port for Objective Caml, called Cash, which can be found at:
`http://pauillac.inria.fr/cash/'
Installing and using scsh
*************************
Now that you have downloaded scsh, you might want to install and use
it. Some help about this subject is provided here.
Compilation problems
====================
Scsh should compile without problems on most Unix platforms.
Particular notes:
1. On MacOS X, with scsh versions older than 0.6.0, you had to specify
the host type to configure, since autoconf was not able to
autodetect it; you had to include `--host=powerpc-apple-bsd' in the
configure commandline.
2. On Linux, compiling versions older than 0.5.2 with the new version
on the glibc caused problems. The 0.5.2 release notes say
"problems with the signal system blowing up builds on some of the
more obscure Unix systems have been fixed").
Is there a "port" of scsh for FreeBSD?
======================================
Installing scsh on FreeBSD is best done by compiling FreeBSD's scsh
"port" (meaning the FreeBSD term of a port, which is an integrated
third-party package) or by getting a binary "package" from a FreeBSD
ftp server. The FreeBSD port is available under `ports/lang/scsh'.
It looks like I do not have enough memory to compile scsh?!?
============================================================
If you get errors like "not enough memory" when building scsh, you
may try to adjust the limits on memory usage imposed by your system.
To do this, you have to use the `ulimit' command under `sh' and
derivatives or the `unlimit' command under `csh' and derivatives
(`tcsh' and the like). See the reference manual of your shell for more
information.
Is there some kind of "contributed code archive" for scsh?
==========================================================
The following resources may be of interest to you:
1. The Scheme Untergrund Library, which collects contributed code for
Scheme 48 and scsh:
`http://www.nongnu.org/sunterlib/'
2. The scsh Wiki on which you can, among other things, share some
code snippets `http://www.scsh.net/cgi-bin/wiki.cgi'
3. The resource page, on the scsh home page
`http://www.scsh.net/resources.html'.
4. The scsh contributed code repository, at
`ftp://ftp.scsh.net/pub/scsh/contrib/', Which currently includes:
1. sunet, an extensible web server written by Olin Shivers with
extensions by Michael Sperber;
2. Functional Postscript, which provides a Scheme interface to
the Postscript page description language;
3. A text markup system by Scott Draves and Jonathan Rees;
4. pgscsh, a socket-level interface to the PostgreSQL
object-relational DBMS, by Eric Marsden.
Go on and send more code.
5. The various Scheme code repositories, which are all listed in the
Scheme FAQ. The two main repositories are the Scheme Repository
at Indiana University:
`http://www.cs.indiana.edu/scheme-repository/home.html'
and the CMU AI Repository, Scheme Section (a.k.a. the CMU Scheme
Repository):
`http://www.cs.cmu.edu/Web/Groups/AI/html/repository.html'.
Also, some useful code is included with Scheme 48 (hash tables
support, sorting functions, etc.) in the Big Scheme module. Please
notice that you will have to open the module before being able to
access its functions. For additional information, check the file
`doc/big-scheme.txt' in the scsh distribution.
If you want to write some code for scsh but you don't know what, you
might want to take a look at the scsh home-page (*note Getting scsh::)
which contains a list of interesting projects.
Can I use "plain" Scheme code with scsh?
========================================
Generally speaking, all of the existing Scheme code can be run
without problem with scsh. There is only *one* possibly annoying
incompatibility between R5RS-compliant interpreters and scsh: Symbols in
scsh are case-sensitive while this is not true for R5RS-compliant
interpreters. This means, for example, that the following expression:
(eq? 'symbol 'Symbol)
evaluates to `#t' with an R5RS-compliant interpreter (including the
original Scheme 48), while it evaluates to `#f' with scsh.
In practice this shouldn't be a big problem, but if you encounter
code that works perfectly with all Scheme interpreters except scsh, then
this may be the reason.
If you want to know the design decision behind this choice, you
should read the technical report describing the design of scsh (*note
Getting the docs::).
There are also other extensions to R5RS in scsh (e.g. C-like escaped
characters in strings) but they shouldn't break existing Scheme code;
you should have them in mind, however, when trying to write portable
Scheme code under scsh.
Can I use scsh as an interactive shell?
=======================================
Well, technically you can: just run the "scsh" command and you will
enter a Scheme 48 session with all scsh functions available. However,
this is definitely not suitable for interactive work: there is no
command-line editing, no command-line history, no file/function name
completion, no terse syntax, etc. All these features are planned, and
Olin has a design for much of them. However, nobody found the time to
implement them yet.
In the meantime, a nice solution is to use a separate tool which
provides some of these features. Here is a partial list of such tools:
1. Emacs: use the `cmuscheme' package, written by Olin. It is now
part of Emacs, but if you don't have it on your system, you may
use the one provided with scsh, which is also a little more
up-to-date (check the directory `emacs'). This mode enables you
to run scsh (or any Scheme interpreter by the way) as an inferior
process. It provides command-line editing, command-line history,
dynamic completion, file-name completion, automatic indentation of
Scheme code and more.
If you want to give it a try right now, just type `C-u M-x
run-scheme', and then enter `scsh' at the prompt.
2. Some terminal emulator that enables input (or output) editing. An
example is the 9term terminal emulator, inspired by the Plan 9
terminal emulator. Check out 9term's home-page at:
`http://www.cs.su.oz.au/~matty/9term/index.html'
3. Any text editor that can run a process in one of its windows. An
example is wily (although it is more than a text editor), inspired
by Plan 9's ACME tool. For more information:
`http://www.cs.yorku.ca/~oz/wily/'
I get "undefined variable" errors when I try to use some functions?!?
=====================================================================
If you get "undefined variable" errors when you use functions from
the big-scheme package or macros like `define-record', then maybe you
didn't open the appropriate packages. To open them, there are two
solutions:
1. use the `,open' command in interactive mode, or
2. use Scheme 48's module system.
The first solution is nice for interactive work, while the second is
the one to use for scripts.
Documentation on the Scheme 48 module system can be found in the
Scheme 48 documentation. Olin Shivers also posted a message with
further explanations to the scsh newsgroup, which is archived at
`http://groups.google.com/groups?selm=qijyawastzo.fsf%40lambda.ai.mit.edu'.
Can I use SLIB (a Scheme library) with scsh?
============================================
Yes, provided that you get (or write) an initialization file for
scsh. Tomas By wrote one that you can get there:
`ftp://ftp.dcs.shef.ac.uk/home/tomas/scsh.init'
By the way, more information about SLIB is available by following
this URL:
`http://www-swiss.ai.mit.edu/~jaffer/SLIB.html'
Some basic I/O functions (like EOF testing) seem not available in scsh?!?
=========================================================================
Don't forget that scsh is built on top of Scheme. Therefore, you
have access to the full power of Scheme in scsh, and that includes some
basic I/O functions, like the test for EOF, etc. However, these
functions are not documented in the scsh manual, but in the official
Scheme specification (R5RS, *note Getting the docs::).
How can I return the eof-object?
================================
Some functions and macros (like the nice AWK macro) take a reader
function as an argument. This reader function is required to return the
eof-object at the end of the input. This is easy when the input is a
port, but much harder when the input is something else (like a list of
lines, etc.). The reason is that R5RS specifies that the eof-object
can't be read by the `read' procedure, and therefore can't be included
literally in your source. However, it can be defined like that:
(define eof-object (read (make-string-input-port "")))
Is there support for protocols like HTTP, SMTP, etc.?
=====================================================
Yes, but it isn't included in the scsh distribution. You will find
it in the contributed code directory for scsh:
`ftp://ftp.scsh.net/pub/scsh/contrib/'
I get strange errors with some network functions?!?
===================================================
If you are using scsh 0.4.2 under Solaris 2 or Irix 5, and the errors
you get look like:
Error: 122
"Operation not supported on transport endpoint"
#{Procedure 9398 %listen}
then you should switch to a newer version of scsh: this was a known bug
of scsh 0.4.2.
If, for some reason, you want to stick with v0.4.2, here is how to
fix the bug:
In scsh's distribution directory, edit the file
`scsh/solaris/netconst.scm' (if you are under Solaris 2 and above) or
`scsh/irix/netconst.scm' (if you are under Irix 5 and above) so that
the following lines:
(define socket-type/stream 1) ; stream socket
(define socket-type/datagram 2) ; datagram socket
(define socket-type/raw 3) ; raw-protocol interface
;;(define socket-type/rdm 4) ; reliably-delivered message
;;(define socket-type/seqpacket 5) ; sequenced packet stream
are replaced by the following lines:
(define socket-type/stream 2) ; stream socket
(define socket-type/datagram 1) ; datagram socket
(define socket-type/raw 4) ; raw-protocol interface
;;(define socket-type/rdm 5) ; reliably-delivered message
;;(define socket-type/seqpacket 6) ; sequenced packet stream
then recompile scsh, by running make in the main directory, and
reinstall it.
How do I get the multiple values returned by a function?
========================================================
This is documented in the R5RS. However, with all these
continuations, the documentation might be a little hard to understand
for newcomers. So here is a little (although not very useful) example
that uses `values' and `call-with-values':
(call-with-values (lambda () (values 6 7)) *)
=> 42
As you can see, the first argument to `call-with-values' is a
procedure which return multiple values, and the second is a procedure
which gets these multiple values as arguments.
Scheme 48 provides another syntax to access multiple values: the
`receive' macro. This macro binds multiple values returned by an
expression to variables, and then evaluates a sequence of expressions
with these bindings active (for Common Lisp fans, this is similar to
`multiple-value-bind'). Here is the above example, rewritten using
`receive':
(receive (x y) (values 6 7) (* x y))
=> 42
For more information on this function, check out
`doc/big-scheme.txt'.
While this may not be evident here, the `receive' macro is often
easier to use than `call-with-values'.
How do I interface scsh with a C function?
==========================================
Use the Scheme 48 facility to interface with C, documented in the
Scheme 48 manual.
What is the syntax of regular expressions?
==========================================
Scsh 0.5.2 introduced support for SREs, or Structural Regular
Expressions. These provide an s-expression notation for building up
and operating on regular expressions. See the SRE section of the manual
for further details.
Standard string-based regexps are also available (and in fact SREs
compile to string-based regexps). Henry Spencer's POSIX regular
expression engine is used to implement the matching. The syntax
accepted by this engine is described in its man page, which can be
found in the scsh distribution, in file `scsh/regexp/regex.7'.
How should I handle errors?
===========================
Scsh raises exceptions instead of passing error status codes via the
`errno' variable (this makes error handling much simpler). You can use
the `with-errno-handler' form to handle these errors gracefully.
Certain error conditions are signalled by calls to the `error'
primitive. If you wish to intercept these conditions gracefully you can
write your own handler. The following example shows how to intercept the
host-not-found condition on DNS lookup.
#!/usr/local/bin/scsh \
-dm -m whnf -e main -s
!#
(define-structure whnf
(export main)
(open scheme scsh handle)
(begin
(define (with-host-not-found* thunk)
(call-with-current-continuation
(lambda (k)
(with-handler
(lambda (condition next)
(cond ((string-match "^name->host-info" (cadr condition))
(display "No such host")
(newline)
(k '()))
(else (next))))
thunk))))
(define-syntax with-host-not-found
(syntax-rules ()
((with-host-not-found ?body ...)
(with-host-not-found* (lambda () ?body ...)))))
(define (main args)
(with-host-not-found
(host-info "foo.bar.com")))))
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