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================ RScheme (release 0.7.3.3) =================
README
This is the README file for RScheme release 0.7.3.3.
What is RScheme?
RScheme is an object-oriented, extended version of the Scheme dialect of
Lisp. RScheme is freely redistributable, and offers reasonable performance
despite being extraordinarily portable. RScheme can be compiled to C, and
the C can then compiled with a normal C compiler to generate machine code.
This can be done from a running system, and the resulting object code can be
dynamically linked into RScheme as a program executes. By default,
however, RScheme compiles to bytecodes which are interpreted by
a (runtime) virtual machine. This ensures that compilation is fast and
keeps code size down. In general, we recommend using the (default) bytecode
code generation system, and only compiling your time-critical code to
machine code. This allows a nice adjustment of space/time tradeoffs.
To the casual user, RScheme appears to be an interpreter. You
can type RScheme code at a read-eval-print loop, and it executes the
code and prints the result. In reality, every expression you type to
the read-eval-print-loop is compiled and the resulting code is executed.
Features of RScheme
RScheme has a lot of useful features:
0. A very liberal license, similar to Tcl's, which allows RScheme to
be used in commercial software with little burden on the user except
to acknowledge the source. See the COPYING file.
1. R4RS (Revised**4 Report on Scheme) compliant Scheme. It's the
full standard Scheme language, making it useful for writing portable
programs, or using programs written for other Scheme dialects.
Scheme is a small but powerful language with garbage
collection, first-class procedures, list processing features,
lexical scope, etc. Various introductory texts on Scheme and
Scheme programming are available.
2. Object system. Everything in RScheme is an object in an object-oriented
system. That is, RScheme is "objects all the way down" rather than
being a hybrid language like C++ or most Lisp object systems---the
built-in R4RS Scheme data types are all classes in the class hierarchy,
as they should be. Currently, only a simple object system with
single inheritance and single dispatching is supported. Our object
system is approximately a subset of TinyCLOS, with some Dylan-like
extensions.
3. Threads. RScheme supports user-level threads portably, even on
operating systems that don't have threads at all. RScheme multiplexes
several user threads onto one OS-level process. These threads
appear to be preemptive from the normal user's point of view.
(For example, nonblocking I/O can be used automatically so that
one thread doing a read from a network socket doesn't block the
whole system;
the reading thread will actually issue a nonblocking read request
and a thread switch will activate a waiting thread. You don't
have to schedule threads "cooperatively" --- just use threads
and specify nonblocking i/o. RScheme does the rest.) [note: by
default, the system uses stdio for input and output. You will require
the facilities of the syscalls package to have a useful threads system
that does console or network I/O]
4. Sockets, globbing, etc. Rscheme has several important features
for writing serious programs, especially under UNIX. It is suitable
for writing networked servers, for example---that's one of the things
it was designed for. RScheme does not currently have as many shell
programming features as Olin Shivers' scsh (the Scheme shell), but
we will add more, and we may port full scsh to RScheme.
5. Separately-compilable and linkable modules. The module system is
pretty straightforward and simple, similar to Scheme-48's module
system and a subset of Dylan's. It makes development easy by
supporting separate compilation and dynamic module linking.
6. Real-time garbage collection, using Wilson and Johnstone's algorithm.
This is a hard real-time garbage collector, in principle, though
the default configuration is only soft real time. This is sufficient
for most purposes, such as writing interactive applications and
network servers---e.g., to avoid spurious network timeouts because
a thread is delayed by a GC.
Unlike most language implementations that compile to C, RScheme does
NOT use conservative pointer finding in its garbage collector.
RScheme's strategy for compiling to C supports precise identification
of root pointers, as well as pointers in heap-allocated objects,
eliminating potential storage leaks and unreliability due to
misidentification of pointers. (We don't want to overstate our
case here, however---conservative pointer finding works surprisingly
well, and Hans Boehm et al. have done an amazing job on free conservative
GC's for use with C, C++, etc. We have avoided dependence on conservative
pointer finding because we simply didn't need it, and the supprot for
precise pointer identification can be helpful for a variety of other
purposes.)
[For more on RTGC, see <http://www.cs.utexas.edu/users/oops> ]
7. Persistence, using the strategy described in Wilson and Kakkad's
"pointer swizzling at page fault time".
Our persistent object store avoids the need to read and write data
to files, for most purposes. Data can simply be saved on disk as
a special kind of virtual memory image, and processes can open
a persistent store and start traversing its data structures in the
usual way---pages of data are faulted into virtual memory as needed,
on demand, with performance comparable to normal paging.
8. The ability to hand-code time-critical routines in C and plug them
into applications written mostly in RScheme. Since at the moment
we can't claim to have a state-of-the-art compiler that generates
awesome code for everything, we allow speed freaks to hand-code
important things in C and integrate them into their programs. (We
discourage the overuse of this feature! RScheme provides acceptable
performance for most programs, and for most parts of most other
programs, so if you overdo it, you're just making life harder for
yourself.)
9. Big number support. For this, you need to have the gmp libraries
and the header file longlong.h from the gmp sources.
Where to get RScheme
If you have found this file on a web server, in a net announcement, or
whatever, and you want RScheme, you can ftp it from ftp.rscheme.org as
pub/rscheme/rs-0.7.3.3.tar.gz.
What's in the Distribution
If you have ftp'd and untar'd the distribution, you probably found
this file at the top level. Also at this level are several subdirectories,
described below.
README This file---the one you're reading.
INSTALL How to configure, compile, and install RScheme
COPYING License for free use
doc This holds the documentation of the system. Included are
a user reference (user_ref.ps) and notes on the design
and implementation (design_and_impl.ps). The documentation
source is not shipped in this distribution.
modules This is the source for the compiler and various other modules,
in RScheme. Most of the subdirectories hold the code for some
particular module.
handc This is miscellaneous handwritten C code, mostly runtime support
code such as the GC and the low-level parts of the thread
system. This source gets copied into `src/' by the top-level
build process.
compiler This contains the "cross" compiler, which is written in the
language implemented by RScheme 0.6, but compiles code for 0.7.
(this is also the off-line module module, rsc, because it works
in 0.7 too)
test This directory holds regression test files
src The is is the C code generated automatically from RScheme code.
The subdirectories correspond to the subdirectories
of `modules/', and hold the C code generated from the RScheme code
in those directories, plus copies of the subdirectories of `handc/'.
bytcodes This contains the definitions of the bytecode operations and
hooks for the bytecode compiler to make use of the low-level
type system.
packages This contains the standard, but optional, packages, such as
the persistent store (rstore) and posix interface package
(syscalls).
Goals of the RScheme project
============================
We have two major goals for RScheme: building a very useful system, and
building a clean and powerful system that can be used to implement
a variety of langauge constructs, or even entirely different languages,
portably and with good performance.
In more detail, our goals are
1. To build a useful language that has all of the basic features that
any good modern programming language should have, and which provides
decent performance without being a big complicated monster. To
be useful, it should be extremely portable. Nobody should have to
worry that the vendor will go out of business and the software won't
be ported to new platforms. Hence, we compile to C. As long as there
are C compilers on new platforms, it should be very easy to port
RScheme and get good performance. We don't expect C to die in the
next few decades---it's the FORTRAN of the next 30 years.
Compiling to C has its drawbacks. C was not designed to be a compiler
intermediate representation for other languages, so it is not the
optimal backend. On the other hand, we have found novel strategies
for mapping RScheme onto C with surprisingly little degradation of
performance. (We don't just use C as a portable assembler, as some
people do. For example, in many cases, nested RScheme expressions
can be compiled to nested C expressions, and the C compiler will do
a good job of register allocation and instruction scheduling.)
GNU C provides several handy extensions, such as the ability to use
hardware registers to hold important global variables. RScheme
will take advantage of these features if the configure process
recognizes an opportunity.
2. To build a useful framework for research and development in programming
language design and implementation. RScheme was originally motivated
by our frustration with existing language implementations, which are
mostly very limited and very inefficient---or big, hairy, and nonnportable
monsters. Typically, implementors of interesting new languages write
whole new compilers for them, because none of the existing compilers
are flexible enough and understandable enough to make it worth reusing
other people's code. As a result, there are a whole lot of mediocre
to bad langauge implementations out there, and that's a shame. People
keep using languages like C and C++ because they know there will be
decent compilers for them everywhere.
We want to leverage C's momentum, and allow better languages to be easily
implemented with reliably good performance on essentially all common
platforms.
-- Donovan Kolbly RScheme Development Group
d.kolbly@rscheme.org
http://www.rscheme.org/~donovan/