NAME
s9 - Scheme Interpreter

USAGE
s9 [-i file | -] [-hv?] [-gqu] [-e expr] [-d file]
[-k cells] [-l file] [-n nodes] [-r expr]
[-- argument ... | [-f] file argument ...]

DESCRIPTION
Scheme 9 from Empty Space is an interpreter for R4RS Scheme with some additional procedures for accessing typical Unix system calls and Unix and Curses library functions (if compiled-in). The s9 command starts the interpreter.

OPTIONS
-h, -v, or -?
Display a brief summary of options.

-i file | -
Load alternative image file `file'. Image files will be searched in the directory specified in S9FES_IMAGE_DIR (see below) except when the file name begins with a `.' or `/' character. When this option is used, it must be the first one of the s9 command.
When `-' is specified as file, no heap image will be loaded, and the core library will be read from the source file `s9.scm' in the current directory.

-d file
Dump heap image to file and exit.

-e expr
Read expression from expr, evaluate it, and print its value. Multiple -e options may be given, which will evaluate from left to right. Interactive mode will not be entered.

-g[g]
Print GC summaries. A single `g' will report pool growth, a second `g' will also include data about pool usage after each collection.

-k N[m]
Set vector limit to N kilo (or mega) cells (-k 0 means no limit; use with care!). An m suffix indicates 2^20*cells, the default is 2^10*cells.

-l program
Load program before entering the REPL or processing -e and -r options or running a final program. May be repeated.

-n N[m]
Set node limit to N kilo (or mega) nodes (-n 0 means no limit; use with care!). An m suffix indicates 2^20*nodes, the default is 2^10*nodes.

-q
Be quiet: skip banners and prompts, exit on errors.

-r expr
Like -e, but no value will print. Used to run expressions for effect.

-u
Short for -k 0 -n 0 (use unlimited memory).

-- [argument ...]
Arguments following -- are not interpreted by S9fES, but passed to the interpreted program, which can extract them using the command-line procedure. This option must be the last one.

[-f] program [argument ...]
Run program and exit (implies -q). When there are any arguments, they are passed to the program, where they can be extracted using the command-line procedure. This option must be the last one. The -f flag is deprecated.

ONLINE HELP
When the interpreter is running and the default heap image is loaded, just type (help) or ,h to invoke the online help system. When the online help system is not loaded, you will have to run the following command first:

(load-from-library "help.scm")

META COMMANDS
In order to facilitate the invocation of frequently-used top-level procedures, s9 provides the following `meta commands' (they work only when entered directly at the s9 prompt):

,a text  =  (apropos "text")
,h text  =  (help "text")
,l file  =  (load-from-library "file")
,q       =  (quit)

The arguments of ,a and ,h are optional.

ADDITIONS
S9fES supports nestable block comments of the form

#| comment |#

Square brackets may be used in the places of parentheses:

(cond [(foo) (bar)])

The same type of bracket must be used on both ends of a list.

These S9fES procedures are not in R4RS:

(bit-op integer1 integer2 integer3 ...) ==> integer
Implement a variety of bitwise operations. See the bit-op help page for details.

(command-line) ==> list
Return a list of command line arguments passed to the a program being interpreted or following the -- command line option.

(delete-file string) ==> unspecific
Delete the file specified in the string argument. If the file does not exist or cannot be deleted, report an error.

(environment-variable string) ==> string | #f
Retrieve the value of the given environment variable. Return #f, if the variable is undefined.

(error string) ==> undefined
(error string object) ==> undefined
Print an error message of the form error: string: object and terminate program execution. When no object argument is given, this part will be omitted in the message.

(eval object) ==> object
Evaluate object in the current environment and return its normal form.

(exponent number) ==> integer
Extract the exponent part from a number. This will be 0 for integers.

(file-exists? string) ==> boolean
Return #t if the file specified in the string argument exists and otherwise #f.

(fold-left proc base list ...) ==> object
Combine the elements of the lists using proc. Combine elements left-associatively. Base is the leftmost element.

(fold-right proc base list ...) ==> object
Combine the elements of the lists using proc. Combine elements right-associatively. Base is the rightmost element.

(gensym) ==> symbol
Return a fresh symbol.

(load-from-library string) ==> unspecific
Attempt to load the file string from each directory listed in the top-level variable *library-path*.

(locate-file string) ==> string | #f
Search for the file string in each directory listed in *library-path* in sequence. When the file can be located, return its full path, else return #f.

(macro-expand object) ==> object
(macro-expand-1 object) ==> object
If object contains a macro application, return its expanded form, else return the object unchanged. Macro-expand-1 expands macros only once while macro-expand expands them recursively.

(mantissa number) ==> integer
Extract the mantissa part from a number. This will be the number itself for integers.

(print object ...) ==> unspecific
Write multiple objects separated by spaces and with a trailing newline.

(require-extension ext ...) ==> unspecific
Require the named extensions to be compiled-in. Signal an error if not all of the required extensions are present. Ext may be of the form (or ext1 ext2 ...). In this case, it is sufficient if at least one of the exts gven in or is present.

(reverse! list) ==> list
Reverse list destructively and return the reverse list.

(set-input-port! input-port) ==> unspecific
Destructively set the current input port.

(set-output-port! output-port) ==> unspecific
Destructively set the current output port.

(stats expr) ==> object
Evaluate the given expression and return a list containing its normal form plus a summary of the resources used to compute that normal form:

- total nodes allocated
- conses allocated
- vector cells allocated
- garbage collections

Each resource count will be returned as a group of integers representing ones, thousands, millions, etc. Note that expr must be quoted or it will be evaluated before passing it to stats.

(symbols) ==> list
Return a list of all defined symbols.

(system-command string) ==> number
Run the given shell command and return its exit code.

(vector-append vector ...) ==> vector
Return a fresh vector containing the concatenation of the given vectors.

(vector-copy vector) ==> vector
(vector-copy vector integer) ==> vector
(vector-copy vector integer1 integer2) ==> vector
(vector-copy vector integer1 integer2 object) ==> vector
Return a copy of the given vector. When integer1 is specified, skip the given number of elements. When integer2 is also specified, copy elements from integer1 up to, but not including, integer2. When integer2 exceeds the size of the original vector, add slots of an unspecific value to the copy. When an object argument is given, fill extra slots with that argument.

(void) ==> unspecific
Return an unspecific value.

Refer to the help pages for more detailed descriptions of the Scheme 9 extension procedures.

SPECIAL VARIABLES
These variables are predefined in the top-level scope of the interpreter.

** (object)
The normal form of the expression most recently evaluated at the top level. Unspecific results and aborted computations do not change the value of **.

*arguments* (list of strings)
A list of command line arguments passed to the Scheme program (not to the interpreter), i.e. the args in s9 file args or s9 -- args.

*epsilon* (real number)
The smallest difference between two real numbers.

*extensions* (list of symbols)
Identifiers naming the compiled-in extensions.

*host-system* (symbol)
The host system running the s9 interpreter: unix, plan9, or unknown.

*image-file* (string or #f)
The name of the heap image file loaded at startup time. When no image is loaded, the variable is set to #f.

*library-path* (list of strings)
A list containing each path contained in the S9FES_LIBRARY_PATH environment variable (see below).

*loading* (boolean)
Set to #t when loading a file, else #f.

*release-date* (string)
A string containing the release date of the S9 interpreter in the form `YYYY-MM-DD'.

MACROS
A macro is a procedure that is applied to its unevaluated arguments. The macro application is replaced with the value returned by the procedure. This happens before the expression containing the macro application is evaluated, so a macro rewrites its own application:

(define-syntax (when p . c)
  `(if ,p (begin ,@c)))
(macro-expand '(when (= 1 1) (display "true") (newline) #t))
  ==>  (if (= 1 1)
           (begin (display "true")
                  (newline)
                  #t))
(when (= 1 1) 1 2 3)  ==>  3

The define-syntax form introduces a new macro:

(define-syntax name procedure) ==> unspecific
(define-syntax (name args ...) body) ==> unspecific

Both of these forms introduce the keyword name and bind it to a procedure. The first form requires the second argument to be a procedure. Like in define forms the second variant implies a procedure definition.

Macros may contain applications of macros that were defined earlier. Macros may not recurse directly, but they may implement recursion internally using letrec or by rewriting their own applications. The following macro, for example, does not work, because d is undefined in the body of d:

(define-syntax (d x) (and (pair? x) (d (cdr x)))) ; wrong

The following version does work, though:

(define-syntax (d x) (and (pair? x) `(d ,(cdr x)))) ; OK

The body of define-syntax may be a syntax-rules transformer, as described in R4RS, if the syntax-rules extension has been loaded.

TECHNICAL DETAILS
S9fES is a bytecode compiler and SECD abstract machine using shallow binding, primitive procedure inlining, and in-situ lambda lifting. It employs an extremely reliable[1] constant-space mark and sweep garbage collector with in-situ vector pool compaction. Memory pools grow on demand. The interpreter uses arbitrary-precision integer arithmetics and decimal-based real number arithmetics.

INTERPRETER START-UP
When the s9 interpreter is started, the following steps will be performed in this order:

Load heap image or library.
The interpreter searches the image directory for a heap image file and loads it. The image directory is taken from the S9FES_IMAGE_DIR environment variable. When the variable is undefined, a compiled-in default will be used. The default image file is named s9.image. An alternative name can be specified with the -i option (see OPTIONS). When no image file is found in S9FES_IMAGE_DIR, a fallback image is loaded from a file named s9.image in the local directory. When no fallback image can be found, an error is signalled and the interpreter will not start.

Initialize extensions.
Any extensions compiled into the interpreter are initialized by calling the nullary procedure ext:ext, where ext is the name of the extension. The procedures are optional. The last `extension' being initialized is S9 itself, so when a procedure named s9:s9 exists, it will be called at this point.

Evaluate command line options.
First all files specified in -l file options will be loaded in the specified order. Then all expressions specified in -e expr and -r expr command line options will be evaluated in the given order. After that, a -d file option will be evaluated, if present. After processing such an option, the interpreter will exit immediately. Finally, if a program is being passed to the interpreter, the program will be run and any options following the program name will be passed to the program. In this case S9 will exit after program execution.

Enter REPL.
Interactive mode is only entered, when no -e, -r, or -d option was given and no program was specified.

ALLOCATION STRATEGY
The S9fES memory pools grow exponentially until the memory limit is reached. When the limit is reached, the current computation is aborted. A memory limit can be specified using the -k, -n, and -u command line options. The limit is specified in units of 1024 nodes/cells (or in units of 1024*1024 nodes/cells by appending an m suffix).

Note that computations may abort before the limit is reached due to the way the pool grows. Use the -g command line option to experiment with pool sizes.

Specifying a limit of zero (or using the -u option) disables the memory limit completely and the interpreter will allocate as much memory as it can get. This option should be used with care.

LIMITATIONS
These parts of R4RS are not implemented:

I/O: char-ready? (this is in the sys-unix extension).
Transcripts: transcript-off, transcript-on.
Rational and complex numbers and related procedures.
The atan procedure does not accept a second argument.
Inexact digits of the form #.

BUGS
You may not quasiquote quasiquote unless in unquote (e.g.: ``x does not work, but `,`x does).
Syntax-rules is not fully hygienic.

FILES
/usr/share/s9fes
The S9fES procedure library (source code).
s9.image
The interpreter heap image.
*.scm
Scheme source code.

ENVIRONMENT
S9FES_IMAGE_DIR
The directory that will be searched for heap image files when the s9 interpreter starts.

S9FES_LIBRARY_PATH
A colon-separated list of directories which will be searched for library files by the load-from-library and locate-file procedures. The value of this variable will be split at the colons and copied to the *library-path* variable when S9 starts.
Default: .:~/.s9fes:/usr/share/s9fes

SIGNALS
SIGINT
Abort input or terminate program execution.
SIGQUIT
Terminate the interpreter process (emergency exit).
"interrupt"
On Plan 9, receiving an interrupt note will abort input or terminate program execution, as SIGINT would on Unix.

FOOTNOTES
[1] See comp.lang.scheme Usenet message
<vhtzl9lupyp.fsf@maharal.csail.mit.edu>
(Thu, 27 Aug 2009 13:27:42 -0400) and its follow-ups.

REFERENCES
The Revised^4 Report on the Algorithmic Language Scheme.
http://www-swiss.ai.mit.edu/~jaffer/r4rs_toc.html

Scheme 9 from Empty Space -- A Guide to Implementing Scheme in C.
Available at Lulu.com, see http://www.t3x.org

AUTHOR
Nils M Holm