Provided by: scheme9_2017.11.09-1_amd64 
NAME
s9 ‐ Scheme Interpreter
USAGE
s9 [‐hv?] [‐i name|‐] [‐gqu] [‐d image] [‐e expr]
[‐k size[m]] [‐l prog] [‐n size[m]] [‐r expr]
[‐f prog [args]] [‐‐ [args]] [prog [args]]
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 name | ‐
Load alternative image file ‘name.image’. When no image
file can be found, try to load ‘name.scm’. The file will
be searched in the entire S9FES_LIBRARY_PATH (see
below). When this option is used, it must be the first
one of the s9 command.
When ‘‐’ is specified as name, no heap image will be
loaded, and the core library will be read from the
source file ‘s9.scm’.
‐d file
Dump heap image to file and exit.
‐e expr
Read expression from argument, evaluate, and print
value. Multiple ‐e options may be given, which will
evaluate from left to right. Interactive mode will not
be entered.
‐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 from the *arguments* variable. This
option must be the last one. The ‐f flag is optional.
‐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) nodes (‐k 0 means
no limit; use with care!).
‐l program
Load program before entering the REPL or processing ‐f
(may be repeated).
‐n N[m]
Set node limit to N kilo (or mega) nodes (‐n 0 means no
limit; use with care!).
‐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
bound to the *arguments* variable instead. This option
must be the last one.
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 = (sys:exit)
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:
(argv integer) ==> string | #f
Retrieve the value of the given command line argument.
Return #f, if there are less than integer+1 arguments.
Arguments start at 0.
(bit‐op integer1 integer2 integer3 ...) ==> integer | #f
Implement a variety of bitwise operations. See the bit‐
op help page for details.
(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.
(environ 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.
(eval object1) ==> object
(eval object1 object2) ==> object
Evaluate object1 in the current environment and return
its normal form. If object2 is also specified, it is
ignored.
(exponent real) ==> integer
Extract the exponent part from a real number.
(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
(gensym symbol) ==> symbol
(gensym string) ==> symbol
Return a fresh symbol. When a string or symbol argument
is given, use it as prefix for the fresh symbol.
(load‐from‐library string) ==> unspecific
Attempt to load the file string from each directory of
S9FES_LIBRARY_PATH.
(locate‐file string) ==> string | #f
Search for the file string in each directory of
S9FES_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 is a list resembling a macro application,
return the expanded form, else return the object.
Macro‐expand‐1 expands macros only once while macro‐
expand expands them recursively.
(mantissa real) ==> integer
Extract the mantissa part from a real number.
(print object ...) ==> unspecific
Write multiple objects separated by spaces.
(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 given
exts 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 form) ==> form
Evaluate the given form and return a list containing its
normal form plus a summary of the resources used to
compute that normal form:
‐ reduction steps
‐ conses allocated
‐ total nodes allocated
‐ garbage collections
Each resource count will be returned as a group of
integers representing ones, thousands, millions, etc.
Note that form must be quoted or it will be evaluated
before passing it to stats.
(symbols) ==> list
Return a list of all defined symbols.
(system 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 unspecific
slots 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 descriptions of the Scheme 9
extension procedures.
SPECIAL VARIABLES
These variables are predefined in the dynamic top‐level
scope of the interpreter.
** (form)
The normal form of the expression most recently
evaluated at the top level.
*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 ‐f
file args.
*extensions* (list of symbols)
Compiled‐in extensions.
*host‐system* (symbol)
The host system running the s9 interpreter: unix, plan9,
or #F (unknown).
*library‐path* (string)
A verbatim copy of the S9FES_LIBRARY_PATH environment
variable (see below).
*loading* (boolean)
Set to #t when loading a file, else #f.
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 tree‐walking interpreter using deep binding and
hashed environments. It employs an extremely reliable[1]
constant‐space mark and sweep garbage collector with in‐situ
string and 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 library.
The interpreter searches its library path (either built‐
in or specified in the S9FES_LIBRARY_PATH environment
variable) for a heap image file or the library source
code. The heap image file is the name of the interpreter
with a .image suffix appended. An alternative name can
be specified with the ‐i option (see OPTIONS). The
default library source code is named s9.scm. First all
directories of the library path are searched for images,
then the directories are searched for library sources.
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.
When a ‐l file option is found, the program contained in
the given file will be loaded. When a ‐f file args
option is found, the program contained in the file will
be run and then S9 will exit. Args will be passed to
the program.
Enter REPL.
Interactive mode is only entered, when no ‐e, ‐f, or ‐r
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.
BUGS
You may not quasiquote quasiquote unless in unquote (e.g.:
‘‘x does not work, but ‘,‘x does).
The macro expander will expand (x) in (cond (x)), if x is a
macro.
Syntax‐rules is not fully hygienic.
Call/cc must be the only argument when used in lambda (or
derived binding syntax, such as let). Not observing this
rule will break the interpreter.
FILES
@S9DIR@
The S9fES procedure library (source code).
@S9DIR@/s9.image
The interpreter heap image.
*.scm
Scheme source code.
ENVIRONMENT
S9FES_LIBRARY_PATH
A colon‐separated list of directories which will be
searched for the s9 library when the interpreter is
launched. The same directories will be searched by the
locate‐file procedure.
Default: .:˜/.s9fes:@S9DIR@
SIGNALS
SIGINT
Abort input or terminate program execution.
SIGQUIT
Terminate the interpreter process (emergency exit).
SIGTERM
Silently terminate the interpreter process.
"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 Revisedˆ4 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