Although Larceny continues to support the R6RS standard, parts
of that standard (notably lexical syntax) have been superseded
by the R7RS (small) standard.  See HOWTO-R7RS.


ABOUT R6RS

In August 2007, the R6RS standard for the Scheme programming
language was ratified by 65.7% of the voters, which exceeded the
required 60% supermajority [1].

The most important thing to know about the R6RS is that it was a
supplement to the R5RS, not a replacement.  The R6RS had nothing
to say about traditional Scheme programs; it just added two new
kinds of Scheme programs, both intended for execution of entire
programs in batch mode.  For interactive read/eval/print loops
and dynamic loading of libraries or traditional Scheme programs,
the relevant standards are IEEE-1178, R5RS, and R7RS [2,3].
Larceny supports all of these standards; for more about R7RS,
see HOWTO-R7RS.

The second most important thing to know about the R6RS is that
R6RS programs are not portable unless they limit themselves to
the standard R6RS libraries and do not define any libraries of
their own.  The R6RS did not define any mechanism for mapping
library names to files or to other representations of library
source or executable code.  Implementations of the R6RS may
define their own implementation-specific mechanisms that allow
R6RS programs to define and to import nonstandard libraries,
but any program that relies on this is non-portable.

R6RS libraries are best regarded as a standard distribution
syntax for program fragments that can be installed locally
using some implementation-specific protocol.


IMPLEMENTATIONS

Larceny v0.95 was the first complete implementation of an
R6RS-compatible system.  When v0.95 was released, another
implementation was 80% complete, and implementors of three
other systems had announced their intention to support the
R6RS within a year.  The implementors of thirteen other
systems had said they did not plan to implement the R6RS in
toto, although some intended to implement selected features
of the R6RS.

As of March 2015, eleven implementations of the R6RS or a subset
have been released.  Eleven implementations of the R7RS or a
subset have also been released.  Larceny is one of at least
three implementations of the R7RS that continue to support the
older R6RS standard.


R6RS PROGRAMS

There are two distinct kinds of R6RS programs.  A top-level
program consists of an import form followed by a sequence
of definitions and expressions.

A Scheme script has almost the same syntax as a top-level
programs, but its first line is ignored.  Scheme scripts were
described by a non-binding and unratified appendix, and are not
supported by all implementations of the R6RS; their syntax was
designed for Unix, and Larceny currently supports Scheme scripts
only on Unix systems.  Experience has shown that Scheme scripts
are not at all portable between different implementations of the
R6RS.


DEFINING LIBRARIES

As an extension to the R6RS, Larceny allows a top-level
program or Scheme script to define R6RS libraries within
the file that contains the top-level program or Scheme
script, before the import form that begins the top-level
program.  These libraries must be arranged so that no
library depends upon libraries that come later in the
file.

This extension is non-portable.


LIBRARY PATH

As an extension to the R6RS, Larceny automatically loads
libraries when they are first imported, provided Larceny
can find the libraries in its library search path.  The
library search path defaults to a path that contains
the predefined libraries listed in HOWTO-ERR5RS, but
more directories can be added to the library search path
using Larceny's --path command-line option.

Autoloaded libraries must be defined in files that follow
Larceny's standard naming conventions for ERR5RS/R6RS
library files.  These naming conventions are described
in Larceny's user manual [4].

This extension is non-portable.


LIBRARY PATH ENVIRONMENT VARIABLE

As an extension to the R6RS, Larceny allows the library
path to be specified by a LARCENY_LIBPATH environment
variable instead of the --path command-line option.

This extension is non-portable.


RUNNING A TOP-LEVEL PROGRAM

To run a Scheme script, just execute it the same way
you'd execute any other Unix script.  For detailed
instructions, see HOWTO-SCRIPT.

To execute a top-level R6RS program that is contained
within a file named pgm, type:

    larceny -r6rs -program pgm

The -program option can be omitted, in which case
Larceny will read the top-level program from standard
input:

    larceny -r6rs < pgm

If you omit the -program option and do not redirect
the standard input, then Larceny will wait patiently
for you to type a complete top-level program into
standard input, terminating it with an end-of-file.

You probably don't want to do that.  Had you wanted to
interact with a read/eval/print loop, you'd be using an
implementation of the R7RS instead of the R6RS.  See
HOWTO-R7RS.


PREDEFINED LIBRARIES

In addition to the standard R6RS libraries, Larceny
predefines a variety of other libraries that R6RS
programs and libraries can import.  For a partial
list of predefined libraries, see Larceny's user
manual [4].

We recommend that R6RS programs use SRFI 99 records
instead of the corresponding R6RS libraries.  SRFI 99
record types interoperate more smoothly with R6RS records
than R6RS record types themselves interoperate with
R6RS records, and the standard reference implementation
of SRFI 99 records should run efficiently in any
R6RS-compatible system that allows new libraries to be
defined at all.


R6RS CONFORMANCE

Larceny is R6RS-compatible but not R6RS-conforming.

That means Larceny is a practical implementation of
the R6RS, capable of running almost any R6RS program
with the efficiency you'd expect from an optimizing
compiler and precise generational garbage collection.

Larceny is non-conforming because it does not perform
all of the checking for non-portable code that the
R6RS requires implementations to perform.  Since R6RS
libraries are non-portable anyway, the R6RS-mandated
checking for other portability problems is overly
prescriptive.

Larceny performs all checks needed to ensure safety
as that concept has traditionally been defined.


RECOMMENDATIONS

If you are writing a program that will run only in Larceny,
then there is no good reason for you to limit yourself to
the R6RS standard.  In Larceny, R7RS programs can use all
useful features of the R6RS without being limited by R6RS
restrictions.

If your goal is to create or to distribute substantial
programs and libraries that will run without modification
in various implementations of the R6RS, then you will have
to learn how to write programs that remain within the
intersection of features that are supported by the set of
R6RS implementations you wish to use.

--------

[1] http://www.r6rs.org/
[2] http://www.schemers.org/Documents/Standards/R5RS/
[3] http://www.scheme-reports.org/
[4] http://larceny.ccs.neu.edu/doc/
